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<h1 align="center">SQL As Understood By SQLite</h1><p><a href="lang.html">[Top]</a></p><h2>VACUUM</h2><p><b><a href="syntax/vacuum-stmt.html">vacuum-stmt:</a></b>
<button id='x1431' onclick='hideorshow("x1431","x1432")'>hide</button></p>
<blockquote id='x1432'>
<img alt="syntax diagram vacuum-stmt" src="images/syntax/vacuum-stmt.gif" />
</blockquote>
<p>
The VACUUM command rebuilds the entire database. There are several
reasons an application might do this:
<ul>
<li> <p> Unless SQLite is running in "auto_vacuum=FULL" mode, when a large
amount of data is deleted from the database file it leaves behind empty
space, or "free" database pages. This means the database file might
be larger than strictly necessary. Running VACUUM to rebuild the
database reclaims this space and reduces the size of the database file.
<li> <p> Frequent inserts, updates, and deletes can cause the database file
to become fragmented - where data for a single table or index is scattered
around the database file. Running VACUUM ensures that each table and
index is largely stored contiguously within the database file. In some
cases, VACUUM may also reduce the number of partially filled pages in
the database, reducing the size of the database file further.
<li> <p> Normally, the database <a href="pragma.html#pragma_page_size">page_size</a> and whether or not the database
supports <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a> must be configured before the database file is
actually created. However, when not in <a href="wal.html">write-ahead log</a> mode, the
<a href="pragma.html#pragma_page_size">page_size</a> and/or <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a> properties of an existing database may be
changed by using the <a href="pragma.html#pragma_page_size">page_size</a> and/or
<a href="pragma.html#pragma_auto_vacuum">pragma auto_vacuum</a> pragmas and then immediately VACUUMing
the database. When in <a href="wal.html">write-ahead log</a> mode, only the <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a>
support property can be changed using VACUUM.
</ul>
<p>VACUUM only works on the main database. It is not possible to VACUUM an
attached database file.
<p>The VACUUM command works by copying the contents of the database into
a temporary database file and then overwriting the original with the
contents of the temporary file. When overwriting the original, a rollback
journal or <a href="wal.html">write-ahead log</a> WAL file is used just as it would be for any
other database transaction. This means that when VACUUMing a database,
as much as twice the size of the original database file is required in free
disk space.
<p>The VACUUM command may change the <a href="lang_createtable.html#rowid">ROWIDs</a> of entries in any
tables that do not have an explicit <a href="lang_createtable.html#rowid">INTEGER PRIMARY KEY</a>.
</p>
<p>A VACUUM will fail if there is an open transaction, or if there are one or
more active SQL statements when it is run.
<p>As of SQLite version 3.1, an alternative to using the VACUUM command to
reclaim space after data has been deleted is auto-vacuum mode, enabled using
the <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a> pragma. When <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a> is enabled for a database
free pages may be reclaimed after deleting data, causing the file to shrink,
without rebuilding the entire database using VACUUM. However, using
<a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a> can lead to extra database file fragmentation. And <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a>
does not compact partially filled pages of the database as VACUUM does.
</p>
