<title>Pragma statements supported by SQLite</title>

<tcl>
proc Section {name {label {}} {keywords {}}} {
  hd_puts "\n<hr />"
  if {$label!=""} {
    hd_fragment $label
    if {$keywords!=""} {
      eval hd_keywords $keywords
    }
  }
  hd_puts "<h1>$name</h1>\n"
}
proc Subsection {args} {
  set f [lindex $args 0]
  hd_fragment pragma_$f
  foreach x $args {
    hd_keywords $x "PRAGMA $x" "$x pragma"
  }
}

</tcl>

<p>The PRAGMA statement is a SQL extension specific to SQLite and used to 
modify the operation of the SQLite library or to query the SQLite library for 
internal (non-table) data. The PRAGMA statement is issued using the same
interface as other SQLite commands (e.g. [SELECT], [INSERT]) but is
different in the following important respects:
</p>
<ul>
<li>Specific pragma statements may be removed and others added in future
    releases of SQLite. There is no guarantee of backwards compatiblity.
<li>No error messages are generated if an unknown pragma is issued.
    Unknown pragmas are simply ignored. This means if there is a typo in 
    a pragma statement the library does not inform the user of the fact.
<li>Some pragmas take effect during the SQL compilation stage, not the
    execution stage. This means if using the C-language [sqlite3_prepare()], 
    [sqlite3_step()], [sqlite3_finalize()] API (or similar in a wrapper 
    interface), the pragma may run during the [sqlite3_prepare()] call,
    not during the [sqlite3_step()] call as normal SQL statements do.
    Or the pragma might run during sqlite3_step() just like normal
    SQL statements.  Whether or not the pragma runs during sqlite3_prepare()
    or sqlite3_step() depends on the pragma and on the specific release
    of SQLite.
<li>The pragma command is specific to SQLite and is very unlikely 
    to be compatible with any other SQL database engine.
</ul>

<p>The available pragmas fall into four basic categories:</p>
<ul>
<li>Pragmas used to <a href="#modify">modify the operation</a> of the 
    SQLite library in some manner, or to query for the current mode of 
    operation.
<li>Pragmas used to <a href="#schema">query the schema</a> of the current 
    database.
<li>Pragmas used to <a href="#version">query or modify the two 
    version counters stored in the database:</a>
    the schema-version and the user-version.
<li>Pragmas used to <a href="#debug">debug the library</a> and verify that
    database files are not corrupted.
</ul>

<tcl>
Section {PRAGMA command syntax} syntax {PRAGMA}

BubbleDiagram pragma-stmt
BubbleDiagram pragma-value
</tcl>

<p>
A pragma can take either zero or one argument.  The argument is may be either
in parentheses or it may be separated from the pragma name by an equal sign.
The two syntaxes yield identical results.
In many pragmas, the argument is a boolean.  The boolean can be one of:
</p>

<center>
<b>1 yes true on<br>0 no false off</b>
</center>

<p>Keyword arguments can optionally appear in quotes.  
(Example:  <tt>'yes' &91;FALSE&93;</tt>.) Some pragmas
takes a string literal as their argument.  When pragma takes a keyword
argument, it will usually also take a numeric equivalent as well.
For example, "0" and "no" mean the same thing, as does "1" and "yes".
When querying the value of a setting, many pragmas return the number
rather than the keyword.</p>

<p>A pragma may have an optional database name before the pragma name.
The database name is the name of an [ATTACH]-ed database.  Or it can be
"main" or "temp" for the main and the TEMP databases.  If the optional
database name is omitted, "main" is assumed.  In some pragmas, the database
name is meaningless and is simply ignored.</p>

<tcl>Section {Pragmas to modify library operation} modify</tcl>
</tcl>

<ul>
<tcl>Subsection auto_vacuum</tcl>
<li><p><b>PRAGMA auto_vacuum;<br>
          PRAGMA auto_vacuum = </b>
           <i>0 | NONE | 1 | FULL | 2 | INCREMENTAL</i><b>;</b></p>

    <p>Query or set the auto-vacuum status in the database.</p>

    <p>The default setting for auto-vacuum is 0 or "none",
    unless the [SQLITE_DEFAULT_AUTOVACUUM] compile-time option is used.
    The "none" setting means that auto-vacuum is disabled.
    When auto-vacuum is disabled and data is deleted data from a database,
    the database file remains the same size.  Unused database file 
    pages are added to a "freelist" and reused for subsequent inserts.  So
    no database file space is lost.  However, the database file does not
    shrink.  In this mode the [VACUUM]
    command can be used to rebuild the entire database file and
    thus reclaim unused disk space.</p>

