<html><head><title>Pragma statements supported by SQLite</title></head>
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<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 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. 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>
<hr />
<a name="syntax"></a>
<h1>PRAGMA command syntax</h1>
<table cellpadding="10">
<tr><td align="right" valign="top">
<i><font color="#ff3434">sql-statement</font></i> ::=</td>
<td><b><font color="#2c2cf0">PRAGMA </font></b><i><font color="#ff3434">name</font></i><b><font color="#2c2cf0"> </font></b>[<b><font color="#2c2cf0">= </font></b><i><font color="#ff3434">value</font></i><b><font color="#2c2cf0"></font></b>]<b><font color="#2c2cf0"> </font></b><big>|</big><b><font color="#2c2cf0"><br>
PRAGMA </font></b><i><font color="#ff3434">function</font></i><b><font color="#2c2cf0"><big>(</big></font></b><i><font color="#ff3434">arg</font></i><b><font color="#2c2cf0"><big>)</big></font></b></td></tr>
</table>
<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>
<hr />
<a name="modify"></a>
<h1>Pragmas to modify library operation</h1>
<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
<table-name/alias><column-name> if the SELECT statement joins
two or
more tables together, or simply <column-name> if the SELECT
statement queries a single table. When the full-column-names flag
is set, such columns are always named <table-name/alias>
<column-name> 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
<table-name/alias>lt;column-name> if the SELECT statement
joins two or more tables together, or simply <column-name> if
the SELECT statement queries a single table. When the short-column-names
flag is set, such columns are always named <column-name>
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_synchronous"></a>
<li><p><b>PRAGMA synchronous;
<br>PRAGMA synchronous = FULL; </b>(2)<b>
<br>PRAGMA synchronous = NORMAL; </b>(1)<b>
<br>PRAGMA synchronous = OFF; </b>(0)</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 slow.
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>In SQLite version 2, the default value is NORMAL. For version 3, the
default was changed to FULL.
</p>
</li>
<a name="pragma_temp_store"></a>
<li><p><b>PRAGMA temp_store;
<br>PRAGMA temp_store = DEFAULT;</b> (0)<b>
<br>PRAGMA temp_store = FILE;</b> (1)<b>
<br>PRAGMA temp_store = MEMORY;</b> (2)</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
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 memory.
When temp_store is FILE (1) temporary tables and indices are stored
in a file. The <a href="#pragma_temp_store_directory">
temp_store_directory</a> pragma can be used to specify the directory
containing this file.
<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
TEMP_STORE to override this pragma setting. The following table summarizes
the interaction of the TEMP_STORE preprocessor macro and the
temp_store pragma:</p>
<blockquote>
<table cellpadding="2" border="1">
<tr><th valign="bottom">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>
<a name="pragma_temp_store_directory"></a>
<li><p><b>PRAGMA temp_store_directory;
<br>PRAGMA temp_store_directory = 'directory-name';</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. This setting lasts for the duration of the current connection
only and resets to its default value for each new connection opened.
<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>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. For
Unix/Linux/OSX, the default is the is the first writable directory found
in the list of: <b>/var/tmp, /usr/tmp, /tmp,</b> and <b>
<i>current-directory</i></b>. For Windows NT, the default
directory is determined by Windows, generally
<b>C:\Documents and Settings\<i>user-name</i>\Local Settings\Temp\</b>.
Temporary files created by SQLite are unlinked immediately after
opening, so that the operating system can automatically delete the
files when the SQLite process exits. Thus, temporary files are not
normally visible through <i>ls</i> or <i>dir</i> commands.</p>
</li>
</ul>
<hr />
<a name="schema"></a>
<h1>Pragmas to query the database schema</h1>
<ul>
<a name="pragma_database_list"></a>
<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>
<a name="pragma_foreign_key_list"></a>
<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>
<a name="pragma_index_info"></a>
<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>
<a name="pragma_index_list"></a>
<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>
<a name="pragma_table_info"></a>
<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>
<hr />
<a name="version"></a>
<h1>Pragmas to query/modify version values</h1>
<ul>
<a name="pragma_schema_version"></a>
<a name="pragma_user_version"></a>
<li><p><b>PRAGMA [database.]schema_version;
<br>PRAGMA [database.]schema_version = </b><i>integer </i><b>;
<br>PRAGMA [database.]user_version;
<br>PRAGMA [database.]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>
<hr />
<a name="debug"></a>
<h1>Pragmas to debug the library</h1>
<ul>
<a name="pragma_integrity_check"></a>
<li><p><b>PRAGMA integrity_check;</b></p>
<p>The command 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 a single string is returned which is
a description of all problems. If everything is in order, "ok" is
returned.</p></li>
<a name="pragma_parser_trace"></a>
<li><p><b>PRAGMA parser_trace = ON; </b>(1)<b>
<br>PRAGMA parser_trace = OFF;</b> (0)</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 without the NDEBUG macro.
</p></li>
<a name="pragma_vdbe_trace"></a>
<li><p><b>PRAGMA vdbe_trace = ON; </b>(1)<b>
<br>PRAGMA vdbe_trace = OFF;</b> (0)</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>
<a name="pragma_vdbe_listing"></a>
<li><p><b>PRAGMA vdbe_listing = ON; </b>(1)<b>
<br>PRAGMA vdbe_listing = OFF;</b> (0)</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. This is like automatically
executing an EXPLAIN prior to each statement. 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>
