hd_keywords {file format redux}

The SQLite Database File Format

This document describes and defines the on-disk database file format used by SQLite.

This is the second document to define the SQLite database file format. The [file format| first document] differs from this one in that it is written in a formal style whereas this document attempts to be more conversational. The two documents were independently written (they have different authors) and hence serve as cross-checks for one another. Both documents should describe exactly the same file format. Any discrepencies between the two documents should be considered a bug.

1.0 The Database File

Most of the time the complete state of an SQLite database is contained a single file on disk called the "main database file".

While performing a transaction, SQLite stores some temporary information in a second file called the "rollback journal". If the application or host computer crashes before completing the transaction, then the rollback journal contains critical state information needed to restore the content of the main database file. When a rollback journal contains information necessary for recovering the state of the database, we say that it is a "hot journal". Hot journals only come up in an error recovery scenario and so are uncommon, but they are part of the state of an SQLite database and so should not be ignored. This document will define the format of a rollback journal, but most of the content of this document will focus on the main database file.

1.1 Pages

The main database file consists of one or more pages. The size of a page is a power of two between 512 and 32768 inclusive. All pages within the same database are the same size. The page size for a database file is determined by the 2-byte big-endian integer located at an offset of 16 bytes from the beginning of the database file.

Pages are numbered beginning with 1. The maximum page number is 2147483646 (231 - 2). The minimum size SQLite database is a single 512-byte page. The maximum size database would be 2147483646 pages at 32768 bytes per page or 70,368,744,112,128 bytes (about 70 terabytes). Usually SQLite will hit the maximum file size limit of the underlying filesystem or disk hardware size limit long before it hits its own internal size limit.

In common use, SQLite databases tend to range in size from a few kilobytes to a few gigabytes.

At any point in time, every page in the main database has a single use which is one of the following:

All reads from and writes to the main database file begin at a page boundary and all writes are an integer number of pages in size. Reads are also usually an integer number of pages in size, with the one exception that when the database is first opened, the first 100 bytes of the database file (the database file header) are read as a sub-page size unit.

Before any information-bearing page of the database is modified, the original unmodified content of that page is written into the rollback journal. If a transaction is interrupted and needs to be rolled back, the rollback journal can then be used to restore the database to its original state. Freelist leaf pages bear no information that would need to be restored on a rollback and so they are not written to the journal prior to modification, in order to reduce disk I/O.

hd_fragment database_header {database header}

1.2 The Database Header

The first 100 bytes of the database file comprise the database file header. The database file header is divided into fields as shown by the table below. All multibyte fields in the database file header are stored with the must significant byte first (big-endian).

Database Header Format
OffsetSizeDescription
016 The header string: "SQLite format 3\000"
162 The database page size in bytes. Must be a power of two between 512 and 32768 inclusive.
181 File format write version. Must be 1.
191 File format read version. Must be 1.
201 Bytes of unused "reserved" space at the end of each page. Usually 0.
211 Maximum embedded payload fraction. Must be 64.
221 Minimum embedded payload fraction. Must be 32.
231 Leaf payload fraction. Must be 32.
244 File change counter.
284 Reserved for expansion. Must be zero.
324 Page number of the first freelist trunk page.
364 Total number of freelist pages.
404 The schema cookie.
444 The schema format number. Currently support schema formats are 1, 2, 3, and 4.
484 Default page cache size.
524 The page number of the largest root b-tree page when in auto-vacuum or incremental-vacuum modes, or zero otherwise.
564 The database text encoding. A value of 1 means UTF-8. A value of 2 means UTF-16le. A value of 3 means UTF-16be.
604 The "user version" as read and set by the [user_version pragma].
644 True (non-zero) for incremental-vacuum mode. False (zero) otherwise.
6832 Reserved for expansion. Must be zero.

Every SQLite database file begins with the following 16 bytes (in hex): 53 51 4c 69 74 65 20 66 6f 72 6d 61 74 20 33 00. This byte sequence corresponds to the UTF-8 string "SQLite format 3" including the nul terminator character at the end.

The file format write version and file format read version at offsets 18 and 19 are intended to allow for enhancements of the file format in future versions of SQLite. In current versions of SQLite, both of these values must be one. If a version of SQLite coded to the current file format specification encounters a database file where the read version is 1 but the write version is greater than one, then the database file must be treated as read-only. If a database file with a read version greater than 1 is encounter, then that database cannot be read or written.

