BlitzNG
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database.mod
tükrözi: https://github.com/bmx-ng/database.mod.git


			
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<title>SQLite Query Language: VACUUM</title>
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<a href="lang.html">
           <h2 align="center">SQL As Understood By SQLite</h2></a><h1>VACUUM</h1><h4><a href="syntaxdiagrams.html#vacuum-stmt">vacuum-stmt:</a></h4><blockquote> <img alt="syntax diagram vacuum-stmt" src="images/syntax/vacuum-stmt.gif"></img> </blockquote>


<p>When an object (table, index, or trigger) is dropped from the 
database, it leaves behind empty space. 
This empty space will be reused the next time new information is
added to the database.  But in the meantime, the database file might
be larger than strictly necessary.  Also, frequent inserts, updates,
and deletes can cause the information in the database to become
fragmented - scrattered out all across the database file rather
than clustered together in one place.</p>

<p>The VACUUM command cleans
the main database by copying its contents to a temporary database file and 
reloading the original database file from the copy.  This eliminates 
free pages,  aligns table data to be contiguous, and otherwise cleans 
up the database file structure.</p>

<p>The VACUUM command may change the 
<a href="lang_createtable.html#rowid">ROWIDs</a> of entries in tables that do
not have an explicit <a href="lang_createtable.html#rowid">INTEGER PRIMARY KEY</a>.</p>

<p>VACUUM only works on the main database.
It is not possible to VACUUM an attached database file.</p>

<p>The VACUUM command will fail if there is an active transaction.
The VACUUM command is a no-op for in-memory databases.</p>

<p>As of SQLite version 3.1, an alternative to using the VACUUM command
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, 
large deletes cause
the size of the database file to shrink.  However, <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a>
also causes excess fragmentation of the database file.  And <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a>
does not compact partially filled pages of the database as VACUUM
does.</p>

<p>The <a href="pragma.html#pragma_page_size">page_size</a> and/or <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a> mode of a database can be changed
by invoking the <a href="pragma.html#pragma_page_size">page_size pragma</a> and/or <a href="pragma.html#pragma_auto_vacuum">auto_vacuum pragma</a> and then
immediately VACUUMing the database.</p>