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ANALYZE
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<h1 id="overview"><span>1. </span>Overview</h1>
<p><b><a href="syntax/analyze-stmt.html">analyze-stmt:</a></b>
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<p> The ANALYZE command gathers statistics about tables and
indices and stores the collected information
in <a href="fileformat2.html#intschema">internal tables</a> of the database where the query optimizer can
access the information and use it to help make better query planning choices.
If no arguments are given, all attached databases are
analyzed. If a schema name is given as the argument, then all tables
and indices in that one database are analyzed.
If the argument is a table name, then only that table and the
indices associated with that table are analyzed. If the argument
is an index name, then only that one index is analyzed.</p>
<a name="req"></a>
<h2 id="recommended_usage_pattern"><span>1.1. </span>Recommended usage pattern</h2>
<p>Applications with long-lived databases that use complex
queries should consider running the following commands just prior
to closing each database connection:
</p><div class="codeblock"><pre>PRAGMA analysis_limit=400;
PRAGMA optimize;
</pre></div>
<p>The <a href="pragma.html#pragma_optimize">optimize pragma</a> is usually a no-op but it will occasionally
run ANALYZE if it seems like doing so will be useful to the query planner.
The <a href="pragma.html#pragma_analysis_limit">analysis_limit pragma</a> limits the scope of any ANALYZE command that
the <a href="pragma.html#pragma_optimize">optimize pragma</a> runs so that it does not consume too many CPU cycles.
The constant "400" can be adjusted as needed. Values between 100 and
1000 work well for most applications.
</p><h1 id="details"><span>2. </span>Details</h1>
<p> The default implementation stores all statistics in a single
table named "<a href="fileformat2.html#stat1tab">sqlite_stat1</a>".
If SQLite is compiled with the
<a href="compile.html#enable_stat4">SQLITE_ENABLE_STAT4</a> option, then additional histogram data is
collected and stored in <a href="fileformat2.html#stat4tab">sqlite_stat4</a>.
Older versions of SQLite would make use of the <a href="fileformat2.html#stat2tab">sqlite_stat2</a> table
or <a href="fileformat2.html#stat3tab">sqlite_stat3</a> table
when compiled with <a href="compile.html#enable_stat2">SQLITE_ENABLE_STAT2</a> or <a href="compile.html#enable_stat3">SQLITE_ENABLE_STAT3</a>,
but all recent versions of
SQLite ignore the sqlite_stat2 and sqlite_stat3 tables.
Future enhancements may create
additional <a href="fileformat2.html#intschema">internal tables</a> with the same name pattern except with
final digit larger than "4".
All of these tables are collectively referred to as "statistics tables".
</p>
<p> The content of the statistics tables can be queried using <a href="lang_select.html">SELECT</a>
and can be changed using the <a href="lang_delete.html">DELETE</a>, <a href="lang_insert.html">INSERT</a>, and <a href="lang_update.html">UPDATE</a> commands.
The <a href="lang_droptable.html">DROP TABLE</a> command works on statistics tables
as of SQLite version 3.7.9. (2011-11-01)
The <a href="lang_altertable.html">ALTER TABLE</a> command does not work on statistics tables.
Appropriate care should be used when changing the content of the statistics
tables as invalid content can cause SQLite to select inefficient
query plans. Generally speaking, one should not modify the content of
the statistics tables by any mechanism other than invoking the
ANALYZE command.
See "<a href="optoverview.html#manctrl">Manual Control Of Query Plans Using SQLITE_STAT Tables</a>" for
further information.</p>
<p> Statistics gathered by ANALYZE are not automatically updated as
the content of the database changes. If the content of the database
changes significantly, or if the database schema changes, then one should
consider rerunning the ANALYZE command in order to update the statistics.</p>
<p> The query planner loads the content of the statistics tables
into memory when the schema is read. Hence, when an application
changes the statistics tables directly, SQLite will not immediately
notice the changes. An application
can force the query planner to reread the statistics tables by running
<b>ANALYZE sqlite_schema</b>. </p>
<a name="autoanalyze"></a>
<h1 id="automatically_running_analyze"><span>3. </span>Automatically Running ANALYZE</h1>
<p>The <a href="pragma.html#pragma_optimize">PRAGMA optimize</a> command will automatically run ANALYZE on individual
tables on an as-needed basis. The recommended practice is for applications
to invoke the <a href="pragma.html#pragma_optimize">PRAGMA optimize</a> statement just before closing each database
connection.</p>
<p>Each SQLite <a href="c3ref/sqlite3.html">database connection</a> records cases when the query planner would
benefit from having accurate results of ANALYZE at hand. These records
are held in memory and accumulate over the life of a database connection.