SQLite has the ability to set aside a small number of extra bytes at the end of every page for use by extensions. These extra bytes are used, for example, by the SQLite Encryption Extension to store a nonce and/or cryptographic checksum associated with each page. The "reserved space" size in the 1-byte integer at offset 20 is the number of bytes of space at the end of each page to reserve for extensions. This value is usually 0. The value can be odd.

hd_fragment usable_size {usable size}

The "usable size" of a database page is the page size specify by the 2-byte integer at offset 16 in the header less the "reserved" space size recorded in the 1-byte integer at offset 20 in the header. The usable size of a page might be an odd number. However, the usable size is not allowed to be less than 480. In other words, if the page size is 512, then the reserved space size cannot exceed 32.

The maximum and minimum embedded payload fractions and the leaf payload fraction values must be 64, 32, and 32. These values were orginally intended to as tunable parameters that could be used to modify the storage format of the b-tree algorithm. However, that functionality is not supported and there are no current plans to add support in the future. Hence, these three bytes are fixed at the values specified.

The file change counter is a 4-byte big-endian integer which is incremented whenever the database file is changed. When two or more processes are reading the same database file, each process can detect database changes from other processes by monitoring the change counter. A process will normally want to flush its database page cache when another process modified the database, since the cache has become stale. The file change counter facilitates this.

Unused pages in the database file are stored on a freelist. The 4-byte big-endian integer at offset 32 stores the page number of the first page of the freelist, or zero if the freelist is empty. The 4-byte big-endian integer at offset 36 stores stores the total number of pages on the freelist.

The schema cookie is a 4-byte big-endian integer at offset 40 that is incremented whenever the database schema changes. A prepared statement is compiled against a specific version of the database schema. Whenever the database schema changes, the statement must be reprepared. Whenever a prepared statement runs, it first checks the schema cookie to make sure the value is the same as when the statement was prepared and if not it aborts to force the statement to be reprepared.

The schema format number is a 4-byte big-endian integer at offset 44. The schema format number is similar to the file format read and write version numbers at offsets 18 and 19 except that the schema format number refers to the high-level SQL formatting rather than the low-level b-tree formatting. Four schema format numbers are currently defined:

  1. Format 1 is understood by all versions of SQLite back to version 3.0.0.
  2. Format 2 adds the ability of rows within the same table to have a varing number of columns, in order to support the [ALTER TABLE | ALTER TABLE ... ADD COLUMN] functionality. Support for reading and writing format 2 was added in SQLite version 3.1.3 on 2005-02-19.
  3. Format 3 adds the ability of extra columns added by [ALTER TABLE | ALTER TABLE ... ADD COLUMN] to have non-NULL default values. This capability was added in SQLite version 3.1.4 on 2005-03-11.
  4. Format 4 causes SQLite to respect the DESC keyword on index declarations. (The DESC keyword is ignored in indices for formats 1, 2, and 3.) Format 4 also adds two new boolean record type values ([serial types] 8 and 9.) Support for format 4 was added in SQLite 3.3.0 on 2006-01-10.

New database files created by SQLite use format 1 by default, so that database files created by newer versions of SQLite can still be read by older versions of SQLite. The [legacy_file_format pragma] can be used to cause SQLite to create new database files using format 4. Future versions of SQLite may begin to create files using format 4 by default.

The 4-byte big-endian signed integer at offset 48 is the suggest cache size in pages for the database file. The value is a suggestion only and SQLite is under no obligation to honor it. The absolute value of the integer is used as the suggested size. The suggested cache size can be set using the [default_cache_size pragma].

The two 4-byte big-endian integers at offsets 52 and 64 are used to manage the [auto_vacuum] and [incremental_vacuum] modes. If the integer at offset 52 is zero then pointer-map (ptrmap) pages are omitted from the database file and neither auto_vacuum nor incremental_vacuum are supported. If the integer at offset 52 is non-zero then it is the page number of the largest root page in the database file, the database file contain ptrmap pages, and the mode must be either auto_vacuum or incremental_vacuum. In this latter case, the integer at offset 64 is true for incremental_vacuum and false for auto_vacuum. If the integer at offset 52 is zero then the integer at offset 64 must also be zero.

