179 lines
6.6 KiB
HTML
179 lines
6.6 KiB
HTML
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<title>The SQLITE_MEMSTAT Virtual Table</title>
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<div class="fancy_title">
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The SQLITE_MEMSTAT Virtual Table
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</div>
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</div>
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<h1 id="overview"><span>1. </span>Overview</h1>
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<p>
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The SQLITE_MEMSTAT extension implements an <a href="vtab.html#epoonlyvtab">eponymous-only virtual table</a> that
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provides SQL access to the <a href="c3ref/status.html">sqlite3_status64()</a> and
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<a href="c3ref/db_status.html">sqlite3_db_status()</a> interfaces.
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</p>
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<p>
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The SQLITE_STMT extension can also be loaded at run-time
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by compiling the extension into a shared library or DLL using the source
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code at <a href="https://sqlite.org/src/file/ext/misc/memstat.c">https://sqlite.org/src/file/ext/misc/memstat.c</a> and following the
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instructions for how to <a href="loadext.html#build">compile loadable extensions</a>.
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</p>
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<h1 id="usage"><span>2. </span>Usage</h1>
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<p>
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The SQLITE_MEMSTAT virtual table is a read-only table that can be
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queried to determine performance characteristics (primarily the
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amount of memory being used) of the current instance of SQLite.
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The SQLITE_MEMSTATE table is essentially a wrapper around the
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C-language APIs <a href="c3ref/status.html">sqlite3_status64()</a> and <a href="c3ref/db_status.html">sqlite3_db_status()</a>.
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If the
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<a href="https://sqlite.org/src/file/ext/misc/memstat.c">memstat.c</a> source
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file is compiled with the -DSQLITE_ENABLE_ZIPVFS option, then SQLITE_MEMSTAT
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will also do some <a href="c3ref/file_control.html">file-control</a> calls to extract
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memory usage information about the
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<a href="https://www.hwaci.com/sw/sqlite/zipvfs.html">ZIPVFS</a> subsystem,
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if that subsystem as been licensed, installed, and is in use.
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</p><p>
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The SQLITE_MEMSTAT table appears to have the following schema:
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</p><div class="codeblock"><pre>CREATE TABLE sqlite_memstat(
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name TEXT,
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schema TEXT,
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value INT,
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hiwtr INT
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);
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</pre></div>
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<p>
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Each row of the SQLITE_MEMSTAT table corresponds to a single call to
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one of the <a href="c3ref/status.html">sqlite3_status64()</a> or <a href="c3ref/db_status.html">sqlite3_db_status()</a> interfaces.
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The NAME column of the row identifies which "verb" was passed to those
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interfaces. For example, if <a href="c3ref/status.html">sqlite3_status64()</a> is invoked with
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<a href="c3ref/c_status_malloc_count.html#sqlitestatusmemoryused">SQLITE_STATUS_MEMORY_USED</a>, then the NAME column is 'MEMORY_USED'.
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Or if <a href="c3ref/db_status.html">sqlite3_db_status()</a> is invoked with <a href="c3ref/c_dbstatus_options.html#sqlitedbstatuscacheused">SQLITE_DBSTATUS_CACHE_USED</a>,
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then the NAME column is "DB_CACHE_USED".
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</p><p>
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The SCHEMA column is NULL, except for cases when the <a href="c3ref/file_control.html">sqlite3_file_control()</a>
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interface is used to interrogate the ZIPVFS backend. As this only happens
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when the memstat.c module is compiled with -DSQLITE_ENABLE_ZIPVFS and when
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<a href="https://www.hwaci.com/sw/sqlite/zipvfs.html">ZIPVFS</a> is in use,
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SCHEMA is usually NULL.
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</p><p>
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The VALUE and HIWTR columns report the current value of the measure and
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its "high-water mark". The high-water mark is the highest value ever seen
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for the measurement, at least since the last reset. The SQLITE_MEMSTAT
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virtual table does not provide a mechanism for resetting the high-water mark.
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</p><p>
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Depending on which parameter is being interrogated, one of the VALUE
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or HIWTR mark measurements might be undefined. For example, only the
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high-water mark is meaningful for <a href="c3ref/c_status_malloc_count.html#sqlitestatusmallocsize">SQLITE_STATUS_MALLOC_SIZE</a>, and
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only the current value is meaningful for <a href="c3ref/c_dbstatus_options.html#sqlitedbstatuscacheused">SQLITE_DBSTATUS_CACHE_USED</a>.
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For rows where one or the other of VALUE or HIWTR is not meaningful,
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that value is returned as NULL.
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the
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interfaces, with the initial
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</p>
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