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<H1><A NAME="SECTION001940000000000000000"> </A><A NAME="23462"> </A>
<BR>
V<SMALL>ACUUM</SMALL>
</H1>
<P>
<A NAME="vacuum_section"> </A>When P<SMALL>OSTGRE</SMALL>SQL updates a row, it
keeps the original copy of the row in the table file and writes a
new one. The original row, marked as expired, is used by other transactions
still viewing the database in its prior state. Deletions are similarly
marked as expired, but not removed from the table file.
<P>
The <SMALL>VACUUM</SMALL> command removes expired rows from the file. In
the process, it moves rows from the end of the table into the expired
spots, thereby compacting the table file.
<P>
You should run <SMALL>VACUUM</SMALL> periodically to clean out expired rows.
For tables that are heavily modified, it is useful to run <SMALL>VACUUM</SMALL>
every night in an automated manner. For tables with few modifications,
<SMALL>VACUUM</SMALL> should be run less frequently. The command exclusively
locks the table while processing.
<P>
You can run <SMALL>VACUUM</SMALL> in two ways. Using <SMALL>VACUUM</SMALL>
alone vacuums all tables in the database. Using <SMALL>VACUUM</SMALL> <EM>tablename</EM>
vacuums a single table.
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<ADDRESS>
<I>Bruce Momjian</I>
<BR><I>2002-01-08</I>
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