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Doc: Update documentation for asynchronous execution.

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Add a note of caution on the performance of asynchronous execution by
postgres_fdw.  Follow-up for commit 27e1f1456.

Stephen Frost, a little bit expanded by me.

Discussion: https://postgr.es/m/20210506171224.GV20766%40tamriel.snowman.net
This commit is contained in:
Etsuro Fujita 2021-05-17 17:30:00 +09:00
parent 07af57dbad
commit 15fcd33e06
1 changed files with 10 additions and 0 deletions
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doc/src/sgml/postgres-fdw.sgml
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@ -401,6 +401,16 @@ OPTIONS (ADD password_required 'false');
A table-level option overrides a server-level option. A table-level option overrides a server-level option.
The default is <literal>false</literal>. The default is <literal>false</literal>.
</para> </para>
<para>
In order to ensure that the data being returned from a foreign server
is consistent, <filename>postgres_fdw</filename> will only open one
connection for a given foreign server and will run all queries against
that server sequentially even if there are multiple foreign tables
involved, unless those tables are subject to different user mappings.
In such a case, it may be more performant to disable this option to
eliminate the overhead associated with running queries asynchronously.
</para>
</listitem> </listitem>
</varlistentry> </varlistentry>