The 4-byte big-endian integer at offset 56 determines the encoding used for all text strings stored in the database. A value of 1 means UTF-8. A value of 2 means UTF-16le. A value of 3 means UTF-16be. No other values are allowed.

The 4-byte big-endian integer at offset 60 is the user version which is set and queried by the [user_version pragma]. The user version is not used by SQLite.

All other bytes of the database file header are reserved for future expansion and must be set to zero.

1.3 The Lock-Byte Page

The lock-byte page is the single page of the database file that contains the bytes at offsets between 1073741824 and 1073742335, inclusive. A database file that is less than or equal to 1073741824 bytes in size contains no lock-byte page. A database file larger than 1073741824 contains exactly one lock-byte page.

The lock-byte page is set aside for use by the operating-system specific VFS implementation in implementing the database file locking primitives. SQLite will not use the lock-byte page; it will never be read or written by the SQLite core, though operating-system specific VFS implementions may choose to read or write bytes on the lock-byte page according to the needs and proclavities of the underlying system. The unix and win32 VFS implementations that come built into SQLite do not write to the lock-byte page, but we are aware of third-party VFS implementations for other operating systems that do sometimes write to the lock-byte page.

1.4 The Freelist

A database file might contain one or more pages that are not in active use. Unused pages can come about, for example, when information is deleted from the database. Unused pages are stored on the freelist and are reused when additional pages are required.

The freelist is organized as a linked list of freelist trunk pages with each trunk pages containing page numbers for zero or more freelist leaf pages.

A freelist trunk page consists of an array of 4-byte big-endian integers. The size of the array is as many integers as will fit in the usable space of a page. The minimum usable space is 480 bytes so the array will always be at least 120 entries in length. The first integer in the array is the page number of the next freelist trunk page in the list or zero if this is the last freelist trunk page. The second integer in the array is the number of leaf page pointers to follow. Call the second integer L. If L is greater than zero then integers with array indexes between 2 and L+1 inclusive contain page numbers for freelist leaf pages.

Freelist leaf pages contain no information. SQLite avoid reading or writing freelist leaf pages in order to reduce disk I/O.

A bug in SQLite versions prior to 3.6.0 caused the database to be reported as corrupt if any last 6 entries in the freelist trunk page array contained non-zero values. Newer versions of SQLite do not have this problem. However, newer versions of SQLite still avoid using the last six entries in the freelist trunk page array in order that database files created by newer versions of SQLite can be read by older versions of SQLite.

The number of freelist pages is stored as a 4-byte big-endian integer in the database header at an offset of 36 from the beginning of the file. The database header also stores the page number of the first freelist trunk page as a 4-byte big-endian integer at an offset of 32 from the beginning of the file.

1.5 B-tree Pages

A b-tree page is either an interior page or a leaf page. A leaf page contains keys and in the case of a table b-tree each key has associated content. An interior page contains K keys without content but with K+1 pointers to child b-tree pages. A "pointer" in an interior b-tree page is just the 31-bit integer page number of the child page.

Define the depth of a leaf b-tree to be 1 and that the depth of any interior b-tree to be one more than the maximum depth of any of its children. In a well-formed database, all children of any one interior b-tree have the same depth.

In an interior b-tree page, the pointers and keys logically alternate with a pointer on both ends. (The previous sentence is to be understood conceptually - the actual layout of the keys and pointers within the page is more complicated and will be described in the sequel.) All keys within the same page are unique and are organized in ascending order from left to right. For any key X, pointers to the left of a X refer to b-tree pages on which alls keys are less than or equal to X. Pointers to the right of X refer to pages where all keys are greater than X.

Within an interior b-tree page, each key and the pointer to its immediate left are combined into a structure called a "cell". The right-most pointer is held separately. A leaf b-tree page has no pointers, but it still uses the cell structure to hold keys for index b-trees or keys and content for table b-trees.

Every b-tree page has at most one parent b-tree page. A b-tree page without a parent is called a root page. A root b-tree page together with the closure of its children form a complete b-tree. It is possible (and in fact rather common) to have a complete b-tree that consists of a single page that is both a leaf and the root. Because there are pointers from parents to children, every page of a complete b-tree can be located if only the root page is known. Hence, b-trees are identified by their root page number.