    <p>When the auto-vacuum mode is 1  or "full", the freelist pages are
    moved to the end of the database file and the database file is truncated
    to remove the freelist pages at every transaction commit.
    Note, however, that auto-vacuum only truncates the freelist pages
    from the file.  Auto-vacuum does not defragment the database nor
    repack individual database pages the way that the
    [VACUUM] command does.  In fact, because
    it moves pages around within the file, auto-vacuum can actually
    make fragmentation worse.</p>

    <p>Auto-vacuuming is only possible if the database stores some
    additional information that allows each database page to be
    traced backwards to its referer.  Therefore, auto-vacuuming must
    be turned on before any tables are created.  It is not possible
    to enable or disable auto-vacuum after a table has been created.</p>

    <p>When the value of auto-vacuum is 2 or "incremental" then the additional
    information needed to do auto-vacuuming is stored in the database file
    but auto-vacuuming does not occur automatically at each commit as it
    does with auto_vacuum=full.  In incremental mode, the separate
    [incremental_vacuum] pragma must
    be invoked to cause the auto-vacuum to occur.</p>

    <p>The database connection can be changed between full and incremental
    autovacuum mode at any time.  However, the connection can only be changed
    "none" to "full" or "incremental" when the database is empty (no tables
    have yet been created) or by running the [VACUUM] command.  To
    change auto-vacuum modes, first use the auto_vacuum pragma to set
    the new desired mode, then invoke the [VACUUM] command to 
    reorganize the entire database file.  To change from "full" or
    "incremental" back to "none" always requires running [VACUUM] even
    on an empty database.
    </p>

    <p>When the auto_vacuum pragma is invoked with no arguments, it
    returns the current auto_vacuum mode.</p>
    </li>

<tcl>Subsection cache_size</tcl>
<li><p><b>PRAGMA cache_size;
       <br>PRAGMA cache_size = </b><i>Number-of-pages</i><b>;</b></p>
    <p>Query or change the suggested maximum number of database disk pages
    that SQLite will hold in memory at once per open database file.  Whether
    or not this suggestion is honored is at the discretion of the
    [sqlite3_pcache_methods | Application Defined Page Cache]. The default
    page cache implemention that is built into SQLite does honor the 
    suggestion if it can, but alternative page caches implementations
    set by the application at run-time may choose to ignore this suggestion.
    The default suggested cache size is 2000.</p>

    <p>When you change the cache size using the cache_size pragma, the
    change only endures for the current session.  The cache size reverts
    to the default value when the database is closed and reopened.  Use
    the [default_cache_size]
    pragma to check the cache size permanently.</p></li>

<tcl>Subsection case_sensitive_like</tcl>
<li><p><b>PRAGMA case_sensitive_like = </b><i>boolean</i><b>;</b></p>
    <p>The default behavior of the [LIKE] operator is to ignore case
    for ASCII characters. Hence, by default <b>'a' LIKE 'A'</b> is
    true.  The case_sensitive_like pragma installs a new application-defined
    LIKE function that can change
    this behavior.  When case_sensitive_like is enabled,
    <b>'a' LIKE 'A'</b> is false but <b>'a' LIKE 'a'</b> is still true.</p>
    </li>

<tcl>Subsection count_changes</tcl>
<li><p><b>PRAGMA count_changes;
       <br>PRAGMA count_changes = </b>boolean</i><b>;</b></p>
    <p>Query or change the count-changes flag. Normally, when the
    count-changes flag is not set, [INSERT], [UPDATE] and [DELETE] statements
    return no data. When count-changes is set, each of these commands 
    returns a single row of data consisting of one integer value - the
    number of rows inserted, modified or deleted by the command. The 
    returned change count does not include any insertions, modifications
    or deletions performed by triggers.</p>

    <p>The use of this pragma is discouraged.  A better way to get the
    row change count is to use the [sqlite3_changes()] or
    [sqlite3_total_changes()] interfaces.</p>

<tcl>Subsection default_cache_size</tcl>
<li><p><b>PRAGMA default_cache_size;
       <br>PRAGMA default_cache_size = </b><i>Number-of-pages</i><b>;</b></p>
    <p>This pragma queries or sets the suggested maximum number of pages
    of disk cache that will be allocated per open database file.
    The difference between this pragma and [cache_size] is that the
    value set here persists across database connections.
    </p></li>


<tcl>Subsection empty_result_callbacks</tcl>
<li><p><b>PRAGMA empty_result_callbacks;
       <br>PRAGMA empty_result_callbacks = </b><i>boolean</i><b>;</b></p>

    <p>Query or change the empty-result-callbacks flag.</p> 

    <p>The empty-result-callbacks flag affects the [sqlite3_exec()] API only.
    Normally, when the empty-result-callbacks flag is cleared, the
    callback function supplied to the [sqlite3_exec()] call is not invoked
    for commands that return zero rows of data. When empty-result-callbacks
    is set in this situation, the callback function is invoked exactly once,
    with the third parameter set to 0 (NULL). This is to enable programs  
    that use the [sqlite3_exec()] API to retrieve column-names even when
    a query returns no data.</p>

