Documentation Source Text

<title>SQLite Database File Format (Version 2)</title>

<h2>SQLite 2.X Database File Format</h2>

<p>
This document describes the disk file format for SQLite versions 2.1
through 2.8.  SQLite version 3.0 and following uses a very different
format which is described separately.
</p>

<h3>1.0 &nbsp; Layers</h3>

<p>
SQLite is implemented in layers.
(See the <a href="arch.html">architecture description</a>.)
The format of database files is determined by three different
layers in the architecture.
</p>

<ul>
<li>The <b>schema</b> layer implemented by the VDBE.</li>
<li>The <b>b-tree</b> layer implemented by btree.c</li>
<li>The <b>pager</b> layer implemented by pager.c</li>
</ul>

<p>
We will describe each layer beginning with the bottom (pager)
layer and working upwards.
</p>

<h3>2.0 &nbsp; The Pager Layer</h3>

<p>
An SQLite database consists of
"pages" of data.  Each page is 1024 bytes in size.
Pages are numbered beginning with 1.
A page number of 0 is used to indicate "no such page" in the
B-Tree and Schema layers.
</p>

<p>
The pager layer is responsible for implementing transactions
with atomic commit and rollback.  It does this using a separate
journal file.  Whenever a new transaction is started, a journal
file is created that records the original state of the database.
If the program terminates before completing the transaction, the next
process to open the database can use the journal file to restore
the database to its original state.
</p>

<p>
The journal file is located in the same directory as the database
file and has the same name as the database file but with the
characters "<tt>-journal</tt>" appended.
</p>

<p>
The pager layer does not impose any content restrictions on the
main database file.  As far as the pager is concerned, each page
contains 1024 bytes of arbitrary data.  But there is structure to
the journal file.
</p>

<p>
A journal file begins with 8 bytes as follows:
0xd9, 0xd5, 0x05, 0xf9, 0x20, 0xa1, 0x63, and 0xd6.
Processes that are attempting to rollback a journal use these 8 bytes
as a sanity check to make sure the file they think is a journal really
is a valid journal.  Prior version of SQLite used different journal
file formats.  The magic numbers for these prior formats are different
so that if a new version of the library attempts to rollback a journal
created by an earlier version, it can detect that the journal uses
an obsolete format and make the necessary adjustments.  This article
describes only the newest journal format - supported as of version
2.8.0.
</p>

<p>
Following the 8 byte prefix is a three 4-byte integers that tell us
the number of pages that have been committed to the journal,
a magic number used for
sanity checking each page, and the
original size of the main database file before the transaction was
started.  The number of committed pages is used to limit how far
into the journal to read.  The use of the checksum magic number is
described below.
The original size of the database is used to restore the database
file back to its original size.
The size is expressed in pages (1024 bytes per page).
</p>

<p>
All three integers in the journal header and all other multi-byte
numbers used in the journal file are big-endian.
That means that the most significant byte
occurs first.  That way, a journal file that is
originally created on one machine can be rolled back by another
machine that uses a different byte order.  So, for example, a
transaction that failed to complete on your big-endian SparcStation
can still be rolled back on your little-endian Linux box.
</p>

<p>
After the 8-byte prefix and the three 4-byte integers, the
journal file consists of zero or more page records.  Each page
record is a 4-byte (big-endian) page number followed by 1024 bytes
of data and a 4-byte checksum.  
The data is the original content of the database page
before the transaction was started.  So to roll back the transaction,
the data is simply written into the corresponding page of the
main database file.  Pages can appear in the journal in any order,
but they are guaranteed to appear only once. All page numbers will be
between 1 and the maximum specified by the page size integer that
appeared at the beginning of the journal.
</p>

<p>
The so-called checksum at the end of each record is not really a
checksum - it is the sum of the page number and the magic number which
was the second integer in the journal header.  The purpose of this
value is to try to detect journal corruption that might have occurred
because of a power loss or OS crash that occurred which the journal
file was being written to disk.  It could have been the case that the
meta-data for the journal file, specifically the size of the file, had
been written to the disk so that when the machine reboots it appears that
file is large enough to hold the current record.  But even though the
file size has changed, the data for the file might not have made it to
the disk surface at the time of the OS crash or power loss.  This means
that after reboot, the end of the journal file will contain quasi-random
garbage data.  The checksum is an attempt to detect such corruption.  If
the checksum does not match, that page of the journal is not rolled back.
</p>

<p>
Here is a summary of the journal file format:
</p>

<ul>
<li>8 byte prefix: 0xd9, 0xd5, 0x05, 0xf9, 0x20, 0xa1, 0x63, 0xd6</li>
<li>4 byte number of records in journal</li>
<li>4 byte magic number used for page checksums</li>
<li>4 byte initial database page count</li>
<li>Zero or more instances of the following:
   <ul>
   <li>4 byte page number</li>
   <li>1024 bytes of original data for the page</li>
   <li>4 byte checksum</li>
   </ul>
</li>
</ul>

<h3>3.0 &nbsp; The B-Tree Layer</h3>

<p>
The B-Tree layer builds on top of the pager layer to implement
one or more separate b-trees all in the same disk file.  The
algorithms used are taken from Knuth's <i>The Art Of Computer
Programming.</i></p>

<p>
Page 1 of a database contains a header string used for sanity
checking, a few 32-bit words of configuration data, and a pointer
to the beginning of a list of unused pages in the database.
All other pages in the
database are either pages of a b-tree, overflow pages, or unused
pages on the freelist.
</p>

<p>
Each b-tree page contains zero or more database entries.
Each entry has an unique key of one or more bytes and data of
zero or more bytes.
Both the key and data are arbitrary byte sequences.  The combination
of key and data are collectively known as "payload".  The current
implementation limits the amount of payload in a single entry to
1048576 bytes.  This limit can be raised to 16777216 by adjusting
a single #define in the source code and recompiling.  But most entries
contain less than a hundred bytes of payload so a megabyte limit seems
more than enough.
</p>

<p>
Up to 238 bytes of payload for an entry can be held directly on
a b-tree page.  Any additional payload is contained on a linked list
of overflow pages.  This limit on the amount of payload held directly
on b-tree pages guarantees that each b-tree page can hold at least
4 entries.  In practice, most entries are smaller than 238 bytes and
thus most pages can hold more than 4 entries.
</p>

<p>
A single database file can hold any number of separate, independent b-trees.
Each b-tree is identified by its root page, which never changes.
Child pages of the b-tree may change as entries are added and removed
and pages split and combine.  But the root page always stays the same.
The b-tree itself does not record which pages are root pages and which
are not.  That information is handled entirely at the schema layer.
</p>

<h4>3.1 &nbsp; B-Tree Page 1 Details</h4>

<p>
Page 1 begins with the following 48-byte string:
</p>

<blockquote><pre>
** This file contains an SQLite 2.1 database **
</pre></blockquote>

<p>
If you count the number of characters in the string above, you will
see that there are only 47.  A '\000' terminator byte is added to
bring the total to 48.
</p>

<p>
A frequent question is why the string says version 2.1 when (as
of this writing) we are up to version 2.7.0 of SQLite and any
change to the second digit of the version is suppose to represent
a database format change.  The answer to this is that the B-tree
layer has not changed any since version 2.1.  There have been
database format changes since version 2.1 but those changes have
all been in the schema layer.  Because the format of the b-tree
layer is unchanged since version 2.1.0, the header string still
says version 2.1.
</p>

<p>
After the format string is a 4-byte integer used to determine the
byte-order of the database.  The integer has a value of
0xdae37528.  If this number is expressed as 0xda, 0xe3, 0x75, 0x28, then
the database is in a big-endian format and all 16 and 32-bit integers
elsewhere in the b-tree layer are also big-endian.  If the number is
expressed as 0x28, 0x75, 0xe3, and 0xda, then the database is in a
little-endian format and all other multi-byte numbers in the b-tree 
layer are also little-endian.  
Prior to version 2.6.3, the SQLite engine was only able to read databases
that used the same byte order as the processor they were running on.
But beginning with 2.6.3, SQLite can read or write databases in any
byte order.
</p>

<p>
After the byte-order code are six 4-byte integers.  Each integer is in the
byte order determined by the byte-order code.  The first integer is the
page number for the first page of the freelist.  If there are no unused
pages in the database, then this integer is 0.  The second integer is
the number of unused pages in the database.  The last 4 integers are
not used by the b-tree layer.  These are the so-called "meta" values that
are passed up to the schema layer
and used there for configuration and format version information.
All bytes of page 1 past beyond the meta-value integers are unused 
and are initialized to zero.
</p>

<p>
Here is a summary of the information contained on page 1 in the b-tree layer:
</p>

<ul>
<li>48 byte header string</li>
<li>4 byte integer used to determine the byte-order</li>
<li>4 byte integer which is the first page of the freelist</li>
<li>4 byte integer which is the number of pages on the freelist</li>
<li>36 bytes of meta-data arranged as nine 4-byte integers</li>
<li>928 bytes of unused space</li>
</ul>

<h4>3.2 &nbsp; Structure Of A Single B-Tree Page</h4>

<p>
Conceptually, a b-tree page contains N database entries and N+1 pointers
to other b-tree pages.
</p>

<blockquote>
<table border=1 cellspacing=0 cellpadding=5>
<tr>
<td align="center">Ptr<br>0</td>
<td align="center">Entry<br>0</td>
<td align="center">Ptr<br>1</td>
<td align="center">Entry<br>1</td>
<td align="center"><b>...</b></td>
<td align="center">Ptr<br>N-1</td>
<td align="center">Entry<br>N-1</td>
<td align="center">Ptr<br>N</td>
</tr>
</table>
</blockquote>

<p>
The entries are arranged in increasing order.  That is, the key to
Entry 0 is less than the key to Entry 1, and the key to Entry 1 is
less than the key of Entry 2, and so forth.  The pointers point to
pages containing additional entries that have keys in between the
entries on either side.  So Ptr 0 points to another b-tree page that
contains entries that all have keys less than Key 0, and Ptr 1
points to a b-tree pages where all entries have keys greater than Key 0
but less than Key 1, and so forth.
</p>

<p>
Each b-tree page in SQLite consists of a header, zero or more "cells"
each holding a single entry and pointer, and zero or more "free blocks"
that represent unused space on the page.
</p>

<p>
The header on a b-tree page is the first 8 bytes of the page.
The header contains the value
of the right-most pointer (Ptr N) and the byte offset into the page
of the first cell and the first free block.  The pointer is a 32-bit
value and the offsets are each 16-bit values.  We have:
</p>

<blockquote>
<table border=1 cellspacing=0 cellpadding=5>
<tr>
<td align="center" width=30>0</td>
<td align="center" width=30>1</td>
<td align="center" width=30>2</td>
<td align="center" width=30>3</td>
<td align="center" width=30>4</td>
<td align="center" width=30>5</td>
<td align="center" width=30>6</td>
<td align="center" width=30>7</td>
</tr>
<tr>
<td align="center" colspan=4>Ptr N</td>
<td align="center" colspan=2>Cell 0</td>
<td align="center" colspan=2>Freeblock 0</td>
</tr>
</table>
</blockquote>

<p>
The 1016 bytes of a b-tree page that come after the header contain
cells and freeblocks.  All 1016 bytes are covered by either a cell
or a freeblock.
</p>

<p>
The cells are connected in a linked list.  Cell 0 contains Ptr 0 and
Entry 0.  Bytes 4 and 5 of the header point to Cell 0.  Cell 0 then
points to Cell 1 which contains Ptr 1 and Entry 1.  And so forth.
Cells vary in size.  Every cell has a 12-byte header and at least 4
bytes of payload space.  Space is allocated to payload in increments
of 4 bytes.  Thus the minimum size of a cell is 16 bytes and up to
63 cells can fit on a single page.  The size of a cell is always a multiple
of 4 bytes.
A cell can have up to 238 bytes of payload space.  If
the payload is more than 238 bytes, then an additional 4 byte page
number is appended to the cell which is the page number of the first
overflow page containing the additional payload.  The maximum size
of a cell is thus 254 bytes, meaning that a least 4 cells can fit into
the 1016 bytes of space available on a b-tree page.
An average cell is usually around 52 to 100 bytes in size with about
10 or 20 cells to a page.
</p>

<p>
The data layout of a cell looks like this:
</p>

<blockquote>
<table border=1 cellspacing=0 cellpadding=5>
<tr>
<td align="center" width=20>0</td>
<td align="center" width=20>1</td>
<td align="center" width=20>2</td>
<td align="center" width=20>3</td>
<td align="center" width=20>4</td>
<td align="center" width=20>5</td>
<td align="center" width=20>6</td>
<td align="center" width=20>7</td>
<td align="center" width=20>8</td>
<td align="center" width=20>9</td>
<td align="center" width=20>10</td>
<td align="center" width=20>11</td>
<td align="center" width=100>12 ... 249</td>
<td align="center" width=20>250</td>
<td align="center" width=20>251</td>
<td align="center" width=20>252</td>
<td align="center" width=20>253</td>
</tr>
<tr>
<td align="center" colspan=4>Ptr</td>
<td align="center" colspan=2>Keysize<br>(low)</td>
<td align="center" colspan=2>Next</td>
<td align="center" colspan=1>Ksz<br>(hi)</td>
<td align="center" colspan=1>Dsz<br>(hi)</td>
<td align="center" colspan=2>Datasize<br>(low)</td>
<td align="center" colspan=1>Payload</td>
<td align="center" colspan=4>Overflow<br>Pointer</td>
</tr>
</table>
</blockquote>

<p>
The first four bytes are the pointer.  The size of the key is a 24-bit
where the upper 8 bits are taken from byte 8 and the lower 16 bits are
taken from bytes 4 and 5 (or bytes 5 and 4 on little-endian machines.)
The size of the data is another 24-bit value where the upper 8 bits
are taken from byte 9 and the lower 16 bits are taken from bytes 10 and
11 or 11 and 10, depending on the byte order.  Bytes 6 and 7 are the
offset to the next cell in the linked list of all cells on the current
page.  This offset is 0 for the last cell on the page.
</p>

<p>
The payload itself can be any number of bytes between 1 and 1048576.
But space to hold the payload is allocated in 4-byte chunks up to
238 bytes.  If the entry contains more than 238 bytes of payload, then
additional payload data is stored on a linked list of overflow pages.
A 4 byte page number is appended to the cell that contains the first
page of this linked list.
</p>

<p>
Each overflow page begins with a 4-byte value which is the
page number of the next overflow page in the list.   This value is
0 for the last page in the list.  The remaining
1020 bytes of the overflow page are available for storing payload.
Note that a full page is allocated regardless of the number of overflow
bytes stored.  Thus, if the total payload for an entry is 239 bytes,
the first 238 are stored in the cell and the overflow page stores just
one byte.
</p>

<p>
The structure of an overflow page looks like this:
</p>

<blockquote>
<table border=1 cellspacing=0 cellpadding=5>
<tr>
<td align="center" width=20>0</td>
<td align="center" width=20>1</td>
<td align="center" width=20>2</td>
<td align="center" width=20>3</td>
<td align="center" width=200>4 ... 1023</td>
</tr>
<tr>
<td align="center" colspan=4>Next Page</td>
<td align="center" colspan=1>Overflow Data</td>
</tr>
</table>
</blockquote>

<p>
All space on a b-tree page which is not used by the header or by cells
is filled by freeblocks.  Freeblocks, like cells, are variable in size.
The size of a freeblock is at least 4 bytes and is always a multiple of
4 bytes.
The first 4 bytes contain a header and the remaining bytes
are unused.  The structure of the freeblock is as follows:
</p>

<blockquote>
<table border=1 cellspacing=0 cellpadding=5>
<tr>
<td align="center" width=20>0</td>
<td align="center" width=20>1</td>
<td align="center" width=20>2</td>
<td align="center" width=20>3</td>
<td align="center" width=200>4 ... 1015</td>
</tr>
<tr>
<td align="center" colspan=2>Size</td>
<td align="center" colspan=2>Next</td>
<td align="center" colspan=1>Unused</td>
</tr>
</table>
</blockquote>

<p>
Freeblocks are stored in a linked list in increasing order.  That is
to say, the first freeblock occurs at a lower index into the page than
the second free block, and so forth.  The first 2 bytes of the header
are an integer which is the total number of bytes in the freeblock.
The second 2 bytes are the index into the page of the next freeblock
in the list.  The last freeblock has a Next value of 0.
</p>

<p>
When a new b-tree is created in a database, the root page of the b-tree
consist of a header and a single 1016 byte freeblock.  As entries are
added, space is carved off of that freeblock and used to make cells.
When b-tree entries are deleted, the space used by their cells is converted
into freeblocks.  Adjacent freeblocks are merged, but the page can still
become fragmented.  The b-tree code will occasionally try to defragment
the page by moving all cells to the beginning and constructing a single
freeblock at the end to take up all remaining space.
</p>

<h4>3.3 &nbsp; The B-Tree Free Page List</h4>

<p>
When information is removed from an SQLite database such that one or
more pages are no longer needed, those pages are added to a list of
free pages so that they can be reused later when new information is
added.  This subsection describes the structure of this freelist.
</p>

<p>
The 32-bit integer beginning at byte-offset 52 in page 1 of the database
contains the address of the first page in a linked list of free pages.
If there are no free pages available, this integer has a value of 0.
The 32-bit integer at byte-offset 56 in page 1 contains the number of
free pages on the freelist.
</p>

<p>
The freelist contains a trunk and many branches.  The trunk of
the freelist is composed of overflow pages.  That is to say, each page
contains a single 32-bit integer at byte offset 0 which
is the page number of the next page on the freelist trunk.
The payload area
of each trunk page is used to record pointers to branch pages. 
The first 32-bit integer in the payload area of a trunk page
is the number of branch pages to follow (between 0 and 254)
and each subsequent 32-bit integer is a page number for a branch page.
The following diagram shows the structure of a trunk freelist page:
</p>