A b-tree page is either a table b-tree page or an index b-tree page. All pages within each complete b-tree are of the same type: either table or index. There is a one-to-one mapping from table b-trees in the database file to (non-virtual) tables in the database schema, including system tables such as sqlite_master. There is one-to-one mapping between index b-trees in the database file and indices in the schema, including implied indices created by uniqueness constraints. The b-tree corresponding to the sqlite_master table always has its root page on a page number of 1. The sqlite_master table contains the root page number for every other table and index in the database file.

Each entry in a table b-tree consists of a 64-bit signed integer key and up to 2147483647 bytes of arbitrary data. Interior table b-trees hold only keys and pointers to children. All data is contained in the table b-tree leaves.

Each entry in an index b-tree consists of an arbitrary key of up to 2147483647 bytes in length and no data.

hd_fragment cell_payload {cell payload}

Define the "payload" of a cell to be the arbitrary length section of the cell. For an index b-tree, the key is always arbitrary in length and hence the payload is the key. There are no arbitrary length elements in the cells of interior table b-tree pages and so those cells have no payload. Table b-tree leaf pages contain arbitrary length content and so for cells on those pages the payload is the content.

When the size of payload for a cell exceeds a certain threshold (to be defined later) then only the first few bytes of the payload are stored on the b-tree page and the balance is stored in a linked list of content overflow pages.

A b-tree page is divided into regions in the following order:

  1. The 100-byte database file header (found on page 1 only)
  2. The 8 or 12 byte b-tree page header
  3. The cell pointer array
  4. Unallocated space
  5. The cell content area
  6. The reserved region.

The 100-byte database file header is found only on page 1, which is always a table b-tree page. All other b-tree pages in the database file omit this 100-byte header.

The reserved region is an area of unused space at the end of every page (except the locking page) that extensions can use to hold per-page information. The size of the reserved region is determined by the one-byte unsigned integer found at an offset of 20 into the database file header. The size of the reserved region is usually zero.

The b-tree page header is 8 bytes in size for leaf pages and 12 bytes for interior pages. All multibyte values in the page header are big-endian. The b-tree page header is composed of the following fields:

B-tree Page Header Format
OffsetSizeDescription
01 A flag indicating the b-tree page type A value of 2 means the page is an interior index b-tree page. A value of 5 means the page is an interior table b-tree page. A value of 10 means the page is a leaf index b-tree page. A value of 13 means the page is a leaf table b-tree page. Any other value is an error.
12 Byte offset into the page of the first freeblock
32 Number of cells on this page
52 Offset to the first byte of the cell content area
71 Number of fragmented free bytes within the cell content area
84 The right-most pointer (interior b-tree pages only)

The cell pointer array of a b-tree page immediately follows the b-tree page header. Let K be the number of cells on the btree. The cell pointer array consists of K 2-byte integer offsets to the cell contents. The cell pointers are arranged in key order with left-most cell (the cell with the smallest key) first and the right-most cell (the cell with the largest key) last.

Cell content is stored in the cell content region of the b-tree page. SQLite strives to place cells as far toward the end of the b-tree page as it can, in order to leave space for future growth of the cell pointer array. The area in between the last cell pointer array entry and the beginning of the first cell is the unallocated region.

A freeblock is a structure used to identify unallocated space within a b-tree page. Freeblocks are organized on a chain. The first 2 bytes of a freeblock are a big-endian integer which is the offset in the b-tree page of the next freeblock in the chain, or zero if the freeblock is the last on the chain. The third and fourth bytes of each freeblock form a bit-endian integers which is the size of the freeblock in bytes, including the 4-byte header. Freeblocks are always connected in order of increasing offset. The second field of the b-tree page header is the offset of the first freeblock, or zero if there are no freeblocks on the page. In a well-formed b-tree page, there will always be at least one cell before the first freeblock.

A freeblock requires at least 4 bytes of space. If there is an isolated group of 1, 2, or 3 unused bytes within the cell content area, those bytes comprise a fragment. The total number of bytes in all fragments is stored in the fifth field of the b-tree page header. In a well-formed b-tree page, the total number of bytes in fragments may not exceed 60.