    <p>This pragma is legacy.  It was created long ago in the early
    days of SQLite before the prepared statement interface was available.
    Do not use this pragma.  It is likely to go away in a future release</p>
   
    

<tcl>Subsection encoding</tcl>
<li><p><b>PRAGMA encoding;
       <br>PRAGMA encoding = "UTF-8";
       <br>PRAGMA encoding = "UTF-16";
       <br>PRAGMA encoding = "UTF-16le";
       <br>PRAGMA encoding = "UTF-16be";</b></p>
    <p>In first form, if the main database has already been
    created, then this pragma returns the text encoding used by the
    main database, one of "UTF-8", "UTF-16le" (little-endian UTF-16
    encoding) or "UTF-16be" (big-endian UTF-16 encoding).  If the main
    database has not already been created, then the value returned is the
    text encoding that will be used to create the main database, if 
    it is created by this session.</p>
    <p>The second and subsequent forms of this pragma are only useful if
    the main database has not already been created. In this case the 
    pragma sets the encoding that the main database will be created with if
    it is created by this session. The string "UTF-16" is interpreted
    as "UTF-16 encoding using native machine byte-ordering".  If the second
    and subsequent forms are used after the database file has already
    been created, they have no effect and are silently ignored.</p>

    <p>Once an encoding has been set for a database, it cannot be changed.</p>

    <p>Databases created by the [ATTACH] command always use the same encoding
    as the main database.</p>
</li>

<tcl>Subsection full_column_names</tcl>
<li><p><b>PRAGMA full_column_names;
       <br>PRAGMA full_column_names = </b><i>boolean</i><b>;</b></p>
    <p>Query or change the full_column_names flag. This flag together 
    with the [short_column_names] flag determine
    the way SQLite assigns names to results returned by [SELECT] statements.
    Result columns are named by applying the following rules in order:
    <ol>
    <li><p>If there is an AS clause on the result, then the name of
        the column is the right-hand side of the AS clause.</p></li>
    <li><p>If the result is a general expression, not a just the name of
        a source table column,
        then the name of the result is a copy of the expression text.</p></li>
    <li><p>If the [short_column_names] pragma is ON, then the name of the
        result is the name of the source table column without the 
        source table name prefix:  COLUMN.</p></li>
    <li><p>If both pragmas [short_column_names] and [full_column_names]
        are OFF then
        case (2) applies for simple queries and case (5) applies for joins.
        </p></li>
    <li><p>The name of the result column is a combination of the source table
        and source column name:  TABLE.COLUMN</p></li>
    </ol>
</li>

<tcl>Subsection fullfsync</tcl>
<li><p><b>PRAGMA fullfsync
       <br>PRAGMA fullfsync = </b><i>boolean</i><b>;</b></p>
    <p>Query or change the fullfsync flag. This flag affects
    determines whether or not the F_FULLFSYNC syncing method is used
    on systems that support it.  The default value is off.  As of this
    writing (2006-02-10) only Mac OS X supports F_FULLFSYNC.
    </p>
</li>

<tcl>Subsection incremental_vacuum</tcl>
<li><p><b>PRAGMA incremental_vacuum</b><i>(N)</i><b>;</b></p>
    <p>The incremental_vacuum pragma causes up to <i>N</i> pages to
    be removed from the freelist.  The database file is truncated by
    the same amount.  The incremental_vacuum pragma has no effect if
    the database is not in
    <a href="#pragma_auto_vacuum">auto_vacuum==incremental</a> mode
    or if there are no pages on the freelist.  If there are fewer than
    <i>N</i> pages on the freelist, or if <i>N</i> is less than 1, or
    if <i>N</i> is omitted entirely, then the entire freelist is cleared.</p>

    <p>As of [version 3.4.0] (the first version that supports
    incremental_vacuum) this feature is still experimental.  Possible
    future changes include enhancing incremental vacuum to do
    defragmentation and node repacking just as the full-blown
    [VACUUM] command does.  And
    incremental vacuum may be promoted from a pragma to a separate
    SQL command, or perhaps some variation on the [VACUUM] command.
    Programmers are cautioned to not become enamored with the
    current syntax or functionality as it is likely to change.</p>
</li>

<tcl>Subsection journal_mode</tcl>
<li><p><b>PRAGMA journal_mode;
       <br>PRAGMA </b><i>database</i><b>.journal_mode;
       <br>PRAGMA journal_mode
              = <i>DELETE | TRUNCATE | PERSIST | MEMORY | OFF</i>
       <br>PRAGMA </b><i>database</i><b>.journal_mode
              = <i>DELETE | TRUNCATE | PERSIST | MEMORY | OFF</i></b></p>

