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Change the docs for PARALLEL option of Vacuum.
The rules to choose the number of parallel workers to perform parallel vacuum operation were not clearly specified. Reported-by: Peter Eisentraut Author: Amit Kapila Backpatch-through: 13, where it was introduced Discussion: https://postgr.es/m/36aa8aea-61b7-eb3c-263b-648e0cb117b7@2ndquadrant.com
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@ -235,22 +235,22 @@ VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ ANALYZE ] [ <replaceable class="paramet
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Perform index vacuum and index cleanup phases of <command>VACUUM</command>
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in parallel using <replaceable class="parameter">integer</replaceable>
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background workers (for the details of each vacuum phase, please
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refer to <xref linkend="vacuum-phases"/>). In plain <command>VACUUM</command>
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(without <literal>FULL</literal>), if the <literal>PARALLEL</literal> option
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is omitted, then the number of workers is determined based on the number of
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indexes on the relation that support parallel vacuum operation and is further
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limited by <xref linkend="guc-max-parallel-maintenance-workers"/>. An index
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can participate in parallel vacuum if and only if the size of the index is
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more than <xref linkend="guc-min-parallel-index-scan-size"/>. Please note
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that it is not guaranteed that the number of parallel workers specified in
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<replaceable class="parameter">integer</replaceable> will be used during
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execution. It is possible for a vacuum to run with fewer workers than
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specified, or even with no workers at all. Only one worker can be used per
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index. So parallel workers are launched only when there are at least
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<literal>2</literal> indexes in the table. Workers for vacuum are launched
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before the start of each phase and exit at the end of the phase. These
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behaviors might change in a future release. This option can't be used with
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the <literal>FULL</literal> option.
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refer to <xref linkend="vacuum-phases"/>). The number of workers used
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to perform the operation is equal to the number of indexes on the
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relation that support parallel vacuum which is limited by the number of
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workers specified with <literal>PARALLEL</literal> option if any which is
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further limited by <xref linkend="guc-max-parallel-maintenance-workers"/>.
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An index can participate in parallel vacuum if and only if the size of the
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index is more than <xref linkend="guc-min-parallel-index-scan-size"/>.
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Please note that it is not guaranteed that the number of parallel workers
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specified in <replaceable class="parameter">integer</replaceable> will be
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used during execution. It is possible for a vacuum to run with fewer
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workers than specified, or even with no workers at all. Only one worker
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can be used per index. So parallel workers are launched only when there
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are at least <literal>2</literal> indexes in the table. Workers for
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vacuum are launched before the start of each phase and exit at the end of
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the phase. These behaviors might change in a future release. This
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option can't be used with the <literal>FULL</literal> option.
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</para>
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</listitem>
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</varlistentry>
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