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Assorted documentation improvements for max_parallel_workers.

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Commit b460f5d669 overlooked a few bits
of documentation that seem like they should mention the new setting.
This commit is contained in:
Robert Haas
2016-12-05 11:03:17 -05:00
parent 2b959d4957
commit 0e50af2453
2 changed files with 23 additions and 4 deletions
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17
doc/src/sgml/parallel.sgml
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@@ -61,14 +61,15 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
session will request a number of <link linkend="bgworker">background
worker processes</link> equal to the number
of workers chosen by the planner. The total number of background
workers that can exist at any one time is limited by
<xref linkend="guc-max-worker-processes">, so it is possible for a
workers that can exist at any one time is limited by both
<xref linkend="guc-max-worker-processes"> and
<xref linkend="guc-max-parallel-workers">, so it is possible for a
parallel query to run with fewer workers than planned, or even with
no workers at all. The optimal plan may depend on the number of workers
that are available, so this can result in poor query performance. If this
occurrence is frequent, considering increasing
<varname>max_worker_processes</> so that more workers can be run
simultaneously or alternatively reducing
<varname>max_worker_processes</> and <varname>max_parallel_workers</>
so that more workers can be run simultaneously or alternatively reducing
<xref linkend="guc-max-parallel-workers-per-gather"> so that the planner
requests fewer workers.
</para>
@@ -203,6 +204,14 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
</para>
</listitem>
<listitem>
<para>
No background workers can be obtained because of the limitation that
the total number of background workers launched for purposes of
parallel query cannot exceed <xref linkend="guc-max-parallel-workers">.
</para>
</listitem>
<listitem>
<para>
The client sends an Execute message with a non-zero fetch count.