The total amount of free space on a b-tree page consists of the size of the unallocated region plus the total size of all freeblocks plus the number of fragmented free bytes. SQLite may from time to time reorganize a b-tree page so that there are no freeblocks or fragment bytes, all unused bytes are contained in the unallocated space region, and all cells are packed tightly at the end of the page. This is called "defragmenting" the b-tree page.

hd_fragment varint {variable-length integer} {varint}

A variable-length integer or "varint" is a static Huffman encoding of 64-bit twos-complement integers that uses less space for small positive values. A varint is between 1 and 9 bytes in length. The varint consists of either zero or more byte which have the high-order bit set followed by a single byte with the high-order bit clear, or nine bytes, whichever is shorter. The lower seven bits of each of the first eight bytes and all 8 bits of the ninth byte are used to reconstruct the 64-bit twos-complement integer. Varints are big-endian: bits taken from the earlier byte of the varint are the more significant and bits taken from the later bytes.

The format of a cell depends on which kind of b-tree page the cell appears on. The following table shows the elements of a cell, in order of appearance, for the various b-tree page types.

Table B-Tree Leaf Cell:

  • A varint which is the total number of bytes of payload, including any overflow
  • A varint which is the integer key, a.k.a. "rowid"
  • The initial portion of the payload that does not spill to overflow pages.
  • A 4-byte big-endian integer page number for the first page of the overflow page list - omitted if all payload fits on the b-tree page.

Table B-Tree Interior Cell:

  • A 4-byte big-ending page number which is the left child pointer.
  • A varint which is the integer key

Index B-Tree Leaf Cell:

  • A varint which is the total number of bytes of key payload, including any overflow
  • The initial portion of the payload that does not spill to overflow pages.
  • A 4-byte big-endian integer page number for the first page of the overflow page list - omitted if all payload fits on the b-tree page.

Index B-Tree Interior Cell:

  • A 4-byte big-ending page number which is the left child pointer.
  • A varint which is the total number of bytes of key payload, including any overflow
  • The initial portion of the payload that does not spill to overflow pages.
  • A 4-byte big-endian integer page number for the first page of the overflow page list - omitted if all payload fits on the b-tree page.

The information above can be recast into a table format as follows:

B-tree Cell Format
Datatype Appears in... Description
Table Leaf Table Interior Index Leaf Index Interior
4-byte integer     Page number of left child
varint   Number of bytes of payload
varint     Rowid
byte array   Payload
4-byte integer   Page number of first overflow page

The amount of payload that spills onto overflow pages also depends on the page type. For the following computations, let U be the usable size of a database page, the total page size less the reserved space at the end of each page. And let P be the payload size.

Table B-Tree Leaf Cell:

If the payload size P is less than U-36 then the entire payload is stored on the b-tree page. Let M be ((U-12)*32/255)-23. If P is greater than U-35 then the number of byte stored on the b-tree page is the lessor of M+((P-M)%(U-4)) and U-35.

Table B-Tree Interior Cell:

Interior pages of table b-trees have no payload and so there is never any payload to spill.

Index B-Tree Leaf Or Interior Cell:

If the payload size P is less than ((U-12)*64/255)-22 then the entire payload is stored on the b-tree page. Let M be ((U-12)*32/255)-23. If P is greater than ((U-12)*64/255)-23 then the number of byte stored on the b-tree page is the lessor of M+((P-M)%(U-4)) and ((U-12)*64/255)-23.

The overflow thresholds are designed to give a minimum fanout of 4 for index b-trees and to make sure enough of the payload is on the b-tree page that the record header can usually be accessed without consulting an overflow page. In hindsight, the designers of the SQLite b-tree logic realize that these thresholds could have been made much simpler. However, the computations cannot be now be changed without resulting in an incompatible file format. And the current computations work well, even if they are a little complex.

1.6 Cell Payload Overflow Pages

When the payload of a b-tree cell is too large for the b-tree page, the surplus is spilled onto overflow pages. Overflow pages form a linked list. The first four bytes of each overflow page are a big-endian integer which is the page number of the next page in the chain, or zero for the final page in the chain. The fifth byte through the last usable byte are used to hold overflow content.

1.7 Pointer Map or Ptrmap Pages

Pointer map or ptrmap pages are extra pages inserted into the database to make the operation of [auto_vacuum] and [incremental_vacuum] modes more efficient. Other page types in the database typically have pointers from parent to child. For example, a interior b-tree page contains pointers to its child b-tree pages and an overflow chain has a pointer from earlier to later links in the chain. A ptrmap page contains linkage information going in the opposite direction, from child to parent.

Ptrmap pages must exist in any database file which has a non-zero largest root b-tree page value at offset 52 in the database header. If the largest root b-tree page value is zero, then the database must not contain ptrmap pages.

