Documentation Source Text

      which these fields and data structures are created, initialized and
      updated.  
-->

  [h2 "Glossary"]
    <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

      <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>

      <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.

    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 "Database File Format Details" database_file_format]

  <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

    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 "File Format Overview" "fileformat_overview"]
    <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 

      <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.

            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

      <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=H30030>
          [fileformat_import_requirement H30030]

      <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=H30040>
          [fileformat_import_requirement H30040]
      <p class=req id=H30050>
          [fileformat_import_requirement H30050]

        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]
        [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);

      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=H30500>
          [fileformat_import_requirement H30500]
    <p class=req id=H30510>
          [fileformat_import_requirement H30510]

    [h3 "Variable Length Integer Format" "varint_format"]
      <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]
        [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]
	[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=H30520>

      <p class=req id=H30750>
          [fileformat_import_requirement H30750]

      <p class=req id=H30760>
          [fileformat_import_requirement H30760]

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


        [h4 "Index B-Tree Content"]
          <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 

        <p class=req id=H30800>
          [fileformat_import_requirement H30800]
 
      [h4 "Record Sort Order" "index_btree_compare_func"]
        <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

          <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.

      <p class=req id=H30900>
          [fileformat_import_requirement H30900]

      <p class=req id=H30910>
          [fileformat_import_requirement H30910]

      <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=H30920>
          [fileformat_import_requirement H30920]

      <p class=req id=H30930>
          [fileformat_import_requirement H30930]

        by child page C(n+1) have values larger than that of I(n), for values
        of n in the same range.

        [Figure tabletree.gif figure_tabletree "Table B-Tree Tree Structure"]

      <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=H31020>
          [fileformat_import_requirement H31020]

      <p class=req id=H31030>
          [fileformat_import_requirement H31030]

      <p class=req id=H31040>
          [fileformat_import_requirement H31040]

      <p class=req id=H31050>
          [fileformat_import_requirement H31050]

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

      [h4 "Table B-Tree Content" table_btree_content]
        <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=H31060>
          [fileformat_import_requirement H31060]

        <p class=req id=H31070>
          [fileformat_import_requirement H31070]

    <p class=req id=H31400>
          [fileformat_import_requirement H31400]
    <p class=req id=H31410>
          [fileformat_import_requirement H31410]
    <p class=req id=H31420>
          [fileformat_import_requirement H31420]



















[h1 "Journal File Format" journal_file_format]



    <p>
      This section describes the format used by an SQLite <i>journal file</i>.

    <p>
      A journal file consists of one or more <i>journal headers</i>, zero
      or more <i>journal records</i> and optionally a <i>master journal
      pointer</i>. Each journal file always begins with a
      <i>journal header</i>, followed by zero or more <i>journal records</i>.
      Following this may be a second <i>journal header</i> followed by a
      second set of zero or more <i>journal records</i> and so on. There
      is no limit to the number of <i>journal headers</i> a journal file
      may contain. Following the <i>journal headers</i> and their accompanying
      sets of <i>journal records</i> may be the optional <i>master journal
      pointer</i>. Or, the file may simply end following the final <i>journal
      record</i>.

    [h2 "Journal Header Format" journal_header_format]

    <p>
      A <i>journal header</i> is <i>sector-size</i> bytes in size, where <i>
      sector-size</i> is the value returned by the xSectorSize method of
      the file handle opened on the database file. Only the first 28 bytes
      of the <i>journal header</i> are used, the remainder may contain garbage
      data. The first 28 bytes of each <i>journal header</i> consists of an 

    <p>
      All <i>journal headers</i> are positioned in the file so that they 
      start at a <i>sector size</i> aligned offset. To achieve this, unused
      space may be left between the start of the second and subsequent
      <i>journal headers</i> and the end of the <i>journal records</i>
      associated with the previous header.

  [h2 "Journal Record Format" journal_record_format]

    <p>
      Each <i>journal record</i> contains the original data for a database page
      modified by the <i>write transaction</i>. If a rollback is required, then
      this data may be used to restore the contents of the database page to the
      state it was in before the <i>write transaction</i> was started.

    <p>
      The set of <i>journal records</i> that follow a <i>journal header</i>
      in a <i>journal file</i> are packed tightly together. There are no
      alignment requirements for <i>journal records</i> as there are for
      <i>journal headers</i>.

  [h2 "Master Journal Pointer"]

    <p>
      To support <i>atomic</i> transactions that modify more than one 
      database file, SQLite sometimes includes a <i>master journal pointer</i>
      record in a <i>journal file</i>. A <i>master journal pointer</i>
      contains the name of a <i>master journal-file</i> along with a 
      check-sum and some well-known values that allow the 

               Finally, the <b>journal magic</b> field always contains a
               well-known 8-byte string value; the same value stored in the
               first 8 bytes of a <i>journal header</i>. The well-known
               sequence of bytes is:
                 <pre>0xd9 0xd5 0x05 0xf9 0x20 0xa1 0x63 0xd7</pre>
    </table>

<!--
\[h1 "Database File Structure Traversal" "database_file_traversal"]

  \[h2 "B-Tree Cursors"]

  <h2>B-Tree Access Strategies</h2>
    <h3>Full Linear Scan</h3>
    <h3>Seek to Value</h3>
    <h3>Range Scan</h3>

  <h2>Retrieving Record Values</h2>

\[h1 "Database File Manipulation" "database_file_manipulation"]

  <h2>Creating a Database</h2>

  <h2>Table B-Trees</h2>
    <h3>Creating a new Table B-Tree</h3>
    <h3>Deleting a Table B-Tree</h3>
    <h3>Adding an Entry to a Table B-Tree</h3>
    <h3>Removing an Entry from a Table B-Tree</h3>

  <h2>Index B-Trees</h2>
    <h3>Creating a new Index B-Tree</h3>
    <h3>Deleting a Index B-Tree</h3>
    <h3>Adding an Entry to a Index B-Tree</h3>
    <h3>Removing an Entry from a Index B-Tree</h3>

  <h2>Overflow Chains</h2>

  <h2>Allocating/Deallocating Pages</h2>
    <h2>Allocating a Page</h2>
    <h2>Deallocating a Page</h2>

  <h2>Auto-Vacuum Commit Operations</h2>

  <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 <i>insert-link</i>. 
    The requirements in <i>insert-link</i> 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 "Database Creation/Initialization" database_initialization]
    <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 "Setting Database Parameters" database_parameters]
    <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 "Table/Index Creation" btree_creation]
      <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"]

      <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"]
        <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 "Page Allocation" page_allocation]
     <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 "Page Deallocation" page_deallocation]
      <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 "Removing a Page From The Free List" page_removal]
      <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 "Database Reorganization (auto-vacuum)" incremental_vacuum]
      <p class=todo>
        Requirements describing incremental vacuum steps. And on-commit
        handling in auto-vacuum databases.
-->

[h1 References]

  <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.