The <a href="pragma.html#pragma_optimize">PRAGMA optimize</a> command looks at those records and runs ANALYZE on only
those tables for which new or updated ANALYZE data seems likely to be useful.
In most cases <a href="pragma.html#pragma_optimize">PRAGMA optimize</a> will not run ANALYZE, but it will occasionally
do so either for tables that have never before been analyzed, or for tables
that have grown significantly since they were last analyzed.</p>
<p>Since the actions of <a href="pragma.html#pragma_optimize">PRAGMA optimize</a> are determined to some extent by
prior queries that have been evaluated on the same database connection, it
is recommended that <a href="pragma.html#pragma_optimize">PRAGMA optimize</a> be deferred until the database connection
is closing and has thus had an opportunity to accumulate as much usage information
as possible. It is also reasonable to set a timer to run <a href="pragma.html#pragma_optimize">PRAGMA optimize</a>
every few hours, or every few days, for database connections that stay open
for a long time.</p>
<p>Applications that desire more control can run <a href="pragma.html#pragma_optimize">PRAGMA optimize(0x03)</a> to
obtain a list of ANALYZE commands that SQLite thinks are appropriate to run,
but without actually running those commands. If the returned set is
non-empty, the application can then make a decision about whether or not
to run the suggested ANALYZE commands, perhaps after prompting the user
for guidance.</p>
<p>The <a href="pragma.html#pragma_optimize">PRAGMA optimize</a> command was first introduced with
SQLite 3.18.0 (2017-03-28) and is a no-op for all prior releases
of SQLite.</p>
<a name="approx"></a>
<h1 id="approximate_analyze_for_large_databases"><span>4. </span>Approximate ANALYZE For Large Databases</h1>
<p>By default, ANALYZE does a full scan of every index. This can be slow for
large databases. So beginning with SQLite version 3.32.0 (2020-05-22), the
<a href="pragma.html#pragma_analysis_limit">PRAGMA analysis_limit</a> command can be used to limit the amount of
scanning performed by ANALYZE, and thus help ANALYZE to run faster,
even on very large database files. We call this running an
"approximate ANALYZE".
</p><p>The recommended usage pattern for the <a href="pragma.html#pragma_analysis_limit">analysis_limit</a> pragma is
like this:
</p><div class="codeblock"><pre>PRAGMA analysis_limit=1000;
</pre></div>
<p>This pragma tells the ANALYZE command to start a full scan
of the index as it normally would. But when the number of rows visited
reaches 1000 (or whatever other limit is specified by the pragma), the
ANALYZE command will begin taking actions to stop the scan. If
the left-most column of the index has changed at least once during the
previous 1000 steps, then the analysis stops immediately. But if the
left-most column has always been the same, then ANALYZE skips ahead to
the first entry with a different left-most column and reads an additional
1000 rows before terminating.
</p><p>The details of the effects of the analysis limit described in the previous
paragraph are subject to change in future versions of SQLite. But the
core idea will remain the same. An analysis limit of N will strive to
limit the number of rows visited in each index to approximately N.
</p><p>Values of N between 100 and 1000 are recommended.
Or, to disable the analysis limit, causing ANALYZE to do a
complete scan of each index, set the analysis limit to 0. The default
value for the analysis limit is 0 for backwards compatibility.
</p><p>The values placed in the sqlite_stat1 table by an approximate ANALYZE
are not exactly the same as what would be computed by an unrestricted
analysis. But they are usually close enough. The index statistics in
the sqlite_stat1 table are approximations in any case, so the fact that
the results of an approximate ANALYZE are slightly different from
a traditional full scan ANALYZE has little practical impact. It is
possible to construct a pathological case where an approximate ANALYZE
is noticeably inferior to a full-scan ANALYZE, but such cases rare in
real-world problems.
</p><p>A good rule of thumb seems to be to always set "PRAGMA analysis_limit=N"
for N between 100 and 1000 prior to running either "ANALYZE" or
"<a href="pragma.html#pragma_optimize">PRAGMA optimize</a>". The results are not quite as precise, but they
are precise enough, and the fact that the results are computed so much
faster means that developers are more likely to compute them. An
approximate ANALYZE is better than not running ANALYZE at all.
</p><h2 id="limitations_of_approximate_analyze"><span>4.1. </span>Limitations of approximate ANALYZE</h2>
<p>The content in the sqlite_stat4 table cannot be computed with
anything less than a full scan. Hence, if a non-zero analysis limit
is specified, the sqlite_stat4 table is not computed.
</p><p align="center"><small><i>This page last modified on <a href="https://sqlite.org/docsrc/honeypot" id="mtimelink" data-href="https://sqlite.org/docsrc/finfo/pages/lang_analyze.in?m=85eb0dec08045a663">2020-06-18 21:18:56</a> UTC </small></i></p>
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