252 lines
8.1 KiB
Plaintext
252 lines
8.1 KiB
Plaintext
<!--
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$PostgreSQL: pgsql/doc/src/sgml/ref/vacuum.sgml,v 1.36 2004/12/02 19:28:48 tgl Exp $
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PostgreSQL documentation
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-->
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<refentry id="SQL-VACUUM">
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<refmeta>
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<refentrytitle id="sql-vacuum-title">VACUUM</refentrytitle>
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<refmiscinfo>SQL - Language Statements</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>VACUUM</refname>
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<refpurpose>garbage-collect and optionally analyze a database</refpurpose>
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</refnamediv>
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<indexterm zone="sql-vacuum">
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<primary>VACUUM</primary>
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</indexterm>
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<refsynopsisdiv>
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<synopsis>
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VACUUM [ FULL | FREEZE ] [ VERBOSE ] [ <replaceable class="PARAMETER">table</replaceable> ]
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VACUUM [ FULL | FREEZE ] [ VERBOSE ] ANALYZE [ <replaceable class="PARAMETER">table</replaceable> [ (<replaceable class="PARAMETER">column</replaceable> [, ...] ) ] ]
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</synopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>
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<command>VACUUM</command> reclaims storage occupied by deleted tuples.
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In normal <productname>PostgreSQL</productname> operation, tuples that
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are deleted or obsoleted by an update are not physically removed from
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their table; they remain present until a <command>VACUUM</command> is
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done. Therefore it's necessary to do <command>VACUUM</command>
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periodically, especially on frequently-updated tables.
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</para>
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<para>
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With no parameter, <command>VACUUM</command> processes every table in the
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current database. With a parameter, <command>VACUUM</command> processes
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only that table.
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</para>
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<para>
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<command>VACUUM ANALYZE</command> performs a <command>VACUUM</command>
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and then an <command>ANALYZE</command> for each selected table. This
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is a handy combination form for routine maintenance scripts. See
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<xref linkend="sql-analyze" endterm="sql-analyze-title">
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for more details about its processing.
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</para>
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<para>
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Plain <command>VACUUM</command> (without <literal>FULL</>) simply reclaims
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space and makes it
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available for re-use. This form of the command can operate in parallel
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with normal reading and writing of the table, as an exclusive lock
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is not obtained. <command>VACUUM
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FULL</command> does more extensive processing, including moving of tuples
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across blocks to try to compact the table to the minimum number of disk
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blocks. This form is much slower and requires an exclusive lock on each
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table while it is being processed.
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</para>
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<para>
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<literal>FREEZE</literal> is a special-purpose option that
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causes tuples to be marked <quote>frozen</quote> as soon as possible,
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rather than waiting until they are quite old. If this is done when there
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are no other open transactions in the same database, then it is guaranteed
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that all tuples in the database are <quote>frozen</> and will not be
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subject to transaction ID wraparound problems, no matter how long the
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database is left unvacuumed.
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<literal>FREEZE</literal> is not recommended for routine use. Its only
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intended usage is in connection with preparation of user-defined template
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databases, or other databases that are completely read-only and will not
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receive routine maintenance <command>VACUUM</> operations.
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See <xref linkend="maintenance"> for details.
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</para>
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</refsect1>
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<refsect1>
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<title>Parameters</title>
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<variablelist>
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<varlistentry>
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<term><literal>FULL</literal></term>
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<listitem>
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<para>
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Selects <quote>full</quote> vacuum, which may reclaim more
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space, but takes much longer and exclusively locks the table.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>FREEZE</literal></term>
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<listitem>
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<para>
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Selects aggressive <quote>freezing</quote> of tuples.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>VERBOSE</literal></term>
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<listitem>
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<para>
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Prints a detailed vacuum activity report for each table.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>ANALYZE</literal></term>
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<listitem>
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<para>
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Updates statistics used by the planner to determine the most
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efficient way to execute a query.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="PARAMETER">table</replaceable></term>
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<listitem>
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<para>
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The name (optionally schema-qualified) of a specific table to
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vacuum. Defaults to all tables in the current database.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="PARAMETER">column</replaceable></term>
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<listitem>
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<para>
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The name of a specific column to analyze. Defaults to all columns.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Outputs</title>
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<para>
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When <literal>VERBOSE</> is specified, <command>VACUUM</> emits
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progress messages to indicate which table is currently being
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processed. Various statistics about the tables are printed as well.
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</para>
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</refsect1>
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<refsect1>
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<title>Notes</title>
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<para>
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We recommend that active production databases be
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vacuumed frequently (at least nightly), in order to
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remove expired rows. After adding or deleting a large number
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of rows, it may be a good idea to issue a <command>VACUUM
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ANALYZE</command> command for the affected table. This will update the
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system catalogs with
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the results of all recent changes, and allow the
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<productname>PostgreSQL</productname> query planner to make better
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choices in planning queries.
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</para>
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<para>
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The <option>FULL</option> option is not recommended for routine use,
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but may be useful in special cases. An example is when you have deleted
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most of the rows in a table and would like the table to physically shrink
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to occupy less disk space. <command>VACUUM FULL</command> will usually
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shrink the table more than a plain <command>VACUUM</command> would.
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</para>
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</refsect1>
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<refsect1>
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<title>Examples</title>
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<para>
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The following is an example from running <command>VACUUM</command> on a
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table in the regression database:
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<programlisting>
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regression=# VACUUM VERBOSE ANALYZE onek;
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INFO: vacuuming "public.onek"
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INFO: index "onek_unique1" now contains 1000 tuples in 14 pages
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DETAIL: 3000 index tuples were removed.
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0 index pages have been deleted, 0 are currently reusable.
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CPU 0.01s/0.08u sec elapsed 0.18 sec.
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INFO: index "onek_unique2" now contains 1000 tuples in 16 pages
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DETAIL: 3000 index tuples were removed.
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0 index pages have been deleted, 0 are currently reusable.
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CPU 0.00s/0.07u sec elapsed 0.23 sec.
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INFO: index "onek_hundred" now contains 1000 tuples in 13 pages
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DETAIL: 3000 index tuples were removed.
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0 index pages have been deleted, 0 are currently reusable.
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CPU 0.01s/0.08u sec elapsed 0.17 sec.
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INFO: index "onek_stringu1" now contains 1000 tuples in 48 pages
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DETAIL: 3000 index tuples were removed.
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0 index pages have been deleted, 0 are currently reusable.
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CPU 0.01s/0.09u sec elapsed 0.59 sec.
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INFO: "onek": removed 3000 tuples in 108 pages
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DETAIL: CPU 0.01s/0.06u sec elapsed 0.07 sec.
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INFO: "onek": found 3000 removable, 1000 nonremovable tuples in 143 pages
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DETAIL: 0 dead tuples cannot be removed yet.
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There were 0 unused item pointers.
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0 pages are entirely empty.
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CPU 0.07s/0.39u sec elapsed 1.56 sec.
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INFO: analyzing "public.onek"
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INFO: "onek": 36 pages, 1000 rows sampled, 1000 estimated total rows
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VACUUM
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</programlisting>
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</para>
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</refsect1>
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<refsect1>
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<title>Compatibility</title>
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<para>
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There is no <command>VACUUM</command> statement in the SQL standard.
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</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<simplelist type="inline">
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<member><xref linkend="app-vacuumdb" endterm="app-vacuumdb-title"></member>
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</simplelist>
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</refsect1>
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</refentry>
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Local variables:
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mode: sgml
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sgml-default-dtd-file:"../reference.ced"
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sgml-local-catalogs:"/usr/lib/sgml/catalog"
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End:
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