<blockquote>
<table border=1 cellspacing=0 cellpadding=5>
<tr>
<td align="center" width=20>0</td>
<td align="center" width=20>1</td>
<td align="center" width=20>2</td>
<td align="center" width=20>3</td>
<td align="center" width=20>4</td>
<td align="center" width=20>5</td>
<td align="center" width=20>6</td>
<td align="center" width=20>7</td>
<td align="center" width=200>8 ... 1023</td>
</tr>
<tr>
<td align="center" colspan=4>Next trunk page</td>
<td align="center" colspan=4># of branch pages</td>
<td align="center" colspan=1>Page numbers for branch pages</td>
</tr>
</table>
</blockquote>

<p>
It is important to note that only the pages on the trunk of the freelist
contain pointers to other pages.  The branch pages contain no
data whatsoever.  The fact that the branch pages are completely
blank allows for an important optimization in the paging layer.  When
a branch page is removed from the freelist to be reused, it is not
necessary to write the original content of that page into the rollback
journal.  The branch page contained no data to begin with, so there is
no need to restore the page in the event of a rollback.  Similarly,
when a page is not longer needed and is added to the freelist as a branch
page, it is not necessary to write the content of that page
into the database file.
Again, the page contains no real data so it is not necessary to record the
content of that page.  By reducing the amount of disk I/O required,
these two optimizations allow some database operations
to go four to six times faster than they would otherwise.
</p>

<h3>4.0 &nbsp; The Schema Layer</h3>

<p>
The schema layer implements an SQL database on top of one or more
b-trees and keeps track of the root page numbers for all b-trees.
Where the b-tree layer provides only unformatted data storage with
a unique key, the schema layer allows each entry to contain multiple
columns.  The schema layer also allows indices and non-unique key values.
</p>

<p>
The schema layer implements two separate data storage abstractions:
tables and indices.  Each table and each index uses its own b-tree
but they use the b-tree capabilities in different ways.  For a table,
the b-tree key is a unique 4-byte integer and the b-tree data is the
content of the table row, encoded so that columns can be separately
extracted.  For indices, the b-tree key varies in size depending on the
size of the fields being indexed and the b-tree data is empty.
</p>

<h4>4.1 &nbsp; SQL Table Implementation Details</h4>

<p>Each row of an SQL table is stored in a single b-tree entry.
The b-tree key is a 4-byte big-endian integer that is the ROWID
or INTEGER PRIMARY KEY for that table row.
The key is stored in a big-endian format so
that keys will sort in numerical order using memcmp() function.</p>

<p>The content of a table row is stored in the data portion of
the corresponding b-tree table.  The content is encoded to allow
individual columns of the row to be extracted as necessary.  Assuming
that the table has N columns, the content is encoded as N+1 offsets
followed by N column values, as follows:
</p>

<blockquote>
<table border=1 cellspacing=0 cellpadding=5>
<tr>
<td>offset 0</td>
<td>offset 1</td>
<td><b>...</b></td>
<td>offset N-1</td>
<td>offset N</td>
<td>value 0</td>
<td>value 1</td>
<td><b>...</b></td>
<td>value N-1</td>
</tr>
</table>
</blockquote>

<p>
The offsets can be either 8-bit, 16-bit, or 24-bit integers depending
on how much data is to be stored.  If the total size of the content
is less than 256 bytes then 8-bit offsets are used.  If the total size
of the b-tree data is less than 65536 then 16-bit offsets are used.
24-bit offsets are used otherwise.  Offsets are always little-endian,
which means that the least significant byte occurs first.
</p>

<p>
Data is stored as a nul-terminated string.  Any empty string consists
of just the nul terminator.  A NULL value is an empty string with no
nul-terminator.  Thus a NULL value occupies zero bytes and an empty string
occupies 1 byte.
</p>

<p>
Column values are stored in the order that they appear in the CREATE TABLE
statement.  The offsets at the beginning of the record contain the
byte index of the corresponding column value.  Thus, Offset 0 contains
the byte index for Value 0, Offset 1 contains the byte offset
of Value 1, and so forth.  The number of bytes in a column value can
always be found by subtracting offsets.  This allows NULLs to be
recovered from the record unambiguously.
</p>

<p>
Most columns are stored in the b-tree data as described above.
The one exception is column that has type INTEGER PRIMARY KEY.
INTEGER PRIMARY KEY columns correspond to the 4-byte b-tree key.
When an SQL statement attempts to read the INTEGER PRIMARY KEY,
the 4-byte b-tree key is read rather than information out of the
b-tree data.  But there is still an Offset associated with the
INTEGER PRIMARY KEY, just like any other column.  But the Value
associated with that offset is always NULL.
</p>

<h4>4.2 &nbsp; SQL Index Implementation Details</h4>

<p>
SQL indices are implement using a b-tree in which the key is used
but the data is always empty.  The purpose of an index is to map
one or more column values into the ROWID for the table entry that
contains those column values.
</p>

<p>
Each b-tree in an index consists of one or more column values followed
by a 4-byte ROWID.  Each column value is nul-terminated (even NULL values)
and begins with a single character that indicates the datatype for that
column value.  Only three datatypes are supported: NULL, Number, and
Text.  NULL values are encoded as the character 'a' followed by the
nul terminator.  Numbers are encoded as the character 'b' followed by
a string that has been crafted so that sorting the string using memcmp()
will sort the corresponding numbers in numerical order.  (See the
sqliteRealToSortable() function in util.c of the SQLite sources for
additional information on this encoding.)  Numbers are also nul-terminated.
Text values consists of the character 'c' followed by a copy of the
text string and a nul-terminator.  These encoding rules result in
NULLs being sorted first, followed by numerical values in numerical
order, followed by text values in lexicographical order.
</p>

<h4>4.4 &nbsp; SQL Schema Storage And Root B-Tree Page Numbers</h4>

<p>
The database schema is stored in the database in a special tabled named
"sqlite_master" and which always has a root b-tree page number of 2.
This table contains the original CREATE TABLE,
CREATE INDEX, CREATE VIEW, and CREATE TRIGGER statements used to define
the database to begin with.  Whenever an SQLite database is opened,
the sqlite_master table is scanned from beginning to end and 
all the original CREATE statements are played back through the parser
in order to reconstruct an in-memory representation of the database
schema for use in subsequent command parsing.  For each CREATE TABLE
and CREATE INDEX statement, the root page number for the corresponding
b-tree is also recorded in the sqlite_master table so that SQLite will
know where to look for the appropriate b-tree.
</p>

<p>
SQLite users can query the sqlite_master table just like any other table
in the database.  But the sqlite_master table cannot be directly written.
The sqlite_master table is automatically updated in response to CREATE
and DROP statements but it cannot be changed using INSERT, UPDATE, or
DELETE statements as that would risk corrupting the database.
</p>

<p>
SQLite stores temporary tables and indices in a separate
file from the main database file.  The temporary table database file
is the same structure as the main database file.  The schema table
for the temporary tables is stored on page 2 just as in the main
database.  But the schema table for the temporary database named
"sqlite_temp_master" instead of "sqlite_master".  Other than the
name change, it works exactly the same.
</p>

<h4>4.4 &nbsp; Schema Version Numbering And Other Meta-Information</h4>

<p>
The nine 32-bit integers that are stored beginning at byte offset
60 of Page 1 in the b-tree layer are passed up into the schema layer
and used for versioning and configuration information.  The meaning
of the first four integers is shown below.  The other five are currently
unused.
</p>

<ol>
<li>The schema version number</li>
<li>The format version number</li>
<li>The recommended pager cache size</li>
<li>The safety level</li>
</ol>

<p>
The first meta-value, the schema version number, is used to detect when
the schema of the database is changed by a CREATE or DROP statement.
Recall that when a database is first opened the sqlite_master table is
scanned and an internal representation of the tables, indices, views,
and triggers for the database is built in memory.  This internal
representation is used for all subsequent SQL command parsing and
execution.  But what if another process were to change the schema
by adding or removing a table, index, view, or trigger?  If the original
process were to continue using the old schema, it could potentially
corrupt the database by writing to a table that no longer exists.
To avoid this problem, the schema version number is changed whenever
a CREATE or DROP statement is executed.  Before each command is
executed, the current schema version number for the database file
is compared against the schema version number from when the sqlite_master
table was last read.  If those numbers are different, the internal
schema representation is erased and the sqlite_master table is reread
to reconstruct the internal schema representation.
(Calls to sqlite_exec() generally return SQLITE_SCHEMA when this happens.)
</p>

<p>
The second meta-value is the schema format version number.  This
number tells what version of the schema layer should be used to
interpret the file.  There have been changes to the schema layer
over time and this number is used to detect when an older database
file is being processed by a newer version of the library.
As of this writing (SQLite version 2.7.0) the current format version
is "4".
</p>

<p>
The third meta-value is the recommended pager cache size as set 
by the DEFAULT_CACHE_SIZE pragma.  If the value is positive it
means that synchronous behavior is enable (via the DEFAULT_SYNCHRONOUS
pragma) and if negative it means that synchronous behavior is
disabled.
</p>

<p>
The fourth meta-value is safety level added in version 2.8.0.
A value of 1 corresponds to a SYNCHRONOUS setting of OFF.  In other
words, SQLite does not pause to wait for journal data to reach the disk
surface before overwriting pages of the database.  A value of 2 corresponds
to a SYNCHRONOUS setting of NORMAL.  A value of 3 corresponds to a
SYNCHRONOUS setting of FULL. If the value is 0, that means it has not
been initialized so the default synchronous setting of NORMAL is used.
</p>

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<h1>Document Overview</h1>

  <h2>Scope and Purpose</h2>

  <p>
    This document is designed to serve two purposes:
  <ul>
    <li>to provide an engineering guide to the file format used by SQLite, and

    <li>to provide system requirements specifying the behaviour of the SQLite
        software modules responsible for creating and manipulating the
        formatted database files.
  </ul>
  <p>
    Exactly how the database file is created and safely updated on the 
    persistent media is outside the scope of this document. As such no
    mention of journal or statement files is made. Database transactions
    are referred to only with respect to those file manipulation operations 
    (i.e. change-counter management and database reorganization in auto-vacuum
    mode) that occur once per transaction. Here we are concerned solely with
    the arrangement of bytes in the database file, not the interactions between
    the SQLite library and the VFS (Virtual File System) interface.  
  <p>
    Similarly, the various interfaces and SQL language features that may
    be used to manipulate the contents of a database are not dealt with
    here. This document describes the effect of various operations
    on the database, such as creating a table or inserting a new record.
    The myriad of ways that these operations or sets of these operations
    may be achieved using SQLite are dealt with elsewhere.
  <p class=todo>
    Add references to the documents that do describe these things. One other
    document will concentrate on the pager module and the way it uses the VFS
    interface to safely create and update database files.  The other will be
    the document that describes the supported SQL language and capabilities.

  <h2>Document and Requirements Organization</h2>
    <p>
      Section <cite>sqlite_database_files</cite> contains simple 
      requirements describing the relationship between SQLite and the
      definition of a <i>well-formed SQLite database file</i>.
    <p>
      Section <cite>database_file_format</cite> describes the various fields
      and sub-structures that make up the SQLite database file format.
    <p>
      Section <cite>database_file_manipulation</cite> describes the way in
      which these fields and data structures are created, initialized and
      updated.  

  <h2>Glossary</h2>
    <table id=glossary>
      <tr><td>Auto-vacuum last root-page<td>
	A page number stored as 32-bit integer at byte offset 52 of the
        database file header (see section <cite>file_header</cite>). In
        an auto-vacuum database, this is the numerically largest 
	<i>root-page</i> number in the database. Additionally, all pages that
	occur before this page in the database are either B-Tree <i>root
        pages</i>, <i>pointer-map pages</i> or the <i>locking page</i>.

      <tr><td>Auto-vacuum database      <td>
	Each database is either an auto-vacuum database or a non auto-vacuum
        database. Auto-vacuum databases feature pointer-map pages (section
        <cite>pointer_map_pages</cite>) and have a non-zero value stored
	as a 4-byte big-endian integer at offset 52 of the file header (section
        <cite>file_header</cite>).
      <tr><td>B-Tree                    <td>
        A B-Tree is a tree structure optimized for offline storage. The table
	and index data in an SQLite database file is stored in B-Tree
        structures.

      <tr><td>B-Tree cell               <td>
        Each database page that is part of a B-Tree structure contains zero
        or more B-Tree cells. A B-Tree cell contains a single B-Tree key value
	(either an integer or database record) and optionally an associated
        database record value.

      <tr><td>B-Tree page               <td>
	A database page that is part of a B-Tree tree structure (not an
        overflow page).

      <tr><td>(B-Tree) page header      <td>
	The 8 (leaf pages) or 12 (internal node pages) byte header that
        occurs at the start of each B-Tree page.

      <tr><td>Cell content area         <td>
        The area within a B-Tree page in which the B-Tree cells are stored.

      <tr><td>(Database) text encoding  <td>
        The text encoding used for all text values in the database file. One
        of UTF-8, big-endian UTF-16 and little-endian UTF-16. The database
        text encoding is defined by a 4 byte field stored at byte offset
        56 of the database file header (see section <cite>file_header</cite>).

      <tr><td>(Database) file header    <td>
        The first 100 bytes of an SQLite database file constitute the
        database file header.

      <tr><td>(Database) page size      <td>
        An SQLite database file is divided into one or more pages of
        page-size bytes each.

      <tr><td>Database record           <td>
        A database record is a blob of data containing the serialized
        representation of an ordered list of one or more SQL values.

      <tr><td>Database record header    <td>
        The first part of each database record contains the database
        record header. It encodes the types and lengths of values stored
        in the record (see section <cite>record_format</cite>).

      <tr><td>Database record data area <td>
        Following the database record header in each database record is
        the database record data area. It contains the actual data (string
        content, numeric value etc.) of all values in the record 
        (see section <cite>record_format</cite>).

      <tr><td>Default pager cache size  <td>
	A 32-bit integer field stored at byte offset 48 of the database file
	header (see section <cite>file_header</cite>).

      <tr><td style="white-space:nowrap">(Database) usable page size <td>
        The number of bytes of each database page that is usable. This
        is the page-size less the number of bytes left unused at the end
        of each page. The number of bytes left unused is governed by the
        value stored at offset 20 of the file header (see section
        <cite>file_header</cite>).

      <tr><td>File format read version  <td>
	Single byte field stored at byte offset 20 of the database file header
        (see section <cite>file_header</cite>).

      <tr><td>File format write version  <td>
	Single byte field stored at byte offset 19 of the database file header
        (see section <cite>file_header</cite>).

      <tr><td>File change counter       <td>
	A 32-bit integer field stored at byte offset 24 of the database file
	header (see section <cite>file_header</cite>). Normally, SQLite
        increments this value each time it commits a transaction.

      <tr><td>Fragment                  <td>
        A block of 3 or less bytes of unused space within the cell content
        area of a B-Tree page.

      <tr><td>Free block                <td>
        A block of 4 or more bytes of unused space within the cell content
        area of a B-Tree page.

      <tr><td>Free block list           <td>
        The linked list of all free blocks on a single B-Tree page (see 
        section <cite>index_btree_page_format</cite>).

      <tr><td>Free page                 <td>
        A page that is not currently being used to store any database data
        or meta data. Part of the free-page list.

      <tr><td>Free page list            <td>
        A data structure within an SQLite database file that links all the
        free-pages together.

      <tr><td>Index B-Tree              <td>
        One of two variants on the B-Tree data structure used within SQLite
        database files. An index B-Tree (section <cite>index_btrees</cite>)
        uses database records as keys.

      <tr><td>Incremental Vacuum flag   <td>
	A 32-bit integer field stored at byte offset 64 of the database file
	header (see section <cite>file_header</cite>). In auto-vacuum 
        databases, if this field is non-zero then the database is not
        automatically compacted at the end of each transaction.

      <tr><td>Locking page              <td>
        The database page that begins at the 1GB (2<sup>30</sup> byte)
        boundary. This page is always left unused.

      <tr><td>Logical database          <td>
        An SQLite database file is a serialized representation of a logical
        database. A logical database consists of the SQL database schema,
        the content of the various tables in the database, and assorted
	database properties that may be set by the user (auto-vacuum,
        page-size, user-cookie value etc.),

      <tr><td>Non-auto-vacuum database  <td>
        Any database that is not an auto-vacuum database. A non-auto-vacuum
        database contains no pointer-map pages and has a zero value stored
	in the 4-byte big-endian integer field at offset 52 of the database
        file header (section <cite>file_header</cite>).

      <tr><td>Overflow chain             <td>
        A linked list of overflow pages across which a single (large)
        database record is stored (see section 
        <cite>overflow_page_chains</cite>).

      <tr><td>Overflow page             <td>
        If a B-Tree cell is too large to store within a B-Tree page, a
	portion of it is stored using a chain of one or more overflow pages
        (see section <cite>overflow_page_chains</cite>).

      <tr><td>Pointer-map page          <td>
	A database page used to store meta data only present in auto-vacuum
        databases (see section <cite>pointer_map_pages</cite>).

      <tr><td>Right child page          <td>
        Each internal B-Tree node page has one or more child pages. The
        rightmost of these (the one containing the largest key values) is
        known as the right child page.

      <tr><td>Root page                 <td>
        A root page is a database page used to store the root node of a
        B-Tree data structure.

      <tr><td>Schema layer file format  <td>
	An integer between 1 and 4 stored as a 4 byte big-endian integer at
	offset 44 of the file header (section <cite>file_header</cite>).
	Certain file format constructions may only be present in databases
        with a certain minimum schema layer file format value.

      <tr><td>Schema table              <td>
	The table B-Tree with root-page 1 used to store database records
        describing the database schema. Accessible as the "sqlite_master" 
        table from within SQLite.

