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Doc: Improve description of the "parallel_commit" option for postgres_fdw. 

It wasn't very clear that if this option was enabled, postgres_fdw would
commit remote transactions in parallel at main-transaction end, and
commit remote subtransactions in parallel at subtransaction end, due to
the references to (sub)transaction and other too specific documentation.
Clarify that.  Also reword to simplify.

Follow-up for commit 04e706d42.

Jonathan S. Katz, reviewed by Justin Pryzby, with some modifications by
me.

Discussion: https://postgr.es/m/1435bfd1-172b-de38-f590-4404a5a62eb0%40postgresql.org
This commit is contained in:
Etsuro Fujita 2022-05-13 18:30:00 +09:00
parent 30ed71e423
commit 8c8d307f82
1 changed files with 22 additions and 18 deletions
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40
doc/src/sgml/postgres-fdw.sgml
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@ -460,10 +460,16 @@ OPTIONS (ADD password_required 'false');
<title>Transaction Management Options</title>
<para>
When multiple remote (sub)transactions are involved in a local
(sub)transaction, by default <filename>postgres_fdw</filename> commits
those remote (sub)transactions one by one when the local (sub)transaction
commits.
As described in the Transaction Management section, in
<filename>postgres_fdw</filename> transactions are managed by creating
corresponding remote transactions, and subtransactions are managed by
creating corresponding remote subtransactions. When multiple remote
transactions are involved in the current local transaction, by default
<filename>postgres_fdw</filename> commits those remote transactions
serially when the local transaction is committed. When multiple remote
subtransactions are involved in the current local subtransaction, by
default <filename>postgres_fdw</filename> commits those remote
subtransactions serially when the local subtransaction is committed.
Performance can be improved with the following option:
</para>
@ -474,26 +480,24 @@ OPTIONS (ADD password_required 'false');
<listitem>
<para>
This option controls whether <filename>postgres_fdw</filename> commits
remote (sub)transactions opened on a foreign server in a local
(sub)transaction in parallel when the local (sub)transaction commits.
This option can only be specified for foreign servers, not per-table.
The default is <literal>false</literal>.
in parallel remote transactions opened on a foreign server in a local
transaction when the local transaction is committed. This setting also
applies to remote and local subtransactions. This option can only be
specified for foreign servers, not per-table. The default is
<literal>false</literal>.
</para>
<para>
If multiple foreign servers with this option enabled are involved in
a local (sub)transaction, multiple remote (sub)transactions opened on
those foreign servers in the local (sub)transaction are committed in
parallel across those foreign servers when the local (sub)transaction
commits.
If multiple foreign servers with this option enabled are involved in a
local transaction, multiple remote transactions on those foreign
servers are committed in parallel across those foreign servers when
the local transaction is committed.
</para>
<para>
For a foreign server with this option enabled, if many remote
(sub)transactions are opened on the foreign server in a local
(sub)transaction, this option might increase the remote server's load
when the local (sub)transaction commits, so be careful when using this
option.
When this option is enabled, a foreign server with many remote
transactions may see a negative performance impact when the local
transaction is committed.
</para>
</listitem>
</varlistentry>