    <p>This pragma queries or sets the journal mode for databases
    associated with the current [database connection].</p>

    <p>The first two forms of this pragma query the current journaling
    mode.  In the first form, the default journal_mode is returned.
    The default journaling mode is the mode used by databases added
    to the connection by subsequent [ATTACH] statements.  The second
    form returns the current journaling mode for a specific database.</p>

    <p>The last two forms change the journaling mode.  The 4th form
    changes the journaling mode for a specific database connection.
    Use "main" for the main database (the database that was opened by
    the original [sqlite3_open()], [sqlite3_open16()], or
    [sqlite3_open_v2()] interface call) and use "temp" for database
    that holds TEMP tables.  The 3rd form changes the journaling mode
    on all databases and it changes the default journaling mode that
    will be used for new databases added by subsequent [ATTACH]
    commands.  The new journal mode is returned.  If the journal mode
    could not be changed, the original journal mode is returned.</p>

    <p>The DELETE journaling mode is the normal behavior.  In the DELETE
    mode, the rollback journal is deleted at the conclusion of each
    transaction.  Indeed, the delete operation is the action that causes
    the transaction to commit.
    (See the documented titled <a href="atomiccommit.html">
    Atomic Commit In SQLite</a> for additional detail.)</p>

    <p>The TRUNCATE journaling mode commits transactions by truncating
    the rollback journal to zero-length instead of deleting it.  On many
    systems, truncating a file is much faster than deleting the file since
    the containing directory does not need to be changed.</p>

    <p>The PERSIST journaling mode prevents the rollback journal from
    being deleted at the end of each transaction.  Instead, the header
    of the journal is overwritten with zeros.  This will prevent other
    database connections from rolling the journal back.  The PERSIST
    journaling mode is useful as an optimization on platforms where
    deleting or truncating a file is much more expensive than overwriting
    the first block of a file with zeros.</p>

    <p>The MEMORY journaling mode stores the rollback journal in 
    volatile RAM.  This saves disk I/O but that the expense of database
    safety and integrity.  If the application using SQLite crashes in
    the middle of a transaction when the MEMORY journaling mode is set,
    then the database file will very likely go corrupt.</p>

    <p>The OFF journaling mode disables the rollback journal completely.
    No rollback journal is ever created and hence there is never a rollback
    journal to delete.  The OFF journaling mode disables the atomic
    commit and rollback capabilities of SQLite. The [ROLLBACK] command
    no longer works; it behaves in an undefined way.  Applications must
    avoid using the [ROLLBACK] command when the journal mode is OFF.
    If the application crashes
    in the middle of a transaction when the OFF journaling mode is
    set, then the database file will very likely go corrupt.</p>

    <p>Note that the journal_mode for an [in-memory database]
    is either MEMORY or OFF and can not be changed to a different value.
    An attempt to change the journal_mode of an [in-memory database] to
    any setting other than MEMORY or OFF is ignored.  Note also that
    the journal_mode cannot be changed while a transaction is active.</p>
</li>

<tcl>Subsection journal_size_limit</tcl>
<li><p><b>
    PRAGMA journal_size_limit<br>
    PRAGMA journal_size_limit = </b><i>N</i> <b>;</b>

  <p>If a database connection is operating in either "exclusive mode"
  (PRAGMA locking_mode=exclusive) or "persistent journal mode"
  (PRAGMA journal_mode=persist) then under certain circumstances
  after committing a transaction the journal file may remain in
  the file-system. This increases efficiency but also consumes
  space in the file-system. After a large transaction (e.g. a VACUUM),
  it may consume a very large amount of space.

  <p>This pragma may be used to limit the size of journal files left
  in the file-system after transactions are committed on a per database
  basis. Each time a transaction is committed, SQLite compares the
  size of the journal file left in the file-system to the size limit
  configured using this pragma. If the journal file is larger than the
  limit allows for, it is truncated to the limit.