      <tr><td>Schema version            <td>
	A 32-bit integer field stored at byte offset 40 of the database file
	header (see section <cite>file_header</cite>). Normally, SQLite
        increments this value each time it modifies the databas schema.

      <tr><td>Table B-Tree              <td>
        One of two variants on the B-Tree data structure used within SQLite
        database files. A table B-Tree (section <cite>table_btrees</cite>)
        uses 64 bit integers as key values and stores an associated database
        record along with each key value.

      <tr><td>User cookie               <td>
	A 32-bit integer field stored at byte offset 60 of the database file
	header (see section <cite>file_header</cite>). Normally, SQLite
        increments this value each time it modifies the databas schema.

      <tr><td>Variable Length Integer   <td>
        A format used for storing 64-bit signed integer values in SQLite 
        database files. Consumes between 1 and 9 bytes of space, depending
        on the precise value being stored.

      <tr><td>Well formed database file <td>
        An SQLite database file that meets all the criteria laid out in
        section <cite>database_file_format</cite> of this document.
    </table>

<h1 id=sqlite_database_files>SQLite Database Files</h1>
 
  <p>
    The bulk of this document, section <cite>database_file_format</cite>,
    contains the definition of a <i>well-formed SQLite database file</i>.
    SQLite is required to create database files that meet this definition.

  <p class=req id=FF0010>
    The system shall ensure that at the successful conclusion of a
    database transaction the contents of the database file constitute
    a <i>well-formed SQLite database file</i>.

  <p>
    Additionally, the database file should contain a serialized version
    of the logical database produced by the transaction. For all but the
    most trivial logical databases, there are many possible serial 
    representations.

  <p class=req id=FF0020>
    The system shall ensure that at the successful conclusion of a
    database transaction the contents of the database file are a valid
    serialization of the contents of the logical SQL database produced
    by the transaction.

  <p>
    Section <cite>database_file_manipulation</cite> contains requirements
    describing in more detail the way in which SQLite manipulates the
    fields and data structures described in section
    <cite>database_file_format</cite> under various circumstances. These
    requirements are to a certain extent derived from the requirements 
    in this section.
  

<h1 id=database_file_format>Database File Format Details</h1>

  <p>
    This section describes the various fields and sub-structures that make up
    the format used by SQLite to serialize a logical SQL database. 
  <p>
    This section does not contain requirements governing the behaviour of any
    software system. Instead, along with the plain language description of the
    file format are a series of succinct, testable statements describing the
    properties of "well-formed SQLite database files".  Some of these
    statements describe the contents of the database file in terms of the
    contents of the logical SQL database that it is a serialization of. e.g.
    "For each SQL table in the database, the database file shall...".
    The contents and properties of a logical database
    include:
  <ul>
    <li>Whether or not the database is an auto-vacuum database, and if
        so whether or not incremental-vacuum is enabled,
    <li>The configured page-size of the database,
    <li>The configured text-encoding of the database,
    <li>The configured user-cookie value,
    <li>The set of database tables, indexs, triggers and views that make
        up the database schema and their properties, and
    <li>The data (rows) inserted into the set of database tables.
  </ul>

  <p class=todo>
    This concept of a logical database should be defined properly 
    in some requirements document so that it can be referenced from
    here and other places. The definition will be something like the
    list of bullet points above.

  <p>
    A well-formed SQLite database file is defined as a file for which
    all of the statements itemized as requirements within this section
    are true. <span class=todo>mention the requirements numbering scheme
    here.</span> A software system that wishes to interoperate with other
    systems using the SQLite database file format should only ever
    output well-formed SQLite databases. In the case of SQLite itself,
    the system should ensure that the database file is well-formed at
    the conclusion of each database transaction.

  <h2 id="fileformat_overview">File Format Overview</h2>
    <p>
      A B-Tree is a data structure designed for offline storage of a set of
      unique key values. It is structured so as to support fast querying 
      for a single key or range of keys. As implemented in SQLite, each
      entry may be associated with a blob of data that is not part of the
      key. For the canonical introduction to the B-Tree and its variants, 
      refer to reference <cite>ref_comer_btree</cite>. The B-Tree 
      implementation in SQLite also adopts some of the enhancements 
      suggested in <cite>ref_knuth_btree</cite>
    <p>
      An SQLite database file contains one or more B-Tree structures. Each
      B-Tree structure stores the data for a single database table or 
      index. Hence each database file contains a single B-Tree to store
      the contents of the <i>sqlite_master</i> table, and one B-Tree
      for each database table or index created by the user. If the database
      uses auto-increment integer primary keys, then the database file
      also contains a B-Tree to store the contents of the automatically 
      created <i>sqlite_sequence</i> table.
    <p>
      SQLite uses two distinct variants of the B-Tree structure. One variant,
      hereafter refered to as a "table B-Tree" uses signed 64-bit integer
      values for keys. Each entry has an associated variable length blob of 
      data used to store a database record (see section
      <cite>record_format</cite>). Each SQLite database file contains one 
      table B-Tree for the schema table and one table B-Tree for each
      additional database table created by the user.
    <p>
      A database record is a blob of data containing an ordered list of
      SQL values (integers, real values, NULL values, blobs or strings).
      For each row in each table at the SQL level, there is an 
      entry in the corresponding table B-Tree structure in the file. The
      entry key is same as the SQL "rowid" or "integer primary key" field
      of the table row. The associated database record is made up of the
      row's column values, in declaration (CREATE TABLE) order.
    <p>
      The other B-Tree variant used by SQLite, hereafter an "index B-Tree"
      uses database records (section <cite>record_format</cite>) as keys.
      For this kind of B-Tree, there is no additional data associated with
      each entry. SQLite databases contain an index B-Tree for each database
      index created by the user. Database indexes may be created by CREATE
      INDEX statements, or by UNIQUE or PRIMARY KEY (but not INTEGER PRIMARY
      KEY) clauses added to CREATE TABLE statements. 
    <p>
      Index B-Tree structures contain one entry for each row in the 
      associated table at the SQL level. The database record used as the
      key consists of the row's value for each of the indexed columns in
      declaration (CREATE INDEX) order, followed by the row's "rowid" or
      "integer primary key" column value.
    <p>
      For example, the following SQL script:
    <pre>
      CREATE TABLE t1(a INTEGER PRIMARY KEY, b, c, d);
      CREATE INDEX i1 ON t1(d, c);

      INSERT INTO t1 VALUES(1, 'triangle', 3, 180, 'green');
      INSERT INTO t1 VALUES(2, 'square',   4, 360, 'gold');
      INSERT INTO t1 VALUES(3, 'pentagon', 5, 540, 'grey');
      ...</pre>
    <p>
      Creates a database file containing three B-Tree structures: one table
      B-Tree to store the <i>sqlite_master</i> table, one table B-Tree to 
      store table "t1", and one index B-Tree to store index "i1". The
      B-Tree structures created for the user table and index are populated
      as shown in figure <cite>figure_examplepop</cite>.
      <center><img src="images/fileformat/examplepop.gif">
      <p><i>Figure <span class=fig id=figure_examplepop></span> - Example B-Tree Data.</i>
      </center>

  <h2>Global Structure</h2>
    <p>
      The following sections and sub-sections describe precisely the format
      used to house the B-Tree structures within an SQLite database file.

    <h3 id="file_header">File Header</h3>
      <p>
        Each SQLite database file begins with a 100-byte header. The header
        file consists of a well known 16-byte sequence followed by a series of
        1, 2 and 4 byte unsigned integers. All integers in the file header (as
        well as the rest of the database file) are stored in big-endian format.
        
      <p>
	The well known 16-byte sequence that begins every SQLite database file
        is:
      <pre>
          0x53 0x51 0x4c 0x69 0x74 0x65 0x20 0x66 0x6f 0x72 0x6d 0x61 0x74 0x20 0x33 0x00</pre>

      <p>
        Interpreted as UTF-8 encoded text, this byte sequence corresponds 
        to the string "SQLite format 3" followed by a nul-terminator byte.

      <p>
        The 1, 2 and 4 byte unsigned integers that make up the rest of the
        database file header are described in the following table.

      <table class=striped>
        <tr><th>Byte Range <th>Byte Size <th width=100%>Description
        <tr><td>16..17 <td>2<td>
            Database page size in bytes. See section 
            <cite>pages_and_page_types</cite> for details.

        <tr><td>18     <td>1<td>
            <p style="margin-top:0">
            File-format "write version". Currently, this field
            is always set to 1. If a value greater than 1 is read by SQLite,
            then the library will only open the file for read-only access.

            <p style="margin-bottom:0">
            This field and the next one are intended to be used for 
            forwards compatibility, should the need ever arise. If in the
            future a version of SQLite is created that uses a file format
            that may be safely read but not written by older versions of
            SQLite, then this field will be set to a value greater than 1
            to prevent older SQLite versions from writing to a file that
            uses the new format. 

        <tr><td>19     <td>1<td>
            <p style="margin-top:0">
             File-format "read version". Currently, this 
            field is always set to 1. If a value greater than 1 is read 
            by SQLite, then the library will refuse to open the database 

            <p style="margin-bottom:0">
            Like the "write version" described above, this field exists
            to facilitate some degree of forwards compatibility, in case
            it is ever required. If a version of SQLite created in the 
            future uses a file format that may not be safely read by older
            SQLite versions, then this field will be set to a value greater
            than 1.

        <tr><td>20     <td>1<td>
            Number of bytes of unused space at the end of each database
            page. Usually this field is set to 0. If it is non-zero, then 
            it contains the number of bytes that are left unused at the
            end of every database page (see section
            <cite>pages_and_page_types</cite> for a description of a
            database page).

        <tr><td>21     <td>1<td>
             Maximum fraction of an index tree page to use for 
            embedded content. This value is used to determine the maximum
            size of a B-Tree cell to store as embedded content on a
            page that is part of an index B-Tree. Refer to section 
            <cite>index_btree_cell_format</cite> for details.

        <tr><td>22     <td>1<td>
            Minimum fraction of an index B-Tree page to use for
            embedded content when an entry uses one or more overflow pages.
            This value is used to determine the portion of a B-Tree cell 
            that requires one or more overflow pages to store as embedded
            content on a page that is part of an index B-Tree. Refer to
            section <cite>index_btree_cell_format</cite> for details.

        <tr><td>23     <td>1<td>
            Minimum fraction of an table B-Tree leaf page to use for
            embedded content when an entry uses one or more overflow pages.
            This value is used to determine the portion of a B-Tree cell 
            that requires one or more overflow pages to store as embedded
            content on a page that is a leaf of a table B-Tree. Refer to
            section <cite>table_btree_cell_format</cite> for details.

        <tr><td>24..27 <td>4<td>
            <p style="margin-top:0">
            The file change counter. Each time a database transaction is
            committed, the value of the 32-bit unsigned integer stored in
            this field is incremented.
            <p style="margin-bottom:0">
            SQLite uses this field to test the validity of its internal
            cache. After unlocking the database file, SQLite may retain
            a portion of the file cached in memory. However, since the file
            is unlocked, another process may use SQLite to modify the 
            contents of the file, invalidating the internal cache of the
            first process. When the file is relocked, the first process can
            check if the value of the file change counter has been modified
            since the file was unlocked. If it has not, then the internal
            cache may be assumed to be valid and may be reused.

        <tr><td>32..35 <td>4<td>
            Page number of first freelist trunk page. 
            For more details, refer to section <cite>free_page_list</cite>.

        <tr><td>36..39 <td>4<td>
            Number of free pages in the database file.
            For more details, refer to section <cite>free_page_list</cite>.

        <tr><td>40..43 <td>4<td>
            The schema version. Each time the database schema is modified (by
            creating or deleting a database table, index, trigger or view)
            the value of the 32-bit unsigned integer stored in this field
            is incremented.

        <tr><td>44..47 <td>4<td>
            <p style="margin-top:0">
	    Schema layer file-format. This value is similar to the
            "read-version" and "write-version" fields at offsets 18 and 19
            of the database file header. If SQLite encounters a database
            with a schema layer file-format value greater than the file-format
            that it understands (currently 4), then SQLite will refuse to
            access the database.
            <p>
            Usually, this value is set to 1. However, if any of the following
            file-format features are used, then the schema layer file-format
            must be set to the corresponding value or greater:
            <ol start=2 style="margin-bottom:0">
              <li> Implicit NULL values at the end of table records 
                   (see section <cite>table_btree_content</cite>).
	      <li> Implicit default (non-NULL) values at the end of table
                   records (see section <cite>table_btree_content</cite>).
	      <li> Descending indexes (see section
                   <cite>index_btree_compare_func</cite>) and Boolean values
		   in database records (see section <cite>record_format</cite>,
                   serial types 8 and 9).
            </ol>
            

        <tr><td>48..51 <td>4<td>
            Default pager cache size. This field is used by SQLite to store
            the recommended pager cache size to use for the database.

        <tr><td>52..55 <td>4<td>
            For auto-vacuum capable databases, the numerically largest 
            root-page number in the database. Since page 1 is always the
	    root-page of the schema table (section <cite>schema_table</cite>),
            this value is always non-zero for auto-vacuum databases. For
            non-auto-vacuum databases, this value is always zero.

        <tr><td>56..59 <td>4<td>
            (constant) Database text encoding. A value of 1 means all 
            text values are stored using UTF-8 encoding. 2 indicates
            little-endian UTF-16 text. A value of 3 means that the database
            contains big-endian UTF-16 text.  

        <tr><td>60..63 <td>4<td>
            The user-cookie value. A 32-bit integer value available to the
            user for read/write access.

        <tr><td>64..67 <td>4<td>
            The incremental-vacuum flag. In non-auto-vacuum databases this
            value is always zero. In auto-vacuum databases, this field is
            set to 1 if "incremental vacuum" mode is enabled. If incremental
            vacuum mode is not enabled, then the database file is reorganized
            so that it contains no free pages (section
            <cite>free_page_list</cite>) at the end of each database
            transaction. If incremental vacuum mode is enabled, then the
            reorganization is not performed until explicitly requested
            by the user.

      </table>
      <p>
        The four byte block beginning at offset 28 is unused. As is the
        32 byte block beginning at offset 68.
      </p>

      <p>
	Some of the following requirements state that certain database header
        fields must contain defined constant values, even though the sqlite 
        database file format is designed to allow various values. This is
        done to artificially constrain the definition of a 
        <i>well-formed database</i> in order to make implementation and 
        testing more practical.

      <p class=req id=FF0030>
	The first 16 bytes of a well-formed database file contain the UTF-8
        encoding of the string "SQLite format 3" followed by a single 
        nul-terminator byte.

      <p>
        Following the 16 byte magic string in the file header is the
	<i>page size</i>, a 2-byte field. See section
        <cite>pages_and_page_types</cite> for details.

      <p class=req id=FF0040>
	The 19th byte (byte offset 18), the <i>file-format write version</i>, 
        of a well-formed database file contains the value 0x01.
      <p class=req id=FF0050>
	The 20th byte (byte offset 19), the <i>file-format read version</i>, 
        of a well-formed database file contains the value 0x01.

      <p class=req id=FF0060>
	The 21st byte (byte offset 20), the number of unused bytes on each
        page, of a well-formed database file shall contain the value 0x00.

      <p class=req id=FF0070>
	The 22nd byte (byte offset 21), the maximum fraction of an index 
        B-Tree page to use for embedded content, of a well-formed database 
        file shall contain the value 0x40.
      <p class=req id=FF0080>
	The 23rd byte (byte offset 22), the minimum fraction of an index 
        B-Tree page to use for embedded content when using overflow pages,
        of a well-formed database file contains the value 0x20.
      <p class=req id=FF0090>
	The 24th byte (byte offset 23), the minimum fraction of a table
        B-Tree page to use for embedded content when using overflow pages,
        of a well-formed database file contains the value 0x20.
      <p class=req id=FF0100>
        The 4 byte block starting at byte offset 24 of a well-formed 
        database file contains the <i>file change counter</i> formatted
        as a 4-byte big-endian integer.

      <p>
        Following the <i>file change counter</i> in the database header are
        two 4-byte fields related to the database file <i>free page list</i>.
        See section <cite>free_page_list</cite> for details.

      <p class=req id=FF0110>
        The 4 byte block starting at byte offset 40 of a well-formed 
        database file contains the <i>schema version</i> formatted
        as a 4-byte big-endian integer.

      <p class=req id=FF0120>
	The 4 byte block starting at byte offset 44 of a well-formed
        database file, the <i>schema layer file format</i>, contains a 
        big-endian integer value between 1 and 4, inclusive.

      <p class=req id=FF0130>
        The 4 byte block starting at byte offset 48 of a well-formed 
        database file contains the <i>default pager cache size</i> formatted
        as a 4-byte big-endian integer.

      <p class=req id=FF0140>
        The 4 byte block starting at byte offset 52 of a well-formed 
        database file contains the <i>auto-vacuum last root-page</i> 
        formatted as a 4-byte big-endian integer. If this value is non-zero,
        the database is said to be an <i>auto-vacuum database</i>.

      <p class=req id=FF0150>
	The 4 byte block starting at byte offset 56 of a well-formed
        database file, the <i>text encoding</i> contains a big-endian integer 
        value between 1 and 3, inclusive.

      <p class=req id=FF0160>
        The 4 byte block starting at byte offset 60 of a well-formed 
        database file contains the <i>user cookie</i> formatted
        as a 4-byte big-endian integer.

      <p class=req id=FF0170>
	The 4 byte block starting at byte offset 64 of a well-formed
        database file, the <i>incremental vaccum flag</i> contains a big-endian 
        integer value between 0 and 1, inclusive.

      <p class=req id=FF0180>
	In a well-formed non-autovacuum database (one with a zero stored
        in the 4-byte big-endian integer value beginning at byte offset
        52 of the database file header, the incremental vacuum flag is
        set to 0.

