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Doc: Further update documentation for asynchronous execution.

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Add a note about asynchronous execution by postgres_fdw when applied to
Append nodes that contain synchronous subplan(s) as well.  Follow-up for
commit 27e1f1456.

Andrey Lepikhov and Etsuro Fujita

Discussion: https://postgr.es/m/58fa2aa5-07f5-80b5-59a1-fec8a349fee7%40postgrespro.ru
This commit is contained in:
Etsuro Fujita 2021-06-08 13:45:00 +09:00
parent 4e47b02834
commit eab8195368
1 changed files with 13 additions and 0 deletions
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doc/src/sgml/postgres-fdw.sgml
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@ -411,6 +411,19 @@ OPTIONS (ADD password_required 'false');
In such a case, it may be more performant to disable this option to
eliminate the overhead associated with running queries asynchronously.
</para>
<para>
Asynchronous execution is applied even when an
<structname>Append</structname> node contains subplan(s) executed
synchronously as well as subplan(s) executed asynchronously.
In such a case, if the asynchronous subplans are ones processed using
<filename>postgres_fdw</filename>, tuples from the asynchronous
subplans are not returned until after at least one synchronous subplan
returns all tuples, as that subplan is executed while the asynchronous
subplans are waiting for the results of asynchronous queries sent to
foreign servers.
This behavior might change in a future release.
</para>
</listitem>
</varlistentry>