  <p>The second form of the pragma listed above is used to set a new limit
  in bytes for the specified database. A negative number implies no limit.
  Both the first and second forms of the pragma listed above return a single
  result row containing a single integer column - the value of the journal
  size limit in bytes. The default limit value is -1 (no limit), which
  may be overridden by defining the preprocessor macro
  [SQLITE_DEFAULT_JOURNAL_SIZE_LIMIT] at compile time.</p>

  <p>This pragma only operates on the single database specified prior
  to the pragma name (or on the "main" database if no database is specified.)
  There is no way to operate on all attached databases using a single
  PRAGMA statement, nor is there a way to set the limit to use for databases
  that will be attached in the future.
</li>


<tcl>Subsection legacy_file_format</tcl>
<li><p><b>PRAGMA legacy_file_format;
       <br>PRAGMA legacy_file_format = <i>boolean</i></b></p>
    <p>This pragma sets or queries the value of the legacy_file_format
    flag.  When this flag is on, new SQLite databases are created in
    a file format that is readable and writable by all versions of
    SQLite going back to 3.0.0.  When the flag is off, new databases
    are created using the latest file format which might not be
    readable or writable by versions of SQLite prior to 3.3.0.</p>

    <p>When the pragma is issued with no argument, it returns the
    setting of the flag.  This pragma does <u>not</u> tell which
    file format the current database is using.  It tells what format
    will be used by any newly created databases.</p>

    <p>This flag only affects newly created databases.  It has no
    effect on databases that already exist.</p>

    <p>The default file format is set by the
    [SQLITE_DEFAULT_FILE_FORMAT] compile-time option.</p>
</li>

<tcl>Subsection locking_mode</tcl>
<li><p><b>PRAGMA locking_mode;
       <br>PRAGMA locking_mode = <i>NORMAL | EXCLUSIVE</i></b></p>
    <p>This pragma sets or queries the database connection locking-mode. 
    The locking-mode is either NORMAL or EXCLUSIVE.

    <p>In NORMAL locking-mode (the default), a database connection
    unlocks the database file at the conclusion of each read or
    write transaction. When the locking-mode is set to EXCLUSIVE, the
    database connection never releases file-locks. The first time the
    database is read in EXCLUSIVE mode, a shared lock is obtained and 
    held. The first time the database is written, an exclusive lock is
    obtained and held.</p>

    <p>Database locks obtained by a connection in EXCLUSIVE mode may be
    released either by closing the database connection, or by setting the
    locking-mode back to NORMAL using this pragma and then accessing the
    database file (for read or write). Simply setting the locking-mode to
    NORMAL is not enough - locks are not be released until the next time
    the database file is accessed.</p>

    <p>There are two reasons to set the locking-mode to EXCLUSIVE. One
    is if the application actually wants to prevent other processes from
    accessing the database file. The other is that a small number of
    filesystem operations are saved by optimizations enabled in this
    mode. This may be significant in embedded environments.</p>

    <p>When the locking_mode pragma specifies a particular database,
    for example:</p>

    <blockquote>
PRAGMA <b>main.</b>locking_mode=EXCLUSIVE;
    </blockquote>

    <p>Then the locking mode applies only to the named database.  If no
    database name qualifier precedes the "locking_mode" keyword then
    the locking mode is applied to all databases, including any new
    databases added by subsequent [ATTACH] commands.</p>

   <p>The "temp" database (in which TEMP tables and indices are stored)
   and [in-memory databases]
   always uses exclusive locking mode.  The locking mode of temp and
   [in-memory databases] cannot
   be changed.  All other databases use the normal locking mode by default
   and are affected by this pragma.</p>
</li>

<tcl>Subsection page_size</tcl>
<li><p><b>PRAGMA page_size;
       <br>PRAGMA page_size = </b><i>bytes</i><b>;</b></p>
    <p>Query or set the page size of the database. The page size
    may only be set if the database has not yet been created. The page
    size must be a power of two greater than or equal to 512 and less
    than or equal to [SQLITE_MAX_PAGE_SIZE].
    The maximum value for [SQLITE_MAX_PAGE_SIZE] is 32768.
    </p>

    <p>When a new database is created, SQLite assigned a default page size
    based on information received from the xSectorSize and 
    xDeviceCharacteristics methods of the [sqlite3_io_methods] object
    of the newly created database file.  The page_size pragma will only
    cause an immediate change in the
    page size if it is issued while the database is still empty, prior 
    to the first CREATE TABLE statement.  As of [version 3.5.8], if
    the page_size pragma is used to specify a new page size just prior to
    running the [VACUUM] command then [VACUUM] will change the page
    size to the new value.</p>

    <p>If SQLite is compiled with the SQLITE_ENABLE_ATOMIC_WRITE option,
    then the default page size is chosen to be the largest page size
    less than or equal to SQLITE_MAX_DEFAULT_PAGE_SIZE for which atomic
    write is enabled according to the
    xDeviceCharacteristics method of the [sqlite3_io_methods] object for
    the database file.  If the SQLITE_ENABLE_ATOMIC_WRITE option is
    disabled or if xDeviceCharacteristics reports no suitable atomic
    write page sizes, then the default page size is the larger of 
    SQLITE_DEFALT_PAGE_SIZE
    and the sector size as reported by the xSectorSize method of the
    [sqlite3_io_methods] object, but not more than 
    SQLITE_MAX_DEFAULT_PAGE_SIZE.  The normal configuration for SQLite
    running on workstations is for atomic write to be
    disabled, for the maximum page size to be set to 32768, for
    SQLITE_DEFAULT_PAGE_SIZE to be 1024, and for the
    maximum default page size to be set to 8192.  The default xSectorSize
    method on workstation implementations always reports a sector size
    of 512 bytes.  Hence, 
    the default page size chosen by SQLite is usually 1024 bytes.</p>
</li>