    <h3 id="pages_and_page_types">Pages and Page Types</h3>
      <p>
        The entire database file is divided into pages, each page consisting
        of <i>page-size</i> bytes, where <i>page-size</i> is the 2-byte 
        integer value stored at offset 16 of the file header (see above).
        The <i>page-size</i> is always a power of two between 512 
        (2<sup>9</sup>) and 32768 (2<sup>15</sup>). SQLite database files
        always consist of an exact number of pages.
      <p>
        Pages are numbered beginning from 1, not 0. Page 1 consists of
        the first <i>page-size</i> bytes of the database file. The file header
        described in the previous section consumes the first 100 bytes of page
        1.
      <p>
        Each page of the database file is one of the following:
      <ul>
        <li><b>A B-Tree page</b>. B-Tree pages are part of the tree 
            structures used to store database tables and indexes.
        <li><b>An overflow page</b>. Overflow pages are used by particularly
            large database records that do not fit on a single B-Tree page.
        <li><b>A free page</b>. Free pages are pages within the database file
            that are not being used to store meaningful data.
        <li><b>A "pointer-map" page</b>. In auto-vacuum capable databases
            (databases for which the 4 byte big-endian integer stored at
            byte offset 52 of the file header is non-zero) some pages are
            permanently designated "pointer-map" pages. See section 
            <cite>pointer_map_pages</cite> for details.
        <li><b>The locking page</b>. The database page that starts at
            byte offset 2<sup>30</sup>, if it is large enough to contain
            such a page, is always left unused.
      </ul>

      <p class=req id=FF0190>
        The <i>database page size</i> of a well-formed database, stored as a
        2-byte big-endian unsigned integer at byte offset 16 of the file, 
        shall be an integer power of 2 between 512 and 32768, inclusive.
      <p class=req id=FF0200>
        The size of a <i>well formed database file</i> shall be an integer
        multiple of the <i>database page size</i>.
      <p class=req id=FF0210>
	Each page of a <i>well formed database file</i> is exactly one of a 
        <i>B-Tree page</i>, an <i>overflow page</i>, a <i>free page</i>, a 
        <i>pointer-map page</i> or the <i>locking page</i>.
      <p class=req id=FF0220>
        The database page that starts at byte offset 2<sup>30</sup>, the
        <i>locking page</i>, is never used for any purpose.
        

    <h3 id=schema_table>The Schema Table</h3>
      <p>
        Apart from being the page that contains the file-header, page 1 of
        the database file is special because it is the root page of the
        B-Tree structure that contains the schema table data. From the SQL
        level, the schema table is accessible via the name "sqlite_master".
      <p>
        The exact format of the B-Tree structure and the meaning of the term
        "root page" is discussed in section <cite>btree_structures</cite>.
        For now, it is sufficient to know that the B-Tree structure is a
        data structure that stores a set of records. Each record is an
        ordered set of SQL values (the format of which is described in
        section <cite>record_format</cite>). Given the root page number of
        the B-Tree structure (which is well known for the schema table), it
        is possible to iterate through the set of records.
      <p>
	The schema table contains a record for each SQL table (including
	virtual tables) except for sqlite_master, and for each index, trigger
	and view in the logical database.  There is also an entry for each
	UNIQUE or PRIMARY KEY clause present in the definition of a database
	table. Each record in the schema table contains exactly 5 values, in
        the following order:

      <table class=striped>
        <tr><th>Field<th>Description
        <tr><td>Schema item type.
	    <td>A string value. One of "table", "index", "trigger" or "view",
		according to the schema item type. Entries associated with
		UNIQUE or PRIMARY KEY clauses have this field set to "index".
        <tr><td>Schema item name.
	    <td>A string value. The name of the database schema item (table,
		index, trigger or view) associated with this record, if any.
		Entries associated with UNIQUE or PRIMARY KEY clauses have
		this field set to a string of the form
		"sqlite_autoindex_&lt;name&gt;_&lt;idx&gt;" where &lt;name&gt;
		is the name of the SQL table and &lt;idx&gt; is an integer
		value.

        <tr><td style="white-space:nowrap">Associated table name.
            <td>A string value. For "table" 
            or "view" records this is a copy of the second (previous) value. 
            For "index" and "trigger" records, this field is set to the name 
            of the associated database table.
        <tr><td style="white-space:nowrap">The "root page" number. 
	    <td>For "trigger" and "view" records, as well as "table" records
		associated with virtual tables, this is set to NULL. For other
		"table" and "index" records (including those associated with
		UNIQUE or PRIMARY KEY clauses), this field contains the root
		page number (an integer) of the B-Tree structure that contains
		the table or index data.
        <tr><td>The SQL statement.
            <td>A string value. The SQL statement used to create the schema
                item (i.e.  the complete text of an SQL "CREATE TABLE"
		statement). This field contains an empty string for table
		entries associated with PRIMARY KEY or UNIQUE clauses.
		<span class=todo>Refer to some document that describes these
	        SQL statements more precisely.</span>
      </table>
      <p>
	Logical database schema items other than non-virtual tables and indexes
	(including indexes created by UNIQUE or PRIMARY key constraints) do not
	require any other data structures to be created within the database
	file.

      <p>
        Tables and indexes on the other hand, are represented within the
	database file by both an entry in the schema table and a B-Tree
	structure stored elsewhere in the file. The specific B-Tree associated
        with each database table or index is identified by its root page
        number, which is stored in the 4th field of the schema table record.
	In a non-auto-vacuum database, the B-Tree root pages may be stored
	anywhere within the database file. For an auto-vacuum database, all
        B-Tree root pages must at all times form a contiguous set starting
	at page 3 of the database file, skipping any pages that are required to
	be used as pointer-map pages (see section
        <cite>pointer_map_pages</cite>).
      <p>
	As noted in section <cite>file_header</cite>, in an auto-vacuum
        database the page number of the page immediately following the
        final root page in the contiguous set of root pages is stored
        as a 4 byte big-endian integer at byte offset 52 of the database
	file header. Unless that page is itself a pointer-map page, in which
        case the page number of the page following it is stored instead.

      <p>
        For example, if the schema of a logical database is created using
        the following SQL statements:
      <pre>
          CREATE TABLE abc(a, b, c);
          CREATE INDEX i1 ON abc(b, c);
          CREATE TABLE main.def(a PRIMARY KEY, b, c, UNIQUE(b, c));
          CREATE VIEW v1 AS SELECT * FROM abc;
      </pre>
      <p>
        Then the schema table would contain a total of 7 records, as follows:

      <table class=striped>
        <tr><th>Field 1<th>Field 2<th>Field 3<th>Field 4<th>Field 5
        <tr><td>table <td>abc <td>abc <td>2 <td>CREATE TABLE abc(a, b, c)
        <tr><td>index <td>i1 <td>abc <td>3 <td>CREATE INDEX i1 ON abc(b, c)
        <tr><td>table <td>def <td>def <td>4 <td>CREATE TABLE def(a PRIMARY KEY, b, c, UNIQUE(b, c))
        <tr><td>index <td>sqlite_autoindex_def_1 <td>def <td>5 <td>
        <tr><td>index <td>sqlite_autoindex_def_2 <td>def <td>6 <td>
        <tr><td>view <td>v1 <td>v1 <td>0 <td>CREATE VIEW v1 AS SELECT * FROM abc
      </table>

      <p class=req id=FF0230>
	In a <i>well-formed database file</i>, the portion of the first 
        database page not consumed by the database file-header (all but the 
        first 100 bytes) contains the root node of a table B-Tree, 
        the <i>schema table</i>.
      <p class=req id=FF0240>
	All records stored in the <i>schema table</i> contain exactly five
        fields.

      <p>The following requirements describe "table" records.

      <p class=req id=FF0250>
        For each SQL table in the database apart from itself 
        ("sqlite_master"), the <i>schema table</i> of a <i>well-formed 
        database file</i> contains an associated record.

      <p class=req id=FF0260>
        The first field of each <i>schema table</i> record associated with an
        SQL table shall be the text value "table".

      <p class=req id=FF0270>
        The second field of each <i>schema table</i> record associated with an
	SQL table shall be a text value set to the name of the SQL table.

      <p class=req id=FF0280>
        In a <i>well-formed database file</i>, the third field of all 
        <i>schema table</i> records associated with SQL tables shall contain 
        the same value as the second field.

      <p class=req id=FF0290>
        In a <i>well-formed database file</i>, the fourth field of all 
	<i>schema table</i> records associated with SQL tables that are not
	virtual tables contains the page number (an integer value) of the root
        page of the associated <i>table B-Tree</i> structure within the 
        database file.

      <p class=req id=FF0300>
        If the associated database table is a virtual table, the fourth
        field of the <i>schema table</i> record shall contain an SQL NULL 
        value.

      <p class=req id=FF0310>
        In a well-formed database, the fifth field of all <i>schema table</i>
	records associated with SQL tables shall contain a "CREATE TABLE"
        or "CREATE VIRTUAL TABLE" statment (a text value).  The details
        of the statement shall be such that executing the statement
	would create a table of precisely the same name and schema as the
        existing database table.

      <p>The following requirements describe "implicit index" records.

      <p class=req id=FF0320>
        For each PRIMARY KEY or UNIQUE constraint present in the definition
        of each SQL table in the database, the schema table of a well-formed 
        database shall contain a record with the first field set to the text 
        value "index", and the second field set to a text value containing a
        string of the form "sqlite_autoindex_&lt;name&gt;_&lt;idx&gt;", where
        &lt;name&gt; is the name of the SQL table and &lt;idx&gt; is an 
        integer value.

      <p class=req id=FF0330>
        In a well-formed database, the third field of all schema table
        records associated with SQL PRIMARY KEY or UNIQUE constraints shall
	contain the name of the table to which the constraint applies (a 
        text value).
      <p class=req id=FF0340>
        In a well-formed database, the fourth field of all schema table
	records associated with SQL PRIMARY KEY or UNIQUE constraints shall
	contain the page number (an integer value) of the root page of the
        associated index B-Tree structure.
      <p class=req id=FF0350>
        In a well-formed database, the fifth field of all schema table
        records associated with SQL PRIMARY KEY or UNIQUE constraints shall
        contain an SQL NULL value.

      <p>The following requirements describe "explicit index" records.

      <p class=req id=FF0360>
	For each SQL index in the database, the schema table of a well-formed
	database shall contain a record with the first field set to the text
	value "index" and the second field set to a text value containing the
        name of the SQL index.
      <p class=req id=FF0370>
        In a well-formed database, the third field of all schema table
        records associated with SQL indexes shall contain the name of the
        SQL table that the index applies to.
      <p class=req id=FF0380>
        In a well-formed database, the fourth field of all schema table
        records associated with SQL indexes shall contain the page number 
        (an integer value) of the root page of the associated index B-Tree
        structure.
      <p class=req id=FF0390>
        In a well-formed database, the fifth field of all schema table
	records associated with SQL indexes shall contain an SQL "CREATE
	INDEX" statement (a text value). The details of the statement shall 
        be such that executing the statement would create an index of 
        precisely the same name and content as the existing database index.

      <p>The following requirements describe "view" records.

      <p class=req id=FF0400>
	For each SQL view in the database, the schema table of a well-formed
	database shall contain a record with the first field set to the text
	value "view" and the second field set to a text value containing the
        name of the SQL view.

      <p class=req id=FF0410>
        In a well-formed database, the third field of all schema table
        records associated with SQL views shall contain the same value as
        the second field.

      <p class=req id=FF0420>
        In a well-formed database, the third field of all schema table
        records associated with SQL views shall contain the integer value 0.

      <p class=req id=FF0430>
        In a well-formed database, the fifth field of all schema table
	records associated with SQL indexes shall contain an SQL "CREATE
	VIEW" statement (a text value). The details of the statement shall 
        be such that executing the statement would create a view of 
        precisely the same name and definition as the existing database view.

      <p>The following requirements describe "trigger" records.

      <p class=req id=FF0440>
	For each SQL trigger in the database, the schema table of a well-formed
	database shall contain a record with the first field set to the text
	value "trigger" and the second field set to a text value containing the
        name of the SQL trigger.

      <p class=req id=FF0450>
        In a well-formed database, the third field of all schema table
        records associated with SQL triggers shall contain the name of the
        database table or view to which the trigger applies.

      <p class=req id=FF0460>
        In a well-formed database, the third field of all schema table
        records associated with SQL triggers shall contain the integer value 0.

      <p class=req id=FF0470>
        In a well-formed database, the fifth field of all schema table
	records associated with SQL indexes shall contain an SQL "CREATE
	TRIGGER" statement (a text value). The details of the statement shall 
        be such that executing the statement would create a trigger of 
        precisely the same name and definition as the existing database trigger.

      <p>The following requirements describe the placement of B-Tree root 
         pages in auto-vacuum databases.

      <p class=req id=FF0480>
        In an auto-vacuum database, all pages that occur before the page
        number stored in the <i>auto-vacuum last root-page</i> field
        of the database file header (see FF0140) must be either B-Tree <i>root
        pages</i>, <i>pointer-map pages</i> or the <i>locking page</i>.

      <p class=req id=FF0490>
        In an auto-vacuum database, no B-Tree <i>root pages</i> may occur
        on or after the page number stored in the <i>auto-vacuum last root-page</i> field
        of the database file header (see FF0140) must be either B-Tree <i>root
        pages</i>, <i>pointer-map pages</i> or the <i>locking page</i>.


 
  <h2 id="btree_structures">B-Tree Structures</h2>
    <p>
      A large part of any SQLite database file is given over to one or more
      B-Tree structures. A single B-Tree structure is stored using one or more
      database pages. Each page contains a single B-Tree node.
      The pages used to store a single B-Tree structure need not form a
      contiguous block. The page that contains the root node of a B-Tree
      structure is known as the "root page".

    <p>
      SQLite uses two distinct variants of the B-Tree structure:
    <ul>
      <li>The <b>table B-Tree</b>, which uses 64-bit integer values for keys. 
	  In a table B-Tree, an associated database record (section
          <cite>record_format</cite>) is stored along with each entry. Table
          B-Tree structures are described in detail in section 
          <cite>table_btrees</cite>.
      <li>The <b>index B-Tree</b>, which uses database records as keys. Index
          B-Tree structures are described in detail in section 
          <cite>index_btrees</cite>.
    </ul>

    <p class=req id=FF0500>
      As well as the <i>schema table</i>, a <i>well-formed database file</i> 
      contains <i>N</i> table B-Tree structures, where <i>N</i> is the 
      number of non-virtual tables in the logical database, excluding the 
      sqlite_master table but including sqlite_sequence and other system 
      tables.
    <p class=req id=FF0510>
      A well-formed database file contains <i>N</i> table B-Tree structures,
      where <i>N</i> is the number of indexes in the logical database,
      including indexes created by UNIQUE or PRIMARY KEY clauses in the
      declaration of SQL tables.

    <h3 id="varint_format">Variable Length Integer Format</h3>
      <p>
	In several parts of the B-Tree structure, 64-bit twos-complement signed
	integer values are stored in the "variable length integer format"
	described here.
      <p>
        A variable length integer consumes from one to nine bytes of space,
        depending on the value stored. Seven bits are used from each of
        the first eight bytes present, and, if present, all eight from
	the final ninth byte. Unless the full nine byte format is used, the
	serialized form consists of all bytes up to and including the first
	byte with the 0x80 bit cleared.
      <p>
	The number of bytes present depends on the position of the most
	significant set bit in the 64-bit word. Negative numbers always have
	the most significant bit of the word (the sign bit) set and so are
	always encoded using the full nine bytes. Positive integers may be
	encoded using less space. The following table shows the 9 different
	length formats available for storing a variable length integer
	value.

      <table class=striped>
        <tr><th>Bytes<th>Value Range<th>Bit Pattern
        <tr><td>1<td>7 bit<td>0xxxxxxx
        <tr><td>2<td>14 bit<td>1xxxxxxx 0xxxxxxx
        <tr><td>3<td>21 bit<td>1xxxxxxx 1xxxxxxx 0xxxxxxx
        <tr><td>4<td>28 bit<td>1xxxxxxx 1xxxxxxx 1xxxxxxx 0xxxxxxx
        <tr><td>5<td>35 bit<td>1xxxxxxx 1xxxxxxx 1xxxxxxx 1xxxxxxx 0xxxxxxx
        <tr><td>6<td>42 bit<td>1xxxxxxx 1xxxxxxx 1xxxxxxx 1xxxxxxx 1xxxxxxx 0xxxxxxx
        <tr><td>7<td>49 bit<td>1xxxxxxx 1xxxxxxx 1xxxxxxx 1xxxxxxx 1xxxxxxx 1xxxxxxx 0xxxxxxx
        <tr><td>8<td>56 bit<td>1xxxxxxx 1xxxxxxx 1xxxxxxx 1xxxxxxx 1xxxxxxx 1xxxxxxx 1xxxxxxx 0xxxxxxx
        <tr><td>9<td>64 bit<td>1xxxxxxx 1xxxxxxx 1xxxxxxx 1xxxxxxx 1xxxxxxx 1xxxxxxx 1xxxxxxx 1xxxxxxx xxxxxxxx
      </table>
      <p>
        When using the full 9 byte representation, the first byte contains
        the 7 most significant bits of the 64-bit value. The final byte of
        the 9 byte representation contains the 8 least significant bits of
        the 64-bit value. When using one of the other representations, the
        final byte contains the 7 least significant bits of the 64-bit value.
        The second last byte, if present, contains the 7 next least signficant
	bits of the value, and so on. The significant bits of the 64-bit
	value for which no storage is provided are assumed to be zero.
      <p>
	When encoding a variable length integer, SQLite usually selects the
        most compact representation that provides enough storage to accomadate
	the most significant set bit of the value. This is not required
        however, using more bytes than is strictly necessary when encoding
        an integer is valid.

