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Update parallel.sgml for Parallel Append

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Patch by me, reviewed by Thomas Munro, in response to a complaint
from Adrien Nayrat.

Discussion: http://postgr.es/m/baa0d036-7349-f722-ef88-2d8bb3413045@anayrat.info
This commit is contained in:
Robert Haas
2018-08-01 08:14:05 -04:00
parent 9200016335
commit ac535cd478
1 changed files with 48 additions and 0 deletions
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doc/src/sgml/parallel.sgml
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@@ -401,6 +401,54 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
</sect2>
<sect2 id="parallel-append">
<title>Parallel Append</title>
<para>
Whenever <productname>PostgreSQL</productname> needs to combine rows
from multiple sources into a single result set, it uses an
<literal>Append</literal> or <literal>MergeAppend</literal> plan node.
This commonly happens when implementing <literal>UNION ALL</literal> or
when scanning a partitioned table. Such nodes can be used in parallel
plans just as they can in any other plan. However, in a parallel plan,
the planner may instead use a <literal>Parallel Append</literal> node.
</para>
<para>
When an <literal>Append</literal> node is used in a parallel plan, each
process will execute the child plans in the order in which they appear,
so that all participating processes cooperate to execute the first child
plan until it is complete and then move to the second plan at around the
same time. When a <literal>Parallel Append</literal> is used instead, the
executor will instead spread out the participating processes as evenly as
possible across its child plans, so that multiple child plans are executed
simultaneously. This avoids contention, and also avoids paying the startup
cost of a child plan in those processes that never execute it.
</para>
<para>
Also, unlike a regular <literal>Append</literal> node, which can only have
partial children when used within a parallel plan, a <literal>Parallel
Append</literal> node can have both partial and non-partial child plans.
Non-partial children will be scanned by only a single process, since
scanning them more than once would produce duplicate results. Plans that
involve appending multiple results sets can therefore achieve
coarse-grained parallelism even when efficient partial plans are not
available. For example, consider a query against a partitioned table
which can be only be implemented efficiently by using an index that does
not support parallel scans. The planner might choose a <literal>Parallel
Append</literal> of regular <literal>Index Scan</literal> plans; each
individual index scan would have to be executed to completion by a single
process, but different scans could be performed at the same time by
different processes.
</para>
<para>
<xref linkend="guc-enable-parallel-append" /> can be used to disable
this feature.
</para>
</sect2>
<sect2 id="parallel-plan-tips">
<title>Parallel Plan Tips</title>