<tcl>Subsection max_page_count</tcl>
<li><p><b>PRAGMA max_page_count;
       <br>PRAGMA max_page_count = </b><i>N</i><b>;</b></p>
    <p>Query or set the maximum number of pages in the database file.
    Both forms of the pragma return the maximum page count.  The second
    form attempts to modify the maximum page count.  The maximum page
    count cannot be reduced below the current database size.
    </p>
</li>

<tcl>Subsection read_uncommitted</tcl>
<li><p><b>PRAGMA read_uncommitted;
       <br>PRAGMA read_uncommitted = </b><i>boolean</i><b>;</b></p>
    <p>Query, set, or clear READ UNCOMMITTED isolation.  The default isolation
    level for SQLite is SERIALIZABLE.  Any process or thread can select
    READ UNCOMMITTED isolation, but SERIALIZABLE will still be used except
    between connections that share a common page and schema cache.
    Cache sharing is enabled using the [sqlite3_enable_shared_cache()] API.
    Cache sharing is disabled by default.
    </p>

    <p>See [SQLite Shared-Cache Mode] for additional information.</p>
</li>

<tcl>Subsection reverse_unordered_selects</tcl>
<li><p><b>PRAGMA reverse_unordered_selects;
       <br>PRAGMA reverse_unordered_selects = </b><i>boolean</i><b>;</b></p>
    <p>When enabled, this PRAGMA causes [SELECT] statements without a
    an ORDER BY clause to emit their results in the reverse order of what
    they normally would.  This can help debug applications that are
    making invalid assumptions about the result order.<p>SQLite makes no
    guarantees about the order of results if a SELECT omits the ORDER BY
    clause.  Even so, the order of results does not change from one
    run to the next, and so many applications mistakenly come to depend
    on the arbitrary output order whatever that order happens to be.  However, 
    sometimes new versions of SQLite will contain optimizer enhancements
    that will cause the output order of queries without ORDER BY clauses
    to shift.  When that happens, applications that depend on a certain
    output order might malfunction.  By running the application multiple
    times with this pragma both disabled and enabled, cases where the
    application makes faulty assumptions about output order can be
    identified and fixed early, reducing problems
    that might be caused by linking against a different version of SQLite.
    </p>
</li>

<tcl>Subsection short_column_names</tcl>
<li><p><b>PRAGMA short_column_names;
       <br>PRAGMA short_column_names = </b><i>boolean</i><b>;</b></p>
    <p>Query or change the short-column-names flag. This flag affects
    the way SQLite names columns of data returned by [SELECT] statements.
    See the [full_column_names] pragma for full details.
    </p>
</li>

<tcl>Subsection synchronous</tcl>
<li><p><b>PRAGMA synchronous;
       <br>PRAGMA synchronous = </b>
          <i>0 | OFF | 1 | NORMAL | 2 | FULL</i><b>;</b></p>

    <p>Query or change the setting of the "synchronous" flag.  
    The first (query) form will return the setting as an 
    integer.  When synchronous is FULL (2), the SQLite database engine will
    pause at critical moments to make sure that data has actually been 
    written to the disk surface before continuing.  This ensures that if
    the operating system crashes or if there is a power failure, the database
    will be uncorrupted after rebooting.  FULL synchronous is very 
    safe, but it is also slower. 
    When synchronous is NORMAL, the SQLite database
    engine will still pause at the most critical moments, but less often
    than in FULL mode.  There is a very small (though non-zero) chance that
    a power failure at just the wrong time could corrupt the database in
    NORMAL mode.  But in practice, you are more likely to suffer
    a catastrophic disk failure or some other unrecoverable hardware
    fault.
    With synchronous OFF (0), SQLite continues without pausing
    as soon as it has handed data off to the operating system.
    If the application running SQLite crashes, the data will be safe, but
    the database might become corrupted if the operating system
    crashes or the computer loses power before that data has been written
    to the disk surface.  On the other hand, some
    operations are as much as 50 or more times faster with synchronous OFF.
    </p>
    <p>The default setting is synchronous=FULL.
    </p>
</li>


<tcl>Subsection temp_store</tcl>
<li><p><b>PRAGMA temp_store;
       <br>PRAGMA temp_store = </b>
            <i>0 | DEFAULT | 1 | FILE | 2 | MEMORY</i><b>;</b></p>