      <table class=striped>
	<tr><th>Decimal<th>Hexadecimal        <th>Variable Length Integer
	<tr><td>43     <td>0x000000000000002B <td>0x2B
	<tr><td>200815 <td>0x000000000003106F <td>0x8C 0xA0 0x6F
        <tr><td>-1     <td>0xFFFFFFFFFFFFFFFF 
            <td>0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF
        <tr><td>-78056 <td>0xFFFFFFFFFFFECD56
            <td>0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFD 0xCD 0x56
      </table>

    <p class=req id=FF0520>
      A 64-bit signed integer value stored in <i>variable length integer</i>
      format consumes from 1 to 9 bytes of space.

    <p class=req id=FF0530>
      The most significant bit of all bytes except the last in a serialized
      <i>variable length integer</i> is always set. Unless the serialized 
      form consumes the maximum 9 bytes available, then the most significant
      bit of the final byte of the representation is always cleared.

    <p class=req id=FF0540>
      The eight least significant bytes of the 64-bit twos-compliment
      representation of a value stored in a 9 byte <i>variable length
      integer</i> are stored in the final byte (byte offset 8) of the
      serialized <i>variable length integer</i>. The other 56 bits are
      stored in the 7 least significant bits of each of the first 8 bytes
      of the serialized <i>variable length integer</i>, in order from
      most significant to least significant.

    <p class=req id=FF0550>
      A <i>variable length integer</i> that consumes less than 9 bytes of
      space contains a value represented as an <i>N</i>-bit unsigned 
      integer, where <i>N</i> is equal to the number of bytes consumed by 
      the serial representation (between 1 and 8) multiplied by 7. The
      <i>N</i> bits are stored in the 7 least significant bits of each
      byte of the serial representation, from most to least significant.
      

    <h3 id="record_format">Database Record Format</h3>
      <p>
        A database record is a blob of data that represents an ordered
        list of one or more SQL values. Database records are used in two
        places in SQLite database files - as the associated data for entries
        in table B-Tree structures, and as the key values in index B-Tree
        structures. The size (number of bytes consumed by) a database record
        depends on the values it contains.
      <p>
        Each database record consists of a short record header followed by 
        a data area. The record header consists of <i>N+1</i> variable
        length integers (see section <cite>varint_format</cite>), where
        <i>N</i> is the number of values stored in the record.
      <p>
        The first variable length integer in a record header contains the
        size of the record header in bytes. The following <i>N</i> variable
        length integer values each describe the type and size of the 
        records corresponding SQL value (the second integer in the record
        header describes the first value in the record, etc.). Integer
        values are interpreted according to the following table:
      <table class=striped>
        <tr><th>Header Value <th>Data type and size
        <tr><td>0 
            <td>An SQL NULL value (type SQLITE_NULL). This value
                consumes zero bytes of space in the record's data area.
        <tr><td>1
            <td>An SQL integer value (type SQLITE_INTEGER), stored as a
                big-endian 1-byte signed integer.
        <tr><td>2
            <td>An SQL integer value (type SQLITE_INTEGER), stored as a
                big-endian 2-byte signed integer.
        <tr><td>3
            <td>An SQL integer value (type SQLITE_INTEGER), stored as a
                big-endian 3-byte signed integer.
        <tr><td>4
            <td>An SQL integer value (type SQLITE_INTEGER), stored as a
                big-endian 4-byte signed integer.
        <tr><td>5
            <td>An SQL integer value (type SQLITE_INTEGER), stored as a
                big-endian 6-byte signed integer.
        <tr><td>6
            <td>An SQL integer value (type SQLITE_INTEGER), stored as an
                big-endian 8-byte signed integer.
        <tr><td>7
            <td>An SQL real value (type SQLITE_FLOAT), stored as an
                8-byte IEEE floating point value.
        <tr><td>8
            <td>The literal SQL integer 0 (type SQLITE_INTEGER). The value 
                consumes zero bytes of space in the record's data area.
                Values of this type are only present in databases with
                a schema file format (the 32-bit integer at byte offset 44
                of the database file header) value of 4 or greater.

        <tr><td>9
            <td>The literal SQL integer 1 (type SQLITE_INTEGER). The value
                consumes zero bytes of space in the record's data area.
                Values of this type are only present in databases with
                a schema file format (the 32-bit integer at byte offset 44
                of the database file header) value of 4 or greater.

        <tr><td style="white-space:nowrap"><i>bytes</i> * 2 + 12
            <td>Even values greater than 12 are used to signify a blob of
                data (type SQLITE_BLOB) (<i>n</i>-12)/2 bytes in length, where
                <i>n</i> is the integer value stored in the record header.
                
        <tr><td style="white-space:nowrap"><i>bytes</i> * 2 + 13
            <td>Odd values greater than 12 are used to signify a string
                (type SQLITE_TEXT) (<i>n</i>-13)/2 bytes in length, where
                <i>n</i> is the integer value stored in the record header.
      </table>
      <p>
        Immediately following the record header is the data for each
        of the record's values. A record containing <i>N</i> values is
        depicted in figure <cite>figure_recordformat</cite>.
      <center><img src="images/fileformat/recordformat.gif">
      <p><i>Figure <span class=fig id=figure_recordformat></span> - Database Record Format.</i>
      </center>
      
      <p>
        For each SQL value in the record, there is a blob of data stored
        in the records data area. If the corresponding integer type value
        in the record header is 0 (NULL), 8 (integer value 0) or 9 (integer
        value 1), then the blob of data is zero bytes in length. Otherwise,
        the length of the data field is as described in the table above.
      <p>
        The data field associated with a string value contains the string
        encoded using the database encoding, as defined in the database
        file header (see section <cite>file_header</cite>). No 
        nul-terminator character is stored in the database.

      <p class=req id=FF0560>
	A <i>database record</i> consists of a <i>database record header</i>,
        followed by <i>database record data</i>. The first part of the
        <i>database record header</i> is a <i>variable length integer</i>
        containing the total size (including itself) of the header in bytes.

      <p class=req id=FF0570>
	Following the length field, the remainder of the <i>database record
	header</i> is populated with <i>N</i> <i>variable length integer</i>
        fields, where <i>N</i> is the number of database values stored in
        the record. 

      <p class=req id=FF0580>
	Following the <i>database record header</i>, the <i>database record
        data</i> is made up of <i>N</i> variable length blobs of data, where
        <i>N</i> is again the number of database values stored in the record.
        The <i>n</i> blob contains the data for the <i>n</i>th value in
        the database record. The size and format of each blob of data is
        encoded in the corresponding <i>variable length integer</i> field
        in the <i>database record header</i>.

      <p class=req id=FF0590>
        A value of 0 stored within the <i>database record header</i> indicates
        that the corresponding database value is an SQL NULL. In this case
        the blob of data in the data area is 0 bytes in size.

      <p class=req id=FF0600>
        A value of 1 stored within the <i>database record header</i> indicates
        that the corresponding database value is an SQL integer. In this case
	the blob of data contains the integer value, formatted as a 1-byte
        big-endian signed integer.
      <p class=req id=FF0610>
        A value of 2 stored within the <i>database record header</i> indicates
        that the corresponding database value is an SQL integer. In this case
	the blob of data contains the integer value, formatted as a 2-byte
        big-endian signed integer.
      <p class=req id=FF0620>
        A value of 3 stored within the <i>database record header</i> indicates
        that the corresponding database value is an SQL integer. In this case
	the blob of data contains the integer value, formatted as a 3-byte
        big-endian signed integer.
      <p class=req id=FF0630>
        A value of 4 stored within the <i>database record header</i> indicates
        that the corresponding database value is an SQL integer. In this case
	the blob of data contains the integer value, formatted as a 4-byte
        big-endian signed integer.
      <p class=req id=FF0640>
        A value of 5 stored within the <i>database record header</i> indicates
        that the corresponding database value is an SQL integer. In this case
	the blob of data contains the integer value, formatted as a 6-byte
        big-endian signed integer.
      <p class=req id=FF0650>
        A value of 6 stored within the <i>database record header</i> indicates
        that the corresponding database value is an SQL integer. In this case
	the blob of data contains the integer value, formatted as a 8-byte
        big-endian signed integer.

      <p class=req id=FF0660>
        A value of 7 stored within the <i>database record header</i> indicates
	that the corresponding database value is an SQL real (floating
	point number). In this case the blob of data contains an 8-byte
        IEEE floating point number, stored in big-endian byte order.

      <p class=req id=FF0670>
        A value of 8 stored within the <i>database record header</i> indicates
	that the corresponding database value is an SQL integer, value 0.
        In this case the blob of data in the data area is 0 bytes in size.

      <p class=req id=FF0680>
        A value of 9 stored within the <i>database record header</i> indicates
	that the corresponding database value is an SQL integer, value 1.
        In this case the blob of data in the data area is 0 bytes in size.

      <p class=req id=FF0690>
	An even value greater than or equal to 12 stored within the 
        <i>database record header</i> indicates that the corresponding 
        database value is an SQL blob field. The blob of data contains the
        value data.  The blob of data is exactly (<i>n</i>-12)/2 bytes 
        in size, where <i>n</i> is the integer value stored in the 
        <i>database record header</i>.

      <p class=req id=FF0700>
	An odd value greater than or equal to 13 stored within the 
        <i>database record header</i> indicates that the corresponding 
        database value is an SQL text field. The blob of data contains the
        value text stored using the <i>database encoding</i>, with no 
        nul-terminator. The blob of data is exactly (<i>n</i>-12)/2 bytes 
        in size, where <i>n</i> is the integer value stored in the 
        <i>database record header</i>.

      <p>
        The following database file properties define restrictions on the 
        integer values that may be stored within a 
        <i>database record header</i>.

      <p class=req id=FF0710>
        In a well-formed database file, if the values 8 or 9 appear within
        any <i>database record header</i> within the database, then the
        <i>schema-layer file format</i> (stored at byte offset 44 of the
        database file header) must be set to 4.
      <p class=req id=FF0720>
        In a well-formed database file, the values 10 and 11, and all
        negative values may not appear within any <i>database record header</i> 
        in the database.

    <h3 id=index_btrees>Index B-Trees</h3>
      <p>
        As specified in section <cite>fileformat_overview</cite>, index 
        B-Tree structures store a unique set of the database records described
        in the previous section. While in some cases, when there are very
        few entries in the B-Tree, the entire structure may fit on a single
        database page, usually the database records must be spread across
        two or more pages. In this case, the pages are organized into a
        tree structure with a single "root" page at the head of the tree.
      <p>
        Within the tree structure, each page is either an internal tree 
        node containing an ordered list of N references to child nodes 
        (page numbers) and N-1 database records, or a leaf node containing 
        M database records. The value of N may be different for each page, but
        is always two or greater. Similarly, each leaf page may have a
        different non-zero positive value for M. The tree is always of
        uniform height, meaning the number of intermediate levels between 
        each leaf node page and the root page is the same.
      <p>
        Within both internal and leaf node pages, the records are stored in
        sorted order. The comparison function used to determine the sort order
        is described in section <cite>index_btree_compare_func</cite>.
      <p>
        Records are distributed throughout the tree such that for each 
        internal node, all records stored in the sub-tree headed by 
        the first child node ( C(0) ) are considered less than 
        the first record stored on the internal node ( R(0) ) by the 
        comparison function described in section
        <cite>index_btree_compare_func</cite>. Similarly all records stored 
        in the sub-tree headed by C(n) are considered greater than R(n-1) but
        less than R(n) for values of n between 1 and N-2, inclusive. All
        records in the sub-tree headed by C(N-1) are greater than the 
        largest record stored on the internal node.
        <center><img src="images/fileformat/indextree.gif">
        <p><i>Figure <span class=fig id=figure_indextree></span> - Index B-Tree Tree Structure.</i>
        </center>
      <p>
        Figure <cite>figure_indextree</cite> depicts one possible record
        distribution for an index B-Tree containing records R1 to R26, assuming
        that for all values of N, <i>R(N+1)&gt;R(N)</i>. In total the B-Tree
        structure uses 11 database file pages. Internal tree nodes contain
        database records and references to child node pages. Leaf nodes contain
        database records only.

      <p class=req id=FF0730>
        The pages in an index B-Tree structures are arranged into a tree
        structure such that all leaf pages are at the same depth.

      <p class=req id=FF0740>
        Each leaf node page in an index B-Tree contains one or more
        B-Tree cells, where each cell contains a database record.

      <p class=req id=FF0750>
        Each internal node page in an index B-Tree contains one or more
        B-Tree cells, where each cell contains a child page number, <i>C</i>,
        and a database record <i>R</i>. All database records stored within
        the sub-tree headed by page <i>C</i> are smaller than record <i>R</i>,
        according to the index sort order (see below). Additionally, unless 
	<i>R</i> is the smallest database record stored on the internal node
	page, all integer keys within the sub-tree headed by <i>C</i> are
	greater than <i>R<sub>-1</sub></i>, where <i>R<sub>-1</sub></i> is the
        largest database record on the internal node page that is smaller 
        than <i>R</i>.

      <p class=req id=FF0760>
        As well as child page numbers associated with B-Tree cells, each
        internal node page in an index B-Tree contains the page number
        of an extra child page, the <i>right-child page</i>. All database
        records stored in all B-Tree cells within the sub-tree headed by the 
        <i>right-child page</i> are greater than all database records 
        stored within B-Tree cells on the internal node page.

      <p>
	The precise way in which index B-Tree pages and cells are formatted is
        described in subsequent sections.


        <h4>Index B-Tree Content</h4>
          <p>
	    The database file contains one index B-Tree for each database index
	    in the logical database, including those created by UNIQUE or
	    PRIMARY KEY clauses in table declarations. Each record stored in
            an index B-Tree contains the same number of fields, the number of
            indexed columns in the database index declaration plus one. 
          <p>
            An index B-Tree contains an entry for each row in its associated
            database table. The fields of the record used as the index B-Tree
            key are copies of each of the indexed columns of the associated 
            database row, in order, followed by the rowid value of the same 
            row. See figure <cite>figure_examplepop</cite> for an example.

        <p class=req id=FF0770>
          In a well-formed database, each index B-Tree contains a single entry
          for each row in the indexed logical database table.

        <p class=req id=FF0780>
	  Each <i>database record</i> (key) stored by an index B-Tree in a
	  well-formed database contains the same number of values, the number
          of indexed columns plus one.

        <p class=req id=FF0790>
	  The final value in each <i>database record</i> (key) stored by an
	  index B-Tree in a well-formed database contains the rowid (an integer
          value) of the corresponding logical database row.

        <p class=req id=FF0800>
          The first <i>N</i> values in each <i>database record</i> (key) 
	  stored in an index B-Tree where <i>N</i> is the number of indexed
	  columns, contain the values of the indexed columns from the
	  corresponding logical database row, in the order specified for the
          index.
 
      <h4 id="index_btree_compare_func">Record Sort Order</h4>
        <p>
          This section defines the comparison function used when database
	  records are used as B-Tree keys for index B-Trees. The comparison
	  function is only defined when both database records contain the same
          number of fields.
        <p>
          When comparing two database records, the first field of one
          record is compared to the first field of the other. If they
          are not equal, the next pair of fields are compared, and so
          on. If all the fields in the database records are equal, then
          the two records are considered equal. Otherwise, the result
          of the comparison is determined by the first pair of inequal 
          fields.
        <p>
          Two database record fields (SQL values) are compared using the
          following rules:
        <ol>
          <li>If both values are NULL, then they are considered equal.
          <li>If one value is a NULL and the other is not, it is considered
              the lesser of the two.
          <li>If both values are either real or integer values, then the
              comparison is done numerically.
          <li>If one value is a real or integer value, and the other is
              a text or blob value, then the numeric value is considered 
              lesser.
          <li>If both values are text, then the collation function is used
              to compare them. The collation function is a property of the
              index column in which the values are found. <span class=todo>
              Link to document with CREATE INDEX syntax.</span>
          <li>If one value is text and the other a blob, the text value
              is considered lesser.
          <li>If both values are blobs, memcmp() is used to determine the 
              results of the comparison function. If one blob is a prefix
              of the other, the shorter blob is considered lesser.
        </ol>
        <p>
	  Each column of a database index may be declared as "descending".
          <span class=todo>Link to document with CREATE INDEX syntax.</span>
          In SQLite database files with a schema layer file-format equal
          to 4, this modifies the order in which the records are stored in
          the corresponding index B-Tree structure. For each index column
          declared as descending, the results of the above comparison 
	  procedure are inverted.
        <p>
	  The columns of database indexes created by UNIQUE or PRIMARY
          KEY clauses are never treated as descending.

        <p class=todo>
          Need requirements style statements for this information. Easier
          to do once collation sequences have been defined somewhere.


      <h4 id=index_btree_page_format>Index B-Tree Page Format</h4>
        <p>
          Each index B-Tree page is divided into four sections that occur
          in order on the page:
        <ul>
          <li> The 8 (leaf node pages) or 12 (internal tree node pages) 
               byte page-header.
          <li> The cell offset array. This is a series of N big-endian 2-byte
               integer values, where N is the number of records stored on 
               the page.
          <li> A block of unused space. This may be 0 bytes in size.
          <li> The cell content area consumes the remaining space on the page.
        </ul>
        <center><img src="images/fileformat/indexpage.gif">
        <p><i>Figure <span class=fig id=figure_indexpage></span> - Index B-Tree Page Data.</i>
        </center>
        <p>
          The 8 (leaf node pages) or 12 (internal tree node pages) byte page
          header that begins each index B-Tree page is made up of a series of 
          1, 2 and 4 byte unsigned integer values as shown in the following
          table. All values are stored in big-endian byte order.