In a database with ptrmap pages, the first ptrmap page is page 2. A ptrmap page consists of an array of 5-byte entries. Let J be the number of 5-byte entries that will fit in the usable space of a page. (In other words, J=U/5.) The first ptrmap page will contain back pointer information for pages 3 through J+2, inclusive. The next pointer map page will be on page J+3 and that ptrmap page will provide back pointer information for pages J+4 through 2*J+3 inclusive. An so forth for the entire database file.

In a database that uses ptrmap pages, all pages at locations identified by the computation in the previous paragraph must be ptrmap page and no other page may be a ptrmap page. Except, if the byte-lock page happens to fall on the same page number as a ptrmap page, then the ptrmap is moved to the following page for that one case.

Each 5-byte entry on a ptrmap page provides back-link information about one of pages that immediately follow the pointer map. If page B is ptrmap page then back-link information about page B+1 is provided by the first entry on the pointer map. Information about page B+2 is provided by the second entry. And so forth.

Each 5-byte ptrmap entry consists of one byte of "page type" information followed by a 4-byte big-endian page number. Five page types are recognized:

  1. A b-tree root page. The page number should be zero.
  2. A freelist page. The page number should be zero.
  3. The first page of a cell payload overflow chain. The page number is the b-tree page that contains the cell whose content has overflowed.
  4. A page in an overflow chain other than the first page. The page number is the prior page of the overflow chain.
  5. A non-root b-tree page. The page number is the parent b-tree page.

In any database file that contains ptrmap pages, all b-tree root pages must come before any non-root b-tree page, cell payload overflow page, or freelist page.

2.0 Schema Layer

The foregoing text describes low-level aspects of the SQLite file format. The b-tree mechanism provides powerful and efficient means of accessing a large data set. This section will describe how the low-level b-tree layer is used to implement higher-level SQL capabilities.

hd_fragment record_format {record format}

2.1 Record Format

The content of a table b-tree leaf page and the key of any index b-tree page was characterized above as an arbitrary sequence of bytes. The prior discussion mentioned one key being less than another, but did not define what "less than" meant. The current section will address these deficiencies.

Payload, be it table content or index keys, is always in the "record format". The record format defines a sequence of values corresponding to to columns in a table or index. The record format specifies the number of columns, the datatype of each column, and the content of each column.

The record format makes extensive use of the [variable-length integer] or [varint] representation of 64-bit signed integers defined above.

hd_fragment serialtype {serial type} {serial types}

A record contains a header and a body, in that order. The header begins with a single varint which determines the total number of bytes in the header. The varint value is the size of the header in byte including the size varint itself. Following the size varint are one or more additional varints, one per column. These additional varints are called "serial type" numbers and determine the datatype of each column, according to the following key:

Serial Type Codes Of The Record Format
Serial TypeContent SizeMeaning
00 NULL
11 8-bit twos-complement integer
22 Big-endian 16-bit twos-complement integer
33 Big-endian 24-bit twos-complement integer
44 Big-endian 32-bit twos-complement integer
56 Big-endian 48-bit twos-complement integer
68 Big-endian 64-bit twos-complement integer
78 Big-endian IEEE 754-2008 64-bit floating point number
80 Integer constant 0. Only available for schema format 4 and higher.
90 Integer constant 1. Only available for schema format 4 and higher.
10,11   Not used. Reserved for expansion.
N≥12 and even (N-12)/2 A string in the database encoding and (N-12)/2 bytes in length. The nul terminator is omitted.
N≥13 and odd (N-13)/2 A BLOB that is (N-13)/2 bytes in length

Note that because of the way varints are defined, the header size varint and serial type varints will usually consist of a single byte. The serial type varints for large strings and BLOBs might extend to two or three byte varints, but that is the exception rather than the rule. The varint format is very efficient at coding the record header.

The values for each column in the record immediately follow the header. Note that for serial types 0, 8, 9, 12, and 13, the value is zero bytes in length. If all columns are of these types then the body section of the record is empty.

2.2 Record Sort Order

The order of keys in an index b-tree is determined by the sort order of the records that the keys represent. Record comparison proceeds column by column. Columns of a record are examined from left to right. The first pair of columns that are not equal determines the relative order of the two records. The sort order of individual columns is as follows:

  1. NULL values (serial type 0) sort first
  2. Numeric values (serial types 1 through 9) sort next and in numeric order
  3. Text values (even serial types 12 and larger) sort next in the order determined by the columns collating function
  4. BLOB values (odd serial types 13 and larger) sort last in order determined by memcmp().