    <p>Query or change the setting of the "<b>temp_store</b>" parameter.
    When temp_store is DEFAULT (0), the compile-time C preprocessor macro
    [SQLITE_TEMP_STORE] is used to determine where temporary tables and indices
    are stored.  When
    temp_store is MEMORY (2) [temporary tables] and indices are kept in
    as if they were pure [in-memory databases] memory.
    When temp_store is FILE (1) [temporary tables] and indices are stored
    in a file.  The [temp_store_directory] pragma can be used to specify
    the directory containing temporary files when
    <b>FILE</b> is specified. When the temp_store setting is changed,
    all existing temporary tables, indices, triggers, and views are
    immediately deleted.</p>

    <p>It is possible for the library compile-time C preprocessor symbol
    [SQLITE_TEMP_STORE] to override this pragma setting.
    The following table summarizes
    the interaction of the [SQLITE_TEMP_STORE] preprocessor macro and the
    temp_store pragma:</p>

    <blockquote>
    <table cellpadding="2" border="1">
    <tr><th valign="bottom">[SQLITE_TEMP_STORE]</th>
        <th valign="bottom">PRAGMA<br>temp_store</th>
        <th>Storage used for<br>TEMP tables and indices</th></tr>
    <tr><td align="center">0</td>
        <td align="center"><em>any</em></td>
        <td align="center">file</td></tr>
    <tr><td align="center">1</td>
        <td align="center">0</td>
        <td align="center">file</td></tr>
    <tr><td align="center">1</td>
        <td align="center">1</td>
        <td align="center">file</td></tr>
    <tr><td align="center">1</td>
        <td align="center">2</td>
        <td align="center">memory</td></tr>
    <tr><td align="center">2</td>
        <td align="center">0</td>
        <td align="center">memory</td></tr>
    <tr><td align="center">2</td>
        <td align="center">1</td>
        <td align="center">file</td></tr>
    <tr><td align="center">2</td>
        <td align="center">2</td>
        <td align="center">memory</td></tr>
    <tr><td align="center">3</td>
        <td align="center"><em>any</em></td>
        <td align="center">memory</td></tr>
    </table>
    </blockquote>
    </li>
    <br>

<tcl>Subsection temp_store_directory</tcl>
<li><p><b>PRAGMA temp_store_directory;
       <br>PRAGMA temp_store_directory = '</b><i>directory-name</i><b>';</b></p>
    <p>Query or change the setting of the "temp_store_directory" - the
    directory where files used for storing [temporary tables] and indices
    are kept.</p>

    <p>When the temp_store_directory setting is changed, all existing temporary
    tables, indices, triggers, and viewers are immediately deleted.  In
    practice, temp_store_directory should be set immediately after the 
    database is opened.</p>

    <p>Changing the temp_store_directory setting is <u>not</u> threadsafe.
    Never change the temp_store_directory setting if another thread
    within the application is running any SQLite interface at the same time.
    Doing so results in undefined behavior.  Changing the temp_store_directory
    setting writes to the [sqlite3_temp_directory] global
    variable and that global variable is not protected by a mutex.</p>

    <p>The value <i>directory-name</i> should be enclosed in single quotes.
    To revert the directory to the default, set the <i>directory-name</i> to
    an empty string, e.g., <i>PRAGMA temp_store_directory = ''</i>.  An
    error is raised if <i>directory-name</i> is not found or is not
    writable. </p>

    <p>The default directory for temporary files depends on the OS.  Some
    OS interfaces may choose to ignore this variable in place temporary
    files in some other directory different from the directory specified
    here.  In that sense, this pragma is only advisory.</p>
    </li>
</ul>

<tcl>Section {Pragmas to query the database schema} schema</tcl>

<ul>
<tcl>Subsection collation_list</tcl>
<li><p><b>PRAGMA collation_list;</b></p>
    <p>Return a list of the collating sequences defined for the current
    database connection.</p></li>

<tcl>Subsection database_list</tcl>
<li><p><b>PRAGMA database_list;</b></p>
    <p>For each open database, invoke the callback function once with
    information about that database.  Arguments include the index and 
    the name the database was attached with.  The first row will be for 
    the main database.  The second row will be for the database used to 
    store temporary tables.</p></li>

<tcl>Subsection foreign_key_list</tcl>
<li><p><b>PRAGMA foreign_key_list(</b><i>table-name</i><b>);</b></p>
    <p>For each foreign key that references a column in the argument
    table, invoke the callback function with information about that
    foreign key. The callback function will be invoked once for each
    column in each foreign key.</p></li>

<tcl>Subsection freelist_count</tcl>
<li><p><b>PRAGMA freelist_count;</b></p>
    <p>Return the number of unused pages in the database file. Running
    a <a href="#pragma_incremental_vacuum">"PRAGMA incremental_vaccum(N);"</a> 
    command with a large value of N will shrink the database file by this
    number of pages. </p></li>