      <table class=striped>
        <tr><th>Byte Range <th>Byte Size <th width=100%>Description
        <tr><td>0     <td>1<td>B-Tree page flags. For an index B-Tree internal 
                               tree node page, this is set to 0x02. For a
                               leaf node page, 0x0A.
        <tr><td>1..2  <td>2<td>Byte offset of first block of free space on 
                               this page. If there are no free blocks on this
                               page, this field is set to 0.
        <tr><td>3..4  <td>2<td>Number of cells (entries) on this page.
        <tr><td>5..6  <td>2<td>Byte offset of the first byte of the cell
                               content area (see figure 
                               <cite>figure_indexpage</cite>), relative to the 
                               start of the page.
        <tr><td>7     <td>1<td>Number of fragmented free bytes on page.
        <tr><td>8..11 <td>4<td>Page number of rightmost child-page (the
                               child-page that heads the sub-tree in which all
                               records are larger than all records stored on
                               this page). This field is not present for leaf
                               node pages.
      </table>
      <p>
        The cell content area, which occurs last on the page, contains one
        B-Tree cell for each record stored on the B-Tree page. On a leaf node
        page, each cell is responsible for storing a database record only. On
        an internal tree node page, each cell contains a database record and
        the corresponding child page number ((R(0) and C(0)) are stored 
        together, for example - the cell record is considered greater than
        all records stored in the sub-tree headed by the child page). The
        final child page number is stored as part of the page header.
      <p>
        The B-Tree cells may be distributed throughout the cell content area
        and may be interspersed with blocks of unused space. They are not
        sorted within the cell content area in any particular order. The
        serialized format of a B-Tree cell is described in detail in 
        section <cite>index_btree_cell_format</cite>.
      <p>
        The byte offset of each cell in the cell content area, relative
        to the start of the page, is stored in the cell offset array. The
        offsets are in sorted order according to the database records stored
        in the corresponding cells. The first offset in the array is the 
        offset of the cell containing the smallest record on the page,
        according to the comparison function defined in section 
        <cite>index_btree_compare_func</cite>.
      <p>
        As well as the block of unused space between the cell offset array and
        the cell content area, which may be any size, there may be small blocks
        of free space interspersed with the B-Tree cells within the cell
        content area. These are classified into two classes, depending on their         size:
      <ul>
        <li>Blocks of free-space consisting of 3 bytes or less are called
            <b>fragments</b>. The total number of bytes consumed by all
            fragments on a page is stored in the 1 byte unsigned integer at
            byte offset 7 of the page header. The total number of fragmented
            bytes on a single page is never greater than 255.
        <li>Blocks of free-space consisting of more than 3 bytes of contiguous
            space are called <b>free blocks</b>. All free blocks on a single
            page are linked together into a singly linked list. The byte
            offset (relative to the start of the page) of the first block in 
            the list is stored in the 2 byte unsigned integer stored at byte
            offset 1 of the page header. The first two bytes of each free
            block contain the byte offset (again relative to the start of
            the page) of the next block in the list stored as a big-endian
            unsigned integer. The first two bytes of the final block in the 
            list are set to zero. The third and fourth bytes of each free
            block contain the total size of the free block in bytes, stored
            as a 2 byte big-endian unsigned integer.
      </ul>

      <p class=req id=FF0810>
	The <i>b-tree page flags</i> field (the first byte) of each database
	page used as an internal node of an index B-Tree structure is set to
        0x02.  
      <p class=req id=FF0820>
	The <i>b-tree page flags</i> field (the first byte) of each database
        page used as a leaf node of an index B-Tree structure is set to 0x0A.

      <p>
        The following requirements describe the <i>B-Tree page header</i>
        present at the start of both index and table B-Tree pages.

      <p class=req id=FF0830>
        The first byte of each database page used as a B-Tree page contains
	the <i>b-tree page flags</i> field. On page 1, the <i>b-tree page
        flags</i> field is stored directly after the 100 byte file header
        at byte offset 100.

      <p class=req id=FF0840>
        The number of B-Tree cells stored on a B-Tree page is stored as a
        2-byte big-endian integer starting at byte offset 3 of the B-Tree 
        page. On page 1, this field is stored at byte offset 103.

      <p class=req id=FF0850>
        The 2-byte big-endian integer starting at byte offset 5 of each
        B-Tree page contains the byte-offset from the start of the page
        to the start of the <i>cell content area</i>, which consumes all space
        from this offset to the end of the usable region of the page.
	On page 1, this field is stored at byte offset 105.  All B-Tree 
        cells on the page are stored within the cell-content area.

      <p class=req id=FF0860>
        On each page used as an internal node a of B-Tree structures, the
        page number of the rightmost child node in the B-Tree structure is
        stored as a 4-byte big-endian unsigned integer beginning at byte
        offset 8 of the database page, or byte offset 108 on page 1.

      <p>
        This requirement describes the cell content offset array. It applies
        to both B-Tree variants.

      <p class=req id=FF0870>
        Immediately following the <i>page header</i> on each B-Tree page is the
        <i>cell offset array</i>, consisting of <i>N</i> 2-byte big-endian
        unsigned integers, where <i>N</i> is the number of cells stored
        on the B-Tree page (FF0840). On an internal node B-Tree page,
        the cell offset array begins at byte offset 12, or on a leaf
        page, byte offset 8. For the B-Tree node on page 1, these
        offsets are 112 and 108, respectively.

      <p class=req id=FF0880>
        The <i>cell offset array</i> and the <i>cell content area</i> (FF0850)
        may not overlap.

      <p class=req id=FF0890>
        Each value stored in the <i>cell offset array</i> must be greater
        than or equal to the offset to the <i>cell content area</i> (FF0850),
        and less than the database <i>page size</i>.

      <p class=req id=FF0900>
	The <i>N</i> values stored within the <i>cell offset array</i> are the
        byte offsets from the start of the B-Tree page to the beginning of 
        each of the <i>N</i> cells stored on the page.

      <p class=req id=FF0910>
	No two B-Tree cells may overlap.

      <p>
	The following requirements govern management of free-space within the
        page content area (both table and index B-Tree pages).

      <p class=req id=FF0920>
	Within the <i>cell content area</i>, all blocks of contiguous
	free-space (space not used by B-Tree cells) greater than 3 bytes in
        size are linked together into a linked list, the <i>free block list</i>.
        Such blocks of free space are known as <i>free blocks</i>.

      <p class=req id=FF0930>
        The first two bytes of each <i>free block</i> contain the offset
        of the next <i>free block</i> in the <i>free block list</i> formatted
	as a 2-byte big-endian integer, relative to the start of the database
        page. If there is no next <i>free block</i>, then the first two 
        bytes are set to 0x00.

      <p class=req id=FF0940>
        The second two bytes (byte offsets 2 and 3) of each <i>free block</i> 
        contain the total size of the <i>free block</i>, formatted as a 2-byte
        big-endian integer.

      <p class=req id=FF0950>
        On all B-Tree pages, the offset of the first <i>free block</i> in the 
        <i>free block list</i>, relative to the start of the database page,
        is stored as a 2-byte big-endian integer starting at byte offset 
        1 of the database page. If there is no first <i>free block</i> 
        (because the <i>free block list</i> is empty), then the two bytes
        at offsets 1 and 2 of the database page are set to 0x00. On page 1,
        this field is stored at byte offset 101 of the page.


      <p class=req id=FF0960>
        Within the cell-content area, all blocks of contiguous free-space 
        (space not used by B-Tree cells) less than or equal to 3 bytes in 
        size are known as <i>fragments</i>. The total size of all 
        <i>fragments</i> on a B-Tree page is stored as a 1-byte unsigned 
        integer at byte offset 7 of the database page. On page 1, this
        field is stored at byte offset 107.

      <h4 id=index_btree_cell_format>Index B-Tree Cell Format</h4>
        <p> 
          For index B-Tree internal tree node pages, each B-Tree cell begins
          with a child page-number, stored as a 4-byte big-endian unsigned
          integer. This field is omitted for leaf pages, which have no 
          children.
        <p> 
          Following the child page number is the total number of bytes 
          consumed by the cell's record, stored as a variable length integer
          (see section <cite>varint_format</cite>). 
        <p> 
          If the record is small enough, it is stored verbatim in the cell.
          A record is deemed to be small enough to be completely stored in
          the cell if it consists of less than:
        <pre>
            <i>max-local</i> := <i>usable-size</i> * (<i>max-embedded-fraction</i> / 255 ) - 23
</pre>
        <p>
          bytes. In the formula above, <i>usable-size</i> is the page-size
          in bytes less the number of unused bytes left at the end of every
          page (as read from byte offset 20 of the file header), and
          <i>max-embedded-fraction</i> is the value read from byte offset 
          21 of the file header.
        <center><img src="images/fileformat/indexshortrecord.gif">
        <p><i>Figure <span class=fig></span> - Small Record Index B-Tree Cell.</i>
        </center>
        <p>
          If the cell record is larger than the maximum size identified by
          the formula above, then only the first part of the record is stored
          within the cell. The remainder is stored in an overflow-chain (see
          section <cite>overflow_page_chains</cite> for details). Following 
          the part of the record stored within the cell is the page number 
          of the first page in the overflow chain, stored as a 4 byte 
          big-endian unsigned integer. The size of the part of the record 
          stored within the B-Tree cell (<i>local-size</i> in figure 
          <cite>figure_indexlongrecord</cite>) is calculated according to the 
          following algorithm:
        <pre>
            <i>min-local</i> := (<i>usable-size</i> - 12) * <i>min-embedded-fraction</i> / 255 - 23
            <i>max-local</i> := (<i>usable-size</i> - 12) * <i>max-embedded-fraction</i> / 255 - 23
            <i>local-size</i> := (<i>record-size</i> - <i>min-local</i>) % (<i>usable-size</i> - 4)
            if( <i>local-size</i> &gt; <i>max-local</i> )
                <i>local-size</i> := <i>min-local</i>
</pre>
        <p>
          In the formula above, <i>usable-size</i> is the page-size
          in bytes less the number of unused bytes left at the end of every
          page (as read from byte offset 20 of the file header), and
          <i>max-embedded-fraction</i> and <i>min-embedded-fraction</i> are
          the values read from byte offsets 21 and 22 of the file header,
          respectively.
        <center><img src="images/fileformat/indexlongrecord.gif">
        <p><i>Figure <span class=fig id=figure_indexlongrecord></span> - 
          Large Record Index B-Tree Cell.</i>
        </center>

      <p class=req id=FF0970>
        Each B-Tree cell belonging to an internal node page of an index 
        B-Tree consists of a 4-byte big-endian unsigned integer, the 
        <i>child page number</i>, followed by a <i>variable length integer</i>
        field, followed by a <i>database record</i>. The 
        <i>variable length integer</i> field contains the length of the
        database record in bytes.

      <p class=req id=FF0980>
	Each B-Tree cell belonging to an leaf page of an index B-Tree 
        consists of a <i>variable length integer</i> field, followed by 
        a <i>database record</i>. The <i>variable length integer</i> field 
        contains the length of the database record in bytes.

      <p class=req id=FF0990>
	If the database record stored in an index B-Tree page is 
        sufficiently small, then the entire cell is stored within the 
        index B-Tree page.  Sufficiently small is defined as equal to or 
        less than <i>max-local</i>, where:
        <code>
            <i>max-local</i> := (<i>usable-size</i> - 12) * 64 / 255 - 23</code>
        
      <p class=req id=FF1000>
        If the database record stored as part of an index B-Tree cell is too
        large to be stored entirely within the B-Tree page (as defined by
        FF0520), then only a prefix of the <i>database record</i> is stored
        within the B-Tree page and the remainder stored in an <i>overflow
        chain</i>. In this case, the database record prefix is immediately
	followed by the page number of the first page of the 
        <i>overflow chain</i>, formatted as a 4-byte big-endian unsigned 
        integer.

      <p class=req id=FF1010>
        When a <i>database record</i> belonging to a table B-Tree cell is
        stored partially within an <i>overflow page chain</i>, the size
        of the prefix stored within the index B-Tree page is <i>N</i> bytes,
        where <i>N</i> is calculated using the following algorithm:
        <code>
            <i>min-local</i> := (<i>usable-size</i> - 12) * 32 / 255 - 23
            <i>max-local</i> := (<i>usable-size</i> - 12) * 64 / 255 - 23
            <i>N</i> := <i>min-local</i> + ((<i>record-size</i> - <i>min-local</i>) % (<i>usable-size</i> - 4))
            if( <i>N</i> &gt; <i>max-local</i> ) <i>N</i> := <i>min-local</i></code>

      <p>
        Requirements FF1010 and FF0990 are similar to the algorithms 
        presented in the text above. However instead of 
        <i>min-embedded-fraction</i> and <i>max-embedded-fraction</i> the
        requirements use the constant values 32 and 64, as well-formed 
        database files are required by FF0080 and FF0070 to store these 
        values in the relevant database file header fields.

    <h3 id=table_btrees>Table B-Trees</h3>
      <p>
        As noted in section <cite>fileformat_overview</cite>, table B-Trees
        store a set of unique 64-bit signed integer keys. Associated with
        each key is a database record. As with index B-Trees, the database
        file pages that make up a table B-Tree are organized into a tree
        structure with a single "root" page at the head of the tree.
      <p>
        Unlike index B-Tree structures, where entries are stored on both
        internal and leaf nodes, all entries in a table B-Tree are stored
        in the leaf nodes. Within each leaf node, keys are stored in sorted
        order.
      <p>
        Each internal tree node contains an ordered list of N references 
        to child pages, where N is some number greater than one. In a 
        similar manner to the way in which an index B-Tree page would 
        contain N-1 records, each internal table B-Tree node page also 
        contains a list of N-1 64-bit signed integer values in sorted order. 
        The keys are distributed throughout the tree such that for all internal
        tree nodes, integer I(n) is equal to the largest key value stored in
        the sub-tree headed by child page C(n) for values of n between 0 and
        N-2, inclusive. Additionally, all keys stored in the sub-tree headed
        by child page C(n+1) have values larger than that of I(n), for values
        of n in the same range.
        <center><img src="images/fileformat/tabletree.gif">
        <p><i>Figure <span class=fig id=figure_tabletree></span> - Table B-Tree Tree Structure.</i>
        </center>
      <p>
        Figure <cite>figure_tabletree</cite> depicts a table B-Tree containing
	a contiguous set of 14 integer keys starting with 1. Each key <i>n</i>
	has an associated database record R<i>n</i>. All the keys and their
	associated records are stored in the leaf pages. The internal node
	pages contain no database data, their only purpose is to provide
	a way to navigate the tree structure.

      <p class=req id=FF1020>
        The pages in a table B-Tree structures are arranged into a tree
        structure such that all leaf pages are at the same depth.

      <p class=req id=FF1030>
        Each leaf page in a table B-Tree structure contains one or more
        B-Tree cells, where each cell contains a 64-bit signed integer key
        value and a database record.

      <p class=req id=FF1040>
        Each internal node page in a table B-Tree structure contains one or 
        more B-Tree cells, where each cell contains a 64-bit signed integer 
	key value, <i>K</i>, and a child page number, <i>C</i>. All integer key
        values in all B-Tree cells within the sub-tree headed by page <i>C</i>
        are less than or equal to <i>K</i>. Additionally, unless <i>K</i>
        is the smallest integer key value stored on the internal node page,
        all integer keys within the sub-tree headed by <i>C</i> are greater
        than <i>K<sub>-1</sub></i>, where <i>K<sub>-1</sub></i> is the largest
        integer key on the internal node page that is smaller than <i>K</i>.

      <p class=req id=FF1050>
        As well as child page numbers associated with B-Tree cells, each
        internal node page in a table B-Tree contains the page number
        of an extra child page, the <i>right-child page</i>. All key values
        in all B-Tree cells within the sub-tree headed by the <i>right-child
        page</i> are greater than all key values stored within B-Tree cells
        on the internal node page.

      <p>
	The precise way in which table B-Tree pages and cells are formatted is
        described in subsequent sections.

      <h4 id=table_btree_content>Table B-Tree Content</h4>
        <p>
	  The database file contains one table B-Tree for each database table
	  in the logical database. Although some data may be duplicated in
          index B-Tree structures, the table B-Tree is the primary location
          of table data.
        <p>
	  The table B-Tree contains exactly one entry for each row in the
	  database table. The integer key value used for the B-Tree entry is
          the value of the "rowid" field of the corresponding logical row 
          in the database table. The database row fields are stored in the
          record associated with the table B-Tree entry, in the same order
          as they appear in the logical database table. The first field in
          the record (see section <cite>record_format</cite>) contains the
          value of the leftmost field in the database row, and so on.
        <p>
	  If a database table column is declared as an INTEGER PRIMARY KEY,
          then it is an alias for the rowid field, which is stored as the 
          table B-Tree key value. Instead of duplicating the integer value
	  in the associated record, the record field associated with the
          INTEGER PRIMARY KEY column is always set to an SQL NULL.
        <p>
	  Finally, if the schema layer file-format is greater than or equal 
          to 2, some of the records stored in table B-Trees may contain
	  less fields than the associated logical database table does columns.
          If the schema layer file-format is exactly 2, then the logical
          database table column values associated with the "missing" fields 
          are SQL NULL. If the schema layer file-format is greater than
          2, then the values associated with the "missing" fields are 
	  determined by the default value of the associated database table 
          columns.
	  <span class=todo>Reference to CREATE TABLE syntax. How are default
          values determined?</span>

        <p class=req id=FF1060>
          In a well-formed database, each table B-Tree contains a single entry
          for each row in the corresponding logical database table.

        <p class=req id=FF1070>
	  The key value (a 64-bit signed integer) for each B-Tree entry is
	  the same as the value of the rowid field of the corresponding
          logical database row.

        <p class=req id=FF1080>
	  The SQL values serialized to make up each <i>database record</i>
	  stored as ancillary data in a table B-Tree shall be the equal to the
	  values taken by the <i>N</i> leftmost columns of the corresponding
	  logical database row, where <i>N</i> is the number of values in the
          database record.

