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<span id="runningHeaderText">PRAGMA</span> </td>
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<h1 class="heading">
SQL As Understood By SQLite</h1>
<h4>
PRAGMA</h4>
<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>
<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>
<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>
<li>The pragma command is unlikely to be compatible with any other SQL engine. </li>
</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>
<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>
<li>Pragmas used to <a href="#version">query or modify the databases two version values</a>,
the schema-version and the user-version. </li>
<li>Pragmas used to <a href="#debug">debug the library</a> and verify that database
files are not corrupted. </li>
</ul>
<hr />
<a name="syntax"></a>
<h1>
PRAGMA command syntax</h1>
<p>
<table cellpadding="10">
<tr>
<td align="right" width="1%" nowrap>
<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>
<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>
<a name="pragma_cache_size"></a></li>
<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>
<a name="pragma_case_sensitive_like"></a></li>
<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>
<a name="pragma_count_changes"></a></li>
<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>
<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>
<a name="pragma_default_synchronous"></a></li>
<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>
<a name="pragma_empty_result_callbacks"></a></li>
<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
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