## Run this Tcl script to generate the pragma.html file.#set rcsid {$Id: pragma.tcl,v 1.17 2006/02/11 01:25:52 drh Exp $}source common.tclheader {Pragma statements supported by SQLite}proc Section {name {label {}}} {  puts "\n<hr />"  if {$label!=""} {    puts "<a name=\"$label\"></a>"  }  puts "<h1>$name</h1>\n"}puts {<p>The <a href="#syntax">PRAGMA command</a> is a special command used to modify the operation of the SQLite library or to query the library for internal (non-table) data. The PRAGMA command is issued using the sameinterface as other SQLite commands (e.g. SELECT, INSERT) but isdifferent in the following important respects:</p><ul><li>Specific pragma statements may be removed and others added in future    releases of SQLite. Use with caution!<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_compile(),     sqlite3_step(), sqlite3_finalize() API (or similar in a wrapper     interface), the pragma may be applied to the library during the     sqlite3_compile() call.<li>The pragma command is unlikely to be compatible with any other SQL    engine.</ul><p>The available pragmas fall into four basic categories:</p><ul><li>Pragmas used to <a href="#schema">query the schema</a> of the current     database.<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="#version">query or modify the databases two     version values</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>}Section {PRAGMA command syntax} syntaxSyntax {sql-statement} {PRAGMA <name> [= <value>] |PRAGMA <function>(<arg>)}puts {<p>The pragmas that take an integer <b><i>value</i></b> also accept symbolic names.  The strings "<b>on</b>", "<b>true</b>", and "<b>yes</b>" are equivalent to <b>1</b>.  The strings "<b>off</b>", "<b>false</b>", and "<b>no</b>" are equivalent to <b>0</b>.  These strings are case-insensitive, and do not require quotes.  An unrecognized string will be treated as <b>1</b>, and will not generate an error.  When the <i>value</i> is returned it is as an integer.</p>}Section {Pragmas to modify library operation} modifyputs {<ul><a name="pragma_auto_vacuum"></a><li><p><b>PRAGMA auto_vacuum;       <br>PRAGMA auto_vacuum = </b><i>0 | 1</i><b>;</b></p>    <p> Query or set the auto-vacuum flag in the database.</p>    <p>Normally, when a transaction that deletes data from a database is    committed, the database file remains the same size. Unused database file     pages are marked as such and reused later on, when data is inserted into     the database. In this mode the <a href="lang_vacuum.html">VACUUM</a>    command is used to reclaim unused space.</p>    <p>When the auto-vacuum flag is set, the database file shrinks when a    transaction that deletes data is committed (The VACUUM command is not    useful in a database with the auto-vacuum flag set). To support this    functionality the database stores extra information internally, resulting    in slightly larger database files than would otherwise be possible.</p>    <p>It is only possible to modify the value of the auto-vacuum flag before    any tables have been created in the database. No error message is     returned if an attempt to modify the auto-vacuum flag is made after    one or more tables have been created.    </p></li><a name="pragma_cache_size"></a><li><p><b>PRAGMA cache_size;       <br>PRAGMA cache_size = </b><i>Number-of-pages</i><b>;</b></p>    <p>Query or change the maximum number of database disk pages that SQLite    will hold in memory at once.  Each page uses about 1.5K of memory.    The default cache size is 2000.  If you are doing UPDATEs or DELETEs    that change many rows of a database and you do not mind if SQLite    uses more memory, you can increase the cache size for a possible speed    improvement.</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 <a href="#pragma_default_cache_size"><b>default_cache_size</b></a>     pragma to check the cache size permanently.</p></li><a name="pragma_case_sensitive_like"></a><li><p><b>PRAGMA case_sensitive_like;       <br>PRAGMA case_sensitive_like = </b><i>0 | 1</i><b>;</b></p>    <p>The default behavior of the LIKE operator is to ignore case    for latin1 characters. Hence, by default <b>'a' LIKE 'A'</b> is    true.  The case_sensitive_like pragma can be turned on to 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><a name="pragma_count_changes"></a><li><p><b>PRAGMA count_changes;       <br>PRAGMA count_changes = </b><i>0 | 1</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><a name="pragma_default_cache_size"></a><li><p><b>PRAGMA default_cache_size;       <br>PRAGMA default_cache_size = </b><i>Number-of-pages</i><b>;</b></p>    <p>Query or change the maximum number of database disk pages that SQLite    will hold in memory at once.  Each page uses 1K on disk and about    1.5K in memory.    This pragma works like the    <a href="#pragma_cache_size"><b>cache_size</b></a>     pragma with the additional    feature that it changes the cache size persistently.  With this pragma,    you can set the cache size once and that setting is retained and reused    every time you reopen the database.</p></li><a name="pragma_default_synchronous"></a><li><p><b>PRAGMA default_synchronous;</b></p>    <p>This pragma was available in version 2.8 but was removed in version    3.0.  It is a dangerous pragma whose use is discouraged.  To help    dissuide users of version 2.8 from employing this pragma, the documentation    will not tell you what it does.</p></li><a name="pragma_empty_result_callbacks"></a><li><p><b>PRAGMA empty_result_callbacks;       <br>PRAGMA empty_result_callbacks = </b><i>0 | 1</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><a name="pragma_encoding"></a><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><a name="pragma_full_column_names"></a><li><p><b>PRAGMA full_column_names;       <br>PRAGMA full_column_names = </b><i>0 | 1</i><b>;</b></p>    <p>Query or change the full-column-names flag. This flag affects    the way SQLite names columns of data returned by SELECT statements    when the expression for the column is a table-column name or the    wildcard "*".  Normally, such result columns are named    &lt;table-name/alias&gt;&lt;column-name&gt; if the SELECT statement joins     two or    more tables together, or simply &lt;column-name&gt; if the SELECT    statement queries a single table. When the full-column-names flag    is set, such columns are always named &lt;table-name/alias&gt;    &lt;column-name&gt; regardless of whether or not a join is performed.    </p>    <p>If both the short-column-names and full-column-names are set,    then the behaviour associated with the full-column-names flag is    exhibited.    </p></li><a name="pragma_fullfsync"></a><li><p><b>PRAGMA fullfsync       <br>PRAGMA fullfsync = </b><i>0 | 1</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><a name="pragma_page_size"></a><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 8192. The upper limit may be modified by setting    the value of macro SQLITE_MAX_PAGE_SIZE during compilation.  The    maximum upper bound is 32768.    </p></li><a name="pragma_read_uncommitted"></a><li><p><b>PRAGMA read_uncommitted;       <br>PRAGMA read_uncommitted = </b><i>0 | 1</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    <a href="capi3ref.html#sqlite3_enable_shared_cache">    sqlite3_enable_shared_cache()</a> API and is only available between    connections running the same thread.  Cache sharing is off by default.    </p></li><a name="pragma_short_column_names"></a><li><p><b>PRAGMA short_column_names;       <br>PRAGMA short_column_names = </b><i>0 | 1</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    when the expression for the column is a table-column name or the    wildcard "*".  Normally, such result columns are named    &lt;table-name/alias&gt;lt;column-name&gt; if the SELECT statement