A collating function for each column is necessary in order to compute the order of text fields. SQLite defines three built-in collating functions:

BINARY Strings are comparied byte by byte using the memcmp() function from the standard C library.
NOCASE Like BINARY except that uppercase ASCII characters ('A' through 'Z') are folded into their lowercase equivalents prior to running the comparison. Note that only ASCII characters are case-folded. NOCASE does not implement a general purpose unicode caseless comparison.
RTRIM Like BINARY except that spaces at the end of the string are elided prior to comparison.

Additional application-specific collating functions can be added to SQLite using the [sqlite3_create_collation()] interface.

The default collating function for all strings is BINARY. Alternative collating functions for table columns can be specified in the [CREATE TABLE] statement using the COLLATE clause on the column definition. When a column is indexed, the same collating function specified in the [CREATE TABLE] statement is used for the column in the index, by default, though this can be overridden using a COLLATE clause in the [CREATE INDEX] statement.

2.3 Representation Of SQL Tables

Each ordinary SQL table in the database schema is represented on disk by a table b-tree. Each entry in the table b-tree corresponds to a row of the SQL table. The [rowid] of the SQL table is the 64-bit signed integer key for each entry in the table b-tree.

The content of each SQL table row is stored in the database file by first combining the values in the various columns into a byte array in the record format, then storing that byte array as the payload in an entry in the table b-tree. The order of values in the record is the same as the order of columns in the SQL table definition. When an SQL table that includes an [INTEGER PRIMARY KEY] column (which aliases the [rowid]) then that column appears in the record as a NULL value. SQLite will know to use the table b-tree key rather than the NULL value when referencing the [INTEGER PRIMARY KEY] column.

If the [affinity] of a column is REAL and that column contains a value that can be converted to an integer without loss of information (if the value contains no fractional part and is not too large to be represented as an integer) then the column may be stored in the record as an integer. SQLite will know to convert the value back to floating point when extracting it from the record.

2.4 Representation Of SQL Indices

Each SQL index, whether explicitly declared via a [CREATE INDEX] statement or implied by a UNIQUE constraint, corresponds to an index b-tree in the database file. There is one entry in index b-tree for each row in the corresponding table. The key to an index b-tree is a record composed of the columns that are being indexed followed by the [rowid] of the table row. Because every row in a table has a unique rowid and all keys in an index contain the rowid, all keys in an index are unique.

There is a one-to-one mapping between rows in a table and entries in each index associated with that table. Corresponding rows int the index and table b-trees share the same rowid value, and contain the same value for all indexed columns.

2.5 Storage Of The SQL Database Schema

Page 1 of a database file is the root page of a table b-tree that holds a special table named "sqlite_master" (or "sqlite_temp_master" in the case of a TEMP database) which stores the complete database schema. The structure of the sqlite_master table is as if it had been created using the following SQL:

CREATE TABLE sqlite_master(
  type text,
  name text,
  tbl_name text,
  rootpage integer,
  sql text
);

The sqlite_master table contains a row for each table, index, view, and trigger in the database schema, except there is no entry for the sqlite_master table itself.

The sqlite_master.type column will be one of the following text strings: 'table', 'index', 'view', or 'trigger' according to the type of object defined. The 'table' string is used for both ordinary and [virtual tables].

The sqlite_master.name column will hold the name of the object. For indices that are automatically created by UNIQUE constraints, the name is "sqlite_autoindex_TABLE_N" where TABLE is replaced by the name of the table that contains the UNIQUE constraint and N is an integer beginning with 1 and increasing by one with each UNIQUE constraint seen.

The sqlite_master.tbl_name column holds the name of a table or view that the object is associated with. For a table or view, the tbl_name column is a copy of the name column. For an index, the tbl_name is the name of the table that is indexed. For a trigger, the tbl_name column stores the name of the table or view that causes the trigger to fire.

The sqlite_master.rootpage column stores the page number of the root b-tree page for tables and indices. For rows that define views, triggers, and virtual tables, the rootpage column is 0 or NULL.