<tcl>Subsection index_info</tcl>
<li><p><b>PRAGMA index_info(</b><i>index-name</i><b>);</b></p>
    <p>For each column that the named index references, invoke the 
    callback function
    once with information about that column, including the column name,
    and the column number.</p></li>

<tcl>Subsection index_list</tcl>
<li><p><b>PRAGMA index_list(</b><i>table-name</i><b>);</b></p>
    <p>For each index on the named table, invoke the callback function
    once with information about that index.  Arguments include the
    index name and a flag to indicate whether or not the index must be
    unique.</p></li>

<tcl>Subsection page_count</tcl>
<li><p><b>PRAGMA page_count;</b></p>
    <p>Return the total number of pages in the database file.</p></li>

<tcl>Subsection table_info</tcl>
<li><p><b>PRAGMA table_info(</b><i>table-name</i><b>);</b></p>
    <p>For each column in the named table, invoke the callback function
    once with information about that column, including the column name,
    data type, whether or not the column can be NULL, and the default
    value for the column.</p></li>
</ul>

<tcl>Section {Pragmas to query/modify version values} version</tcl>

<ul>
<tcl>Subsection schema_version user_version</tcl>
<li><p><b>PRAGMA schema_version; 
       <br>PRAGMA schema_version = </b><i>integer </i><b>;
       <br>PRAGMA user_version;
       <br>PRAGMA user_version = </b><i>integer </i><b>;</b>

  
<p>    The pragmas schema_version and user_version are used to set or get
       the value of the schema-version and user-version, respectively. Both
       the schema-version and the user-version are 32-bit signed integers
       stored in the database header.</p>
  
<p>    The schema-version is usually only manipulated internally by SQLite.  
       It is incremented by SQLite whenever the database schema is modified 
       (by creating or dropping a table or index). The schema version is 
       used by SQLite each time a query is executed to ensure that the 
       internal cache of the schema used when compiling the SQL query matches 
       the schema of the database against which the compiled query is actually 
       executed.  Subverting this mechanism by using "PRAGMA schema_version" 
       to modify the schema-version is potentially dangerous and may lead 
       to program crashes or database corruption. Use with caution!</p>
  
<p>    The user-version is not used internally by SQLite. It may be used by
       applications for any purpose.</p>
</li>
</ul>

<tcl>Section {Pragmas to debug the library} debug</tcl>

<ul>
<tcl>Subsection integrity_check</tcl>
<li><p><b>PRAGMA integrity_check;
    <br>PRAGMA integrity_check(</b><i>integer</i><b>)</b></p>
    <p>This pragma does an integrity check of the entire database.  It
    looks for out-of-order records, missing pages, malformed records, and
    corrupt indices.
    If any problems are found, then strings are returned (as multiple
    rows with a single column per row) which describe
    the problems.  At most <i>integer</i> errors will be reported
    before the analysis quits.  The default value for <i>integer</i>
    is 100.  If no errors are found, a single row with the value "ok" is
    returned.</p></li>

<tcl>Subsection quick_check</tcl>
<li><p><b>PRAGMA quick_check;
    <br>PRAGMA quick_check(</b><i>integer</i><b>)</b></p>
    <p>The pragma is like [integrity_check] except that it does not verify
    that index content matches table content.  By skipping the verification
    of index content, quick_check is able to run much faster than
    integrity_check.  Otherwise the two pragmas are the same.
    </p></li>

<tcl>Subsection parser_trace</tcl>
<li><p><b>PRAGMA parser_trace = </b><i>boolean</i><b>; </b></p>

    <p>Turn tracing of the SQL parser inside of the
    SQLite library on and off.  This is used for debugging.
    This only works if the library is compiled with the SQLITE_DEBUG
    compile-time option.
    </p></li>

<tcl>Subsection vdbe_trace</tcl>
<li><p><b>PRAGMA vdbe_trace = </b><i>boolean</i><b>;</b></p>

    <p>Turn tracing of the virtual database engine inside of the
    SQLite library on and off.  This is used for debugging.  See the 
    <a href="vdbe.html#trace">VDBE documentation</a> for more 
    information.</p></li>

<tcl>Subsection vdbe_listing</tcl>
<li><p><b>PRAGMA vdbe_listing = </b><i>boolean</i><b>;</b></p>

    <p>Turn listings of virtual machine programs on and off.
    With listing is on, the entire content of a program is printed
    just prior to beginning execution.  The statement
    executes normally after the listing is printed.
    This is used for debugging.  See the 
    <a href="vdbe.html#trace">VDBE documentation</a> for more 
    information.</p></li>
</ul>
