8000 Assorted documentation improvements for max_parallel_workers. · postgres/postgres@0e50af2 · GitHub
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    robertmhaasrobertmhaas
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    Assorted documentation improvements for max_parallel_workers.
    Commit b460f5d overlooked a few bits of documentation that seem like they should mention the new setting.
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    ‎doc/src/sgml/config.sgml

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    @@ -1990,6 +1990,12 @@ include_dir 'conf.d'
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    same or higher value than on the master server. Otherwise, queries
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    will not be allowed in the standby server.
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    </para>
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    <para>
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    When changing this value, consider also adjusting
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    <xref linkend="guc-max-parallel-workers"> and
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    <xref linkend="guc-max-parallel-workers-per-gather">.
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    </para>
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    </listitem>
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    </varlistentry>
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    @@ -2047,6 +2053,10 @@ include_dir 'conf.d'
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    parallel queries. The default value is 8. When increasing or
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    decreasing this value, consider also adjusting
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    <xref linkend="guc-max-parallel-workers-per-gather">.
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    Also, note that a setting for this value which is higher than
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    <xref linkend="guc-max-worker-processes"> will have no effect,
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    since parallel workers are taken from the pool of worker processes
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    established by that setting.
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    </para>
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    </listitem>
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    </varlistentry>

    ‎doc/src/sgml/parallel.sgml

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    @@ -61,14 +61,15 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
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    session will request a number of <link linkend="bgworker">background
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    worker processes</link> equal to the number
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    of workers chosen by the planner. The total number of background
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    workers that can exist at any one time is limited by
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    <xref linkend="guc-max-worker-processes">, so it is possible for a
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    workers that can exist at any one time is limited by both
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    <xref linkend="guc-max-worker-processes"> and
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    <xref linkend="guc-max-parallel-workers">, so it is possible for a
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    parallel query to run with fewer workers than planned, or even with
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    no workers at all. The optimal plan may depend on the number of workers
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    that are available, so this can result in poor query performance. If this
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    occurrence is frequent, considering increasing
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    <varname>max_worker_processes</> so that more workers can be run
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    simultaneously or alternatively reducing
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    <varname>max_worker_processes</> and <varname>max_parallel_workers</>
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    so that more workers can be run simultaneously or alternatively reducing
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    <xref linkend="guc-max-parallel-workers-per-gather"> so that the planner
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    requests fewer workers.
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    </para>
    @@ -203,6 +204,14 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
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    </para>
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    </listitem>
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    <listitem>
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    <para>
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    No background workers can be obtained because of the limitation that
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    the total number of background workers launched for purposes of
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    parallel query cannot exceed <xref linkend="guc-max-parallel-workers">.
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    </para>
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    </listitem>
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    <listitem>
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    <para>
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    The client sends an Execute message with a non-zero fetch count.

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