The sqlite_master.sql column stores SQL text that describes the object. This SQL text is a [CREATE TABLE], [CREATE VIRTUAL TABLE], [CREATE INDEX], [CREATE VIEW], or [CREATE TRIGGER] statement that if evaluated against the database file when it is the main database of a [database connection] would recreated the object. The text is usually a copy of the original statement used to create the object but with normalizations applied so that the text conforms to the following rules:

The text in the sqlite_master.sql column is a copy of the original CREATE statement text that created the object, except normalized as described above and as modified by subsequent [ALTER TABLE] statements.

For indices that are automatically created by UNIQUE constraints, the sqlite_master.sql field is NULL.

hd_fragment rollbackjournal {rollback journal format}

3.0 The Rollback Journal

The rollback journal is a file associated with each SQLite database fail that hold information used to restore the database file to its initial state during the course of a transaction. The rollback journal file is always located in the directory as the database file and has the same name as the database file but with the string "-journal" appended. There can only be a single rollback journal associated with a give database and hence there can only be one write transaction open against a single database at one time.

If a transaction is aborted due to an application crash, an operating system crash, or a hardware power failure or crash, then the database may be left in an inconsistent state. The next time SQLite attempts to open the database file, the presence of the rollback journal file will be detected and the journal will be automatically played back to restore the database to its state at the start of the incomplete transaction.

A rollback journal is only considered to be valid if it exists and contains a valid header. Hence a transaction can be committed in one of three ways:

  1. The rollback journal file can be deleted,
  2. The rollback journal file can be truncated to zero length, or
  3. The header of the rollback journal can be overwritten with invalid header text (for example, all zeros).
These three ways of committing a transaction correspond to the DELETE, TRUNCATE, and PERSIST settings, respectively, of the [journal_mode pragma].

A valid rollback journal begins with a header in the following format:

Rollback Journal Header Format
OffsetSizeDescription
0 8 Header string: 0xd9, 0xd5, 0x05, 0xf9, 0x20, 0xa1, 0x63, 0xd7
8 4 The "Page Count" - The number of pages in the next segment of the journal, or -1 to mean all content to the end of the file
12 4 A random nonce for the checksum
16 4 Initial size of the database in pages
20 4 Size of a disk sector assumed by the process that wrote this journal.
24 4 Size of pages in this journal.

A rollback journal header is padded with zeros out to the size of a single sector (as defined by the sector size integer at offset 20). The header is in a sector by itself so that if a power loss occurs while writing the sector, information that follows the header will be (hopefully) undamaged.

After the header and zero padding are zero or more page records. Each page record stores a copy of the content of a page from the database file before it was changed. The same page may not appear more than once within a single rollback journal. To rollback an incomplete transaction, a process has merely to read the rollback journal from beginning to end and write pages found in the journal back into the database file at the appropriate location.

Let the database page size (the value of the integer at offset 24 in the journal header) be N. Then the format of a page record is as follows:

Rollback Journal Page Record Format
OffsetSizeDescription
0 4 The page number in the database file
4 N Original content of the page prior to the start of the transaction
N+4 4 Checksum

The checksum is an unsigned 32-bit integer computed as follows:

  1. Initialize the checksum to the checksum nonce value found in the journal header at offset 12.
  2. Initialize index X to be N-200 (where N is the size of a database page in bytes.
  3. Interpret the four bytes at offset X into the page as a 4-byte big-endian unsigned integer. Add the value of that integer to the checksum.
  4. Subtrace 200 from X.
  5. If X is greater than or equal to zero, go back to step 3.

The checksum value is used to guard against incomplete writes of a journal page record following a power failure. A different random nonce is used each time a trasaction is started in order to minimize the risk that unwritten sectors might by chance contain data from the same page that was a part of prior journals. By changing the nonce for each transaction, stale data on disk will still generate an incorrect checksum and be detected with high probability. The checksum only uses a sparce sample of 32-bit words from the data record for performance reasons - design studies during the planning phases of SQLite 3.0.0 showed a significant performance hit in checksumming the entire page.

Let the page count value at offset 8 in the journal header by M. If M is greater than zero then after M page records the journal file may be zero padded out to the next multiple of the sector size and another journal header may be inserted. All journal headers within the same journal must contain the same database page size and sector size.

If M is -1 in the initial journal header, then the number of page records that follow is computed by figure out how many page records will fit in the available space of the remainder of the journal file.