        <p class=req id=FF1090>
	    If a logical database table column is declared as an "INTEGER
            PRIMARY KEY", then instead of its integer value, an SQL NULL
	    shall be stored in its place in any database records used as
            ancillary data in a table B-Tree.

        <p>The following database properties discuss table B-Tree records 
           with implicit (default) values.

          <p class=req id=FF1100>
	    If the database <i>schema layer file-format</i> (the value stored 
            as a 4-byte integer at byte offset 44 of the file header) is 1, 
            then all database records stored as ancillary data in a table 
            B-Tree structure have the same number of fields as there are 
            columns in the corresponding logical database table.

          <p class=req id=FF1110>
	    If the database <i>schema layer file-format</i> value is two or
            greater and the rightmost <i>M</i> columns of a row contain SQL NULL
            values, then the corresponding record stored as ancillary data in
	    the table B-Tree has between <i>N</i>-<i>M</i> and <i>N</i> fields,
	    where <i>N</i> is the number of columns in the logical database
            table.

          <p class=req id=FF1120>
	    If the database <i>schema layer file-format</i> value is three or
	    greater and the rightmost <i>M</i> columns of a row contain their
            default values according to the logical table declaration, then the
	    corresponding record stored as ancillary data in the table B-Tree
	    may have as few as <i>N</i>-<i>M</i> fields, where <i>N</i> is the
            number of columns in the logical database table.

      <h4>Table B-Tree Page Format</h4>
        <p>
          Table B-Tree structures use the same page format as index B-Tree 
          structures, described in section <cite>index_btree_page_format</cite>,
          with the following differences:
        <ul>
          <li>The first byte of the page-header, the "flags" field, is set to 
              0x05 for internal tree node pages, and 0x0D for leaf pages.
          <li>The content and format of the B-Tree cells is different. See
              section <cite>table_btree_cell_format</cite> for details.
          <li>The format of page 1 is the same as any other table B-Tree,
              except that 100 bytes less than usual is available for content.
              The first 100 bytes of page 1 is consumed by the database
              file header.
        </ul>

      <p class=req id=FF1130>
	In a <i>well-formed database file</i>, the first byte of each page used
        as an internal node of a table B-Tree structure is set to 0x05.
      <p class=req id=FF1140>
	In a <i>well-formed database file</i>, the first byte of each page used
        as a leaf node of a table B-Tree structure is set to 0x0D.
        
      <p>
        Most of the requirements specified in section 
        <cite>index_btree_page_format</cite> also apply to table B-Tree 
        pages. The wording of the requirements make it clear when this is
        the case, either by refering to generic "B-Tree pages" or by
        explicitly stating that the statement applies to both "table and
        index B-Tree pages".

      <h4 id=table_btree_cell_format>Table B-Tree Cell Format</h4>
        <p>
          Cells stored on internal table B-Tree nodes consist of exactly two 
          fields. The associated child page number, stored as a 4-byte
          big-endian unsigned integer, followed by the 64-bit signed integer
          value, stored as a variable length integer (section 
          <cite>varint_format</cite>). This is depicted graphically in figure
          <cite>figure_tablenodecell</cite>.
        <center><img src="images/fileformat/tablenodecell.gif">
        <p><i>Figure <span class=fig id=figure_tablenodecell></span> - Table B-Tree Internal Node Cell.</i>
        </center>
        <p>
          Cells of table B-Tree leaf pages are required to store a 64-bit
          signed integer key and its associated database record. The first
          two fields of all table B-Tree leaf page cells are the size of
          the database record, stored as a <i>variable length integer</i>
          (see section <cite>varint_format</cite>), followed by the key
          value, also stored as a <i>variable length integer</i>. For 
          sufficiently small records, the entire record is stored in the 
          B-Tree cell following the record-size field. In this case, 
          sufficiently small is defined as less than or equal to:
        <pre>
          max-local := <i>usable-size</i> - 35
</pre>
        <p>
          bytes. Where <i>usable-size</i> is defined as the page-size
          in bytes less the number of unused bytes left at the end of every
          page (as read from byte offset 20 of the file header), and
          <i>max-embedded-fraction</i> is the value read from byte offset 
          21 of the file header. This scenario, where the entire record is
          stored within the B-Tree cell, is depicted in figure
          <cite>figure_tableshortrecord</cite>.
        <center><img src="images/fileformat/tableshortrecord.gif">
        <p><i>Figure <span class=fig id=figure_tableshortrecord></span> - Table B-Tree Small Record Leaf Node Cell.</i>
        </center>

        <p>
          If the record is too large to be stored entirely within the B-Tree
          cell, then the first part of it is stored within the cell and the
          remainder in an overflow chain (see section
          <cite>overflow_page_chains</cite>). The size of the part of the 
          record stored within the B-Tree cell (<i>local-size</i> in figure
          <cite>figure_tablelongrecord</cite>) is calculated according to
          the following algorithm (a similar procedure to that used to
          calculate the portion of an index B-Tree key to store within the cell
          when an overflow chain is required):
        <pre>
            <i>min-local</i> := (<i>usable-size</i> - 12) * <i>min-embedded-fraction</i> / 255 - 23
            <i>max-local</i> := <i>usable-size</i> - 35
            <i>local-size</i> := <i>min-local</i> + (<i>record-size</i> - <i>min-local</i>) % (<i>usable-size</i> - 4)
            if( <i>local-size</i> &gt; <i>max-local</i> )
                <i>local-size</i> := <i>min-local</i>
</pre>
        <p>
          In this case, <i>min-embedded-fraction</i> is the value read from
          byte offset 22 of the file header. The layout of the cell in this
          case, when an overflow-chain is required, is shown in figure
          <cite>figure_tablelongrecord</cite>.

        <center><img src="images/fileformat/tablelongrecord.gif">
        <p><i>Figure <span class=fig id=figure_tablelongrecord></span> - Table B-Tree Large Record Leaf Node Cell.</i>
        </center>

        <p>
          If the leaf page is page 1, then the value of <i>usable-size</i> is
          as it would be for any other B-Tree page, even though the actual
          usable size is 100 bytes less than this for page 1 (because the
          first 100 bytes of the page is consumed by the database file
          header).

        <p>
          The following requirements describe the format of table B-Tree 
          cells, and the distribution thereof between B-Tree and overflow
          pages.

        <p class=req id=FF1150>
          B-Tree cells belonging to table B-Tree internal node pages consist
	  of exactly two fields, a 4-byte big-endian unsigned integer
          immediately followed by a <i>variable length integer</i>. These
          fields contain the child page number and key value respectively
          (see FF1030).

        <p class=req id=FF1160>
          B-Tree cells belonging to table B-Tree leaf node pages consist
	  of three fields, two <i>variable length integer</i> values
          followed by a database record. The size of the database record
          in bytes is stored in the first of the two 
          <i>variable length integer</i> fields. The second of the two
          <i>variable length integer</i> fields contains the 64-bit signed 
          integer key (see FF1030).

        <p class=req id=FF1170>
          If the size of the record stored in a table B-Tree leaf page cell 
          is less than or equal to (<i>usable page size</i>-35) bytes, then 
          the entire cell is stored on the B-Tree leaf page. In a well-formed
	  database, <i>usable page size</i> is the same as the database 
          <i>page size</i>.

        <p class=req id=FF1180>
          If a table B-Tree cell is too large to be stored entirely on
          a leaf page (as defined by FF1170), then a prefix of the cell
          is stored on the leaf page, and the remainder stored in an
          <i>overflow page chain</i>. In this case the cell prefix
          stored on the B-Tree leaf page is immediately followed by a 
          4-byte big-endian unsigned integer containing the page number
          of the first overflow page in the chain.

        <p class=req id=FF1190>
          When a table B-Tree cell is stored partially in an 
          <i>overflow page chain</i>, the prefix stored on the B-Tree
          leaf page consists of the two <i>variable length integer</i> fields,
          followed by the first <i>N</i> bytes of the database record, where
          <i>N</i> is determined by the following algorithm:
      <code>
        <i>min-local</i> := (<i>usable-size</i> - 12) * 255 / 32 - 23
        <i>max-local</i> := (<i>usable-size</i> - 35)
        <i>N</i> := <i>min-local</i> + (<i>record-size</i> - <i>min-local</i>) % (<i>usable-size</i> - 4)
        if( <i>N</i> &gt; <i>max-local</i> ) N := <i>min-local</i>
      </code>
        
        <p>
          Requirement FF1190 is very similar to the algorithm presented in
          the text above. Instead of <i>min-embedded-fraction</i>, it uses
          the constant value 32, as well-formed database files are required
          by FF0090 to store this value in the relevant database file 
          header field.

    <h3 id="overflow_page_chains">Overflow Page Chains</h3>
      <p>
        Sometimes, a database record stored in either an index or table 
        B-Trees is too large to fit entirely within a B-Tree cell. In this
        case part of the record is stored within the B-Tree cell and the
        remainder stored on one or more overflow pages. The overflow pages
        are chained together using a singly linked list. The first 4 bytes
        of each overflow page is a big-endian unsigned integer value 
        containing the page number of the next page in the list. The 
        remaining usable database page space is available for record data.
      <center><img src="images/fileformat/overflowpage.gif">
      <p><i>Figure <span class=fig id=figure_overflowpage></span> - Overflow Page Format.</i>
      </center>
      <p>
        The scenarios in which overflow pages are required and the number
        of bytes stored within the B-Tree cell in each are described for
        index and table B-Trees in sections 
        <cite>index_btree_cell_format</cite> and
        <cite>table_btree_cell_format</cite> respectively. In each case 
        the B-Tree cell also stores the page number of the first page in
        a linked list of overflow pages.
      <p>
        The amount of space available for record data on each overflow 
        page is:
      <pre>
        <i>available-space</i> := <i>usable-size</i> - 4
</pre>
      <p>
        Where <i>usable-size</i> is defined as the page-size in bytes less the
        number of unused bytes left at the end of every page (as read from 
        byte offset 20 of the file header).
      <p>
        Each overflow page except for the last one in the linked list 
        contains <i>available-space</i> bytes of record data. The last
        page in the list contains the remaining data, starting at byte
        offset 4. The value of the "next page" field on the last page
        in an overflow chain is undefined.

      <p class=req id=FF1200>
	A single <i>overflow page</i> may store up to <i>available-space</i>
        bytes of database record data, where <i>available-space</i> is equal
        to (<i>usable-size</i> - 4).

      <p class=req id=FF1210>
        When a database record is too large to store within a B-Tree page
        (see FF1170 and FF1000), a prefix of the record is stored within
        the B-Tree page and the remainder stored across <i>N</i> overflow
        pages. In this case <i>N</i> is the minimum number of pages required 
        to store the portion of the record not stored on the B-Tree page,
        given the maximum payload per overflow page defined by FF1200.

      <p class=req id=FF1220>
        The list of overflow pages used to store a single database record
        are linked together in a singly linked list known as an 
	<i>overflow chain</i>. The first four bytes of each page except the
        last in an <i>overflow chain</i> are used to store the page number
	of the next page in the linked list, formatted as an unsigned
        big-endian integer. The first four bytes of the last page in an
        <i>overflow chain</i> are set to 0x00.

      <p class=req id=FF1230>
        Each overflow page except the last in an <i>overflow chain</i> 
	contains <i>N</i> bytes of record data starting at byte offset 4 of
	the page, where <i>N</i> is the maximum payload per overflow page, 
        as defined by FF1200. The final page in an <i>overflow chain</i> 
        contains the remaining data, also starting at byte offset 4.

  <h2 id=free_page_list>The Free Page List</h2>
    <p>
      Sometimes, after deleting data from the database, SQLite removes pages
      from B-Tree structures. If these pages are not immediately required
      for some other purpose, they are placed on the free page list. The
      free page list contains those pages that are not currently being
      used to store any valid data.
    <p>
      Each page in the free-list is classified as a free-list trunk page 
      or a free-list leaf page. All trunk pages are linked together into
      a singly linked list (in the same way as pages in an overflow chain
      are - see section <cite>overflow_page_chains</cite>). The first four
      bytes of each trunk page contain the page number of the next trunk
      page in the list, formatted as an unsigned big-endian integer. If
      the trunk page is the last page in the linked list, the first four
      bytes are set to zero.
    <p>
      Bytes 4 to 7 of each free-list trunk page contain the number of
      references to free-list leaf pages (page numbers) stored on the
      free-list trunk page. Each leaf page on the free-list is referenced
      by exactly one trunk page.
    <p>
      The remaining space on a free-list trunk page is used to store the
      page numbers of free-list leaf pages as 4 byte big-endian integers. 
      Each free-list trunk page contains up to:
    <pre>
        <i>max-leaf-pointers</i> := (<i>usable-size</i> - 8) / 4
</pre>
    <p>
      pointers, where <i>usable-size</i> is defined as the page-size in bytes
      less the number of unused bytes left at the end of every page (as read
      from byte offset 20 of the file header).
    <center><img src="images/fileformat/freelistpage.gif">
    <p><i>Figure <span class=fig id=figure_freelistpage></span> - Free List Trunk Page Format.</i>
    </center>
    <p>
      All trunk pages in the free-list except for the first contain the 
      maximum possible number of references to leaf pages. <span class=todo>Is this actually true in an auto-vacuum capable database?</span> The page number
      of the first page in the linked list of free-list trunk pages is 
      stored as a 4-byte big-endian unsigned integer at offset 32 of the
      file header (section <cite>file_header</cite>).

    <p class=req id=FF1240>
      All <i>free pages</i> in a <i>well-formed database file</i> are part of
      the database <i>free page list</i>.

    <p class=req id=FF1250>
      Each free page is either a <i>free list trunk</i> page or a
      <i>free list leaf</i> page.

    <p class=req id=FF1260>
      All <i>free list trunk</i> pages are linked together into a singly
      linked list. The first 4 bytes of each page in the linked list 
      contains the page number of the next page in the list, formatted
      as an unsigned big-endian integer. The first 4 bytes of the last
      page in the linked list are set to 0x00.

    <p class=req id=FF1270>
      The second 4 bytes of each <i>free list trunk</i> page contains
      the number of </i>free list leaf</i> page numbers stored on the free list
      trunk page, formatted as an unsigned big-endian integer.

    <p class=req id=FF1280>
      Beginning at byte offset 8 of each <i>free list trunk</i> page are
      <i>N</i> page numbers, each formatted as a 4-byte unsigned big-endian 
      integers, where <i>N</i> is the value described in requirement FF1270.

    <p class=req id=FF1290>
      All page numbers stored on all <i>free list trunk</i> pages refer to
      database pages that are <i>free list leaves</i>.

    <p class=req id=FF1300>
      The page number of each <i>free list leaf</i> page in a well-formed
      database file appears exactly once within the set of pages numbers
      stored on <i>free list trunk</i> pages.

    <p>The following statements govern the two 4-byte big-endian integers
       associated with the <i>free page list</i> structure in the database
       file header.

    <p class=req id=FF1310>
      The total number of pages in the free list, including all <i>free list
      trunk</i> and <i>free list leaf</i> pages, is stored as a 4-byte unsigned
      big-endian integer at offset 36 of the database file header.

    <p class=req id=FF1320>
      The page number of the first page in the linked list of <i>free list
      trunk</i> pages is stored as a 4-byte big-endian unsigned integer at
      offset 32 of the database file header. If there are no <i>free list
      trunk</i> pages in the database file, then the value stored at
      offset 32 of the database file header is 0.
  

    
     

  <h2 id=pointer_map_pages>Pointer Map Pages</h2>
    <p>
      Pointer map pages are only present in auto-vacuum capable databases.
      A database is an auto-vacuum capable database if the value stored 
      at byte offset 52 of the file-header is non-zero.
    <p>
      If they are present, the pointer-map pages together form a lookup 
      table that can be used to determine the type and "parent page" of
      any page in the database, given its page number. The lookup table
      classifies pages into the following categories:
    <table class=striped>
      <tr><th>Page Type <th>Byte Value <th>Description
      <tr><td style="white-space:nowrap">B-Tree Root Page<td>0x01
          <td>The page is the root page of a table or index B-Tree structure.
              There is no parent page number in this case, the value stored
              in the pointer map lookup table is always zero.
      <tr><td>Free Page<td>0x02
          <td>The page is part of the free page list (section
              <cite>free_page_list</cite>). There is no parent page in this
              case, zero is stored in the lookup table instead of a parent
              page number.
      <tr><td>Overflow type 1<td>0x03
          <td>The page is the first page in an overflow chain. The parent
              page is the B-Tree page containing the B-Tree cell to which
              the overflow chain belongs.
      <tr><td style="white-space:nowrap">Overflow type 2<td>0x04
          <td>The page is part of an overflow chain, but is not the first
              page in that chain. The parent page is the previous page in
              the overflow chain linked-list.
      <tr><td>B-Tree Page<td>0x05
          <td>The page is part of a table or index B-Tree structure, and is 
              not an overflow page or root page. The parent page is the page
              containing the parent tree node in the B-Tree structure.
    </table>
    <p>
      Pointer map pages themselves do not appear in the pointer-map lookup
      table. Page 1 does not appear in the pointer-map lookup table either.

    <center><img src="images/fileformat/pointermapentry.gif">
    <p><i>Figure <span class=fig id=figure_pointermapentry></span> - Pointer Map Entry Format.</i>
    </center>
    <p>
      Each pointer-map lookup table entry consumes 5 bytes of space. 
      The first byte of each entry indicates the page type, according to the 
      key described in the table above. The following 4 bytes store the 
      parent page number as a big-endian unsigned integer. This format is
      depicted in figure <cite>figure_pointermapentry</cite>. Each 
      pointer-map page may therefore contain:
    <pre>
        <i>num-entries</i> := <i>usable-size</i> / 5
</pre>
    <p>
      entries, where <i>usable-size</i> is defined as the page-size in bytes
      less the number of unused bytes left at the end of every page (as read
      from byte offset 20 of the file header).
    <p>
      Assuming the database is auto-vacuum capable, page 2 is always a 
      pointer map page. It contains the pointer map lookup table entries for
      pages 3 through (2 + <i>num-entries</i>), inclusive. The first 5 bytes
      of page 2 contain the pointer map lookup table entry for page 3. Bytes
      5 through 9, inclusive, contain the pointer map lookup table entry
      for page 4, and so on.
    <p>
      The next pointer map page in the database is page number (3 +
      <i>num-entries</i>), which contains the pointer map entries for pages
      (4 + <i>num-entries</i>) through (3 + 2 * <i>num-entries</i>) 
      inclusive. In general, for any value of <i>n</i> greater than zero, 
      the following page is a pointer-map page that contains lookup 
      table entries for the <i>num-entries</i> pages that follow it in the
      database file:
    <pre>
        <i>pointer-map-page-number</i> := 2 + <i>n</i> * <i>num-entries</i>
</pre>


    <p class=req id=FF1330>
      Non auto-vacuum databases do not contain pointer map pages.

    <p class=req id=FF1340>
      In an auto-vacuum database file, every <i>(num-entries + 1)</i>th 
      page beginning with page 2 is designated a pointer-map page, where 
      <i>num-entries</i> is calculated as:
      <code>
        <i>num-entries</i> := <i>database-usable-page-size</i> / 5
      </code>

    <p class=req id=FF1350>
      In an auto-vacuum database file, each pointer-map page contains
      a pointer map entry for each of the <i>num-entries</i> (defined by 
      FF1340) pages that follow it, if they exist.

    <p class=req id=FF1360>
      Each pointer-map page entry consists of a 1-byte page type and a 
      4-byte page parent number, 5 bytes in total.

    <p class=req id=FF1370>
	Pointer-map entries are packed into the pointer-map page in order,
        starting at offset 0. The entry associated with the database 
        page that immediately follows the pointer-map page is located at
        offset 0. The entry for the following page at offset 5 etc.

    <p>
      The following requirements govern the content of pointer-map entries.

    <p class=req id=FF1380>
	For each page except page 1 in an auto-vacuum database file that is 
	the root page of a B-Tree structure, the page type of the 
      corresponding pointer-map entry is set to the value 0x01 and the
      parent page number is zero.
    <p class=req id=FF1390>
	For each page that is a part of an auto-vacuum database file free-list, 
	the page type of the corresponding pointer-map entry is set to the
      value 0x02 and the parent page number is zero.
    <p class=req id=FF1400>
	For each page in a well-formed auto-vacuum database that is the first
	page in an overflow chain, the page type of the corresponding
      pointer-map entry is set to 0x03 and the parent page number field
      is set to the page number of the B-Tree page that contains the start
      of the B-Tree cell stored in the overflow-chain.
    <p class=req id=FF1410>
	For each page that is the second or a subsequent page in an overflow
	chain, the page type of the corresponding pointer-map entry is set to
	0x04 and the parent page number field is set to the page number of the
      preceding page in the overflow chain.
    <p class=req id=FF1420>
	For each page that is not a root page but is a part of a B-Tree tree
	structure (not part of an overflow chain), the page type of the
	corresponding pointer-map entry is set to the value 0x05 and the parent
	page number field is set to the page number of the parent node in the
        B-Tree structure.


<h1 id="database_file_manipulation">Database File Manipulation</h1>

  <p class=todo>
    Better to do other higher level requirements before this.
<!--
  <p>
    The previous section described the format of a valid SQLite database
    file. This section describes the way in which a database file is
    transitioned between valid states by SQLite to effect various 
    operations, for example creating a table or inserting a database
    record.
  <p>
    SQLite manipulates database files using combinations of the following
    operations:
  <ol>
    <li>Database Initialization (see section
        <cite>database_initialization</cite>).
    <li>Modification of a global parameter stored in the database file 
        file-header (see section <cite>database_parameters</cite>).
    <li>Creation of a new table.
    <li>Creation of a new index.
    <li>Creation of a new trigger or view.
    <li>Destruction of a database table.
    <li>Destruction of a database index.
    <li>Destruction of a trigger or view.
    <li>Insertion of an index entry.
    <li>Removal of an index entry.
    <li>Insertion of a table entry.
    <li>Removal of a table entry.
  </ol>
  <p>
    For example, execution of a single "UPDATE" statement may result in 
    many entries being removed and inserted into a table B-Tree and several
    index B-Trees. 
  <p>
    The list of operations above itemizes inserting and removing entries 
    from index and table B-Trees separately. Requirements specifying the
    set of database file operations required by various SQL statements
    may be found in [XXX]. The requirements in [XXX] are written so as to
    ensure that the constraints relating the content of a table B-Tree and 
    its associated index B-Trees are met (see section YYY). By contrast the
    requirements specifying the behaviour of the system when a 
    schema item (table, index, view or trigger) is added to or removed 
    from a database describe both the schema (sqlite_master) table
    modifications and any additional modifications made to other parts
    of the database file.

  <p class=todo>
    Fix this XXX reference. And add any other references to SQLiteRT
    requirements documents that may specify requirements in terms of these
    operations.
  <p class=todo>
    VACUUM? Auto-vacuum steps?

  <h2 id=database_initialization>Database Creation/Initialization</h2>
    <p>
      As noted in section <cite>database_file_format</cite> a zero-length 
      file is a valid empty SQLite database. The first time such a
      database is written to, SQLite initializes the the first page of
      the database file as described by the following requirements, creating
      a one-page empty SQLite database file.

    <p class=req>
      When initializing a new database file, SQLite shall set the
      size of the file to <i>page-size</i> bytes, where <i>page-size</i>
      is the database page size in bytes.

    <p class=req>
      When initializing a new database file, SQLite shall set the
      first 16 bytes of the database file to the following values, from
      first (byte offset 0) to last (byte offset 15): 0x53 0x51 0x4c 
      0x69 0x74 0x65 0x20 0x66 0x6f 0x72 0x6d 0x61 0x74 0x20 0x33 0x00.
    <p>
      The 16 byte values specified in the above requirement are the UTF-8
      encoding of the string "SQLite file format 3" followed by a single
      nul-terminator byte.

    <p class=req>
      When initializing a new database file, SQLite shall store the page 
      size in bytes as a 2-byte big-endian unsigned integer at byte 
      offset 16 of the new database file.
    <p class=todo>
      Some requirement to say where the initial page-size comes from. Probably
      a reference to the SQL level requirements documenting the page-size
      pragma.
    <p class=todo>
      Requirements for the other fields of the database header. Also to
      describe how the part of page 1 after the header is initialized.

  <h2 id=database_parameters>Setting Database Parameters</h2>
    <p>
      The database file-header contains three values that the system may
      be required to update in response to the execution of SQL pragma
      statements. These are:
    <ul>
      <li>The default pager-cache size,
      <li>The user-cookie value,
      <li>The incremental-vacuum flag.
    </ul>
    <p>
      Requirements specified in <cite>sql_sqlitert_requirements</cite> 
      specify the various scenarios in which the system is required to set
      one of the above three values. The following requirements explain 
      more specifically what the system has to do to achieve this.
    <p class=req>
      When required to set the default pager-cache size of a database, the
      system shall store the new value as a 4-byte big-endian unsigned 
      integer starting at byte offset 48 of the database file.
    <p class=req>
      When required to set the user-cookie value of a database, the
      system shall store the new value as a 4-byte big-endian unsigned 
      integer starting at byte offset 60 of the database file.
    <p class=req>
      When required to set the incremental vacuum flag of a database, the
      system shall store the new value as a 4-byte big-endian unsigned 
      integer starting at byte offset 64 of the database file.

  <h2>Creating and Deleting B-Tree Structures</h2>
    <h3 id=btree_creation>Table/Index Creation</h3>
      <p class=req>
        When a new table or index is added to a non auto-vacuum database file,
        the system shall initialize a newly allocated database page as the root
        page of an empty table or index B-Tree, respectively.
      <p class=todo>
        Requirements describing in detail how an empty root page is initialized.

      <p class=req>
        When a new table is added to an auto-vacuum database file, the
        system shall initialize the database page immediately following 
        the root page of the B-Tree structure with the numerically largest
        root page number, skipping any pointer-map pages, as the root page of
        an empty table B-Tree.  
        <p class=subreq>
          When adding a new table or index to an auto-vacuum database file,
          if the selected root page exists and is part of the database 
          free-list, the system shall remove it from the free-list.
        <p class=subreq>
          When adding a new table or index to an auto-vacuum database file,
          if the selected root page exists and is not part of the database
          free-list, then the system shall move the current contents of the 
          page to a newly allocated page. The system shall update all existing 
          references to the page within the database file to refer to the
          new page number.
        <p class=todo>
          The above requirement needs to be tested for a B-Tree page, an
          overflow page that is at the start of an overflow chain and an
          overflow page that is not at the start of a chain. Maybe each
          page type should have its own requirement.
        <p class=subreq>
          When adding a new table or index to an auto-vacuum database file,
          the system shall update the pointer-map page entries corresponding
          to both the new root page and, if the existing content of the page
          was moved, the page to which the existing content was moved.

      <p class=req>
        When a new table is added to a database file, the system shall 
        add a new entry to the table B-Tree with page 1 as its root page (the
        sqlite_master table). 
      <p class=todo>
        Requirements for the contents of the new sqlite_master row.

    <h3>Table/Index Destruction</h3>
  <h2>Creating and Deleting Other Schema Items</h2>

  <h2>Data Manipulation</h2>
    <p>
      This section describes the ways in which the system is required to
      manipulate B-Tree structures within a database file. Various 
      operations at the SQL level require the system to insert or remove
      entries from both table and index B-Trees. <span class=todo>It would be
      good to reference some other requirements document here.</span>

    <h3>Inserting Records</h3>

    <h4>Table B-Tree Inserts</h4>

      <p class=req>
        When required to insert a new entry into a table B-Tree, the system
        shall format a new table B-Tree leaf node cell containing the 
        integer key value and accompanying database record, and add the
        new cell to a leaf node of the table B-Tree structure.
      <p>
        The format of a "table B-Tree leaf node cell", as specified in the
        above requirement, is described in section 
        <cite>table_btree_cell_format</cite>.The following sub-requirements
        state that the system shall "attempt to insert a cell" into a given
        page. This operation may fail if there is not enough room on the
        selected page.
        <p class=subreq>
          When inserting a a new table B-Tree leaf node cell, if the table 
          B-Tree is empty or consists of a single page only, the system shall
          attempt to insert the new cell into the root page of the B-Tree.
        <p class=subreq>
          When inserting a a new table B-Tree leaf node cell, if the table 
          B-Tree constructor consists of more than one page, then the system
          shall attempt to insert the new cell into the leaf node page that
          currently contains the largest key value that is smaller than
          the key value of the cell being inserted.
        <p class=todo>
          Finish this.

    <h4>Index B-Tree Inserts</h4>
        <p class=todo>
          Finish this.
  
    <h3>Removing Records</h3>
        <p class=todo>
          Finish this.

  <h2>Page Management</h2>
    <p>
      When a new table or index is created, or when a new entry is added
      to a table or index that does not fit into one of the B-Tree structures
      existing pages, SQLite is required to select a new, presently unused,
      database page to use for the new data. The new page may be obtained
      using one of two methods:
    <ul>
      <li>The size of the database file may be extended by <i>page-size</i>
          bytes, creating a new page at the end of the current database file.
      <li>A database page that is currently part of the free page list 
          (refer to section <cite>free_page_list</cite>) may be removed from
          the free-list and reused.
    </ul>
    <p>
      This process of selecting a new page to use is refered to as
      <i>allocating a new page</i>. See section <cite>page_allocation</cite>
      for further details.
    <p>
      Similarly, when a table or index is removed from the database, or
      when a B-Tree structure is reorganized such that it consumes fewer
      pages, database pages may become unused. In this case the unused pages
      must be added to the database free-list. This process is known as
      <i>freeing a page</i>. See section <cite>page_deallocation</cite> for
      details.
    <p>
      Sometimes, when operating on an auto-vacuum database, the system is
      required to remove a specific page from the free page list. This
      can occur:
    <ul>
      <li>When a new database table or index is created (section 
        <cite>btree_creation</cite>), or
      <li>during an incremental-vacuum operation (section 
        <cite>incremental_vacuum</cite>).
    </ul>
    <p>
      The requirements found in this section specify the manner in which
      the system is required to manipulate the contents of database 
      free-list pages to achieve this are found in section
      <cite>page_removal</cite>.

    <h3 id=page_allocation>Page Allocation</h3>
     <p>
       If the database free-list is empty, then the new page is allocated
       by extending the database file:

     <p class=req>
       When SQLite allocates a new database page, if the database free 
       page list is completely empty, the page shall be allocated by 
       extending the database file.
       <p class=subreq>
         If extending the database file by page-size bytes means that the
         final page of the database file would be a pointer-map page, SQLite
         shall extend the database file by (2 * page-size) bytes. The final
         page of the database file after it has been extended shall be used
         as the new (allocated) page.
       <p class=subreq>
         Otherwise, SQLite shall extend the database file by page-size bytes.
         The final page of the database file after it has been extended shall
         be used as the new (allocated) page.
     <p>
       In the above requirements, the condition "would be a pointer-map 
       page" is only true if both of the following are true:
     <ul>
       <li>The database the page belongs to is an auto-vacuum database, and
       <li>The page number is equal to <i>(2 + n * (usable-size / 5))</i>
           for some integer <i>n</i>.
     </ul>
     <p>
       Otherwise, if the database free page list is not empty when SQLite
       is required to allocate a new database page, then the page is
       allocated by removing a page from the free-list for reuse.

     <p class=req>
       When SQLite allocates a new database page, if the database free page
       list is not empty, then SQLite shall allocate the page by reusing
       a page that is currently linked into the database free page list.
       The page shall be removed from the free-list before it is resused.
       <p class=subreq>
         When allocating a page from the free-list, if the first page 
         in the free-list trunk has no leaves, then SQLite shall use this
         page as the new (allocated) page. Otherwise, SQLite shall select
         one of the leaves belonging to the first page in the free-list
         trunk, remove its entry from the trunk page and use it as the
         new (allocated) page.
       <p class=todo>
         How do we pick a single leaf from the set of leaves on the trunk
         page?
       <p class=subreq>
         If the page removed from the free-list is the first page in the
         free-list trunk, SQLite shall update the 4-byte integer value stored
         at byte offset 32 to contain the page number of the second page 
         in the free-list trunk if it exists, or zero if the freelist is 
         now empty.
       <p class=subreq>
         After removing a page from the free-list, SQLite shall update 
         the 4-byte integer value stored at byte offset 36 of the database 
         file header to reflect the new number of pages in the database 
         free page list (one less than before).

    <h3 id=page_deallocation>Page Deallocation</h3>
      <p class=req>
        If SQLite is required to free a database page when the free-list 
        is complete empty, or when the first page of the free-list trunk
        is completely full, SQLite shall use the freed page as the new 
        head of the free-list trunk. 
        <p class=subreq>
          When a newly freed page is made the head of the free-list trunk,
          the system shall update the 4-byte integer value stored at byte 
          offset 32 to store the page number of the new free-list trunk head.
        <p class=subreq>
          When a newly freed page is made the new head of the free-list trunk,
          the system shall store the page number of the previous head page
          as a big-endian integer in the first 4 bytes of the page data, or
          zero if the free-list was previously empty.
        <p class=subreq>
          When a newly freed page is made the new head of the free-list trunk,
          the system shall set the second block of 4 bytes on the page to 
          zero (indicating that the free-list trunk page currently has no
          leaves).

      <p class=req>
        If SQLite is required to free a database page when the first
        page of the free-list trunk exists and has enough free space for
        another leaf, the freed page shall become a leaf of the first
        page in the free-list trunk.
      <p class=req>
        After removing a page from the free-list, SQLite shall update 
        the 4-byte integer value stored at byte offset 36 of the database 
        file header to reflect the new number of pages in the database 
        free page list (one less than before).

    <h3 id=page_removal>Removing a Page From The Free List</h3>
      <p class=req>
        When the system is required to remove a specific page from the 
        database free-list, and that page is a free-list leaf page, the
        system shall remove the specified leaf page number from the
        relevant trunk page.
      <p class=req>
        When the system is required to remove a specific page from the 
        database free-list, and that page is an empty free-list trunk page,
        the system shall remove the specified page from the free-list
        trunk linked list.
      <p class=req>
        When the system is required to remove a specific page from the 
        database free-list, and that page is a non-empty free-list trunk 
        page, the system shall move the contents of the trunk page
        to its first leaf page, remove the first leaf entry from the new
        trunk page, then link the new trunk page into the free-list trunk
        in place of the page being removed.

    <h3 id=incremental_vacuum>Database Reorganization (auto-vacuum)</h3>
      <p class=todo>
        Requirements describing incremental vacuum steps. And on-commit
        handling in auto-vacuum databases.
-->

<h1>References</h1>

  <table id="refs" style="width:auto; margin: 1em 5ex">
    <tr><td style="width:5ex" id="ref_comer_btree">[1]<td>
     Douglas Comer, <u>Ubiquitous B-Tree</u>, ACM Computing Surveys (CSUR),
     v.11 n.2, pages 121-137, June 1979.
    <tr><td style="width:5ex" id="ref_knuth_btree">[2]<td>
     Donald E. Knuth, <u>The Art Of Computer Programming, Volume 3:
     "Sorting And Searching"</u>, pages 473-480. Addison-Wesley
     Publishing Company, Reading, Massachusetts.
    <tr><td style="width:5ex" id="capi_sqlitert_requirements">[3]<td>
      C API Requirements Document.
    <tr><td style="width:5ex" id="sql_sqlitert_requirements">[4]<td>
      SQL Requirements Document.
    <tr><td style="width:5ex" id="io_sqlitert_requirements">[5]<td>
      File IO Requirements Document.
  </table>