| From: | Petr Jelinek <petr(dot)jelinek(at)2ndquadrant(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de>, pgsql-hackers(at)postgresql(dot)org, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> |
| Subject: | Re: logical replication busy-waiting on a lock |
| Date: | 2017-05-29 19:41:26 |
| Message-ID: | b38ee88b-45ad-19c8-b261-8eac5ae77e89@2ndquadrant.com |
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| Lists: | pgsql-hackers |
On 29/05/17 21:28, Andres Freund wrote:
>
> On May 29, 2017 12:25:35 PM PDT, Petr Jelinek <petr(dot)jelinek(at)2ndquadrant(dot)com> wrote:
>>
>> Looking at the code more, the xid is only used as parameter for
>> SnapBuildBuildSnapshot() which never does anything with that parameter,
>> I wonder if it's really needed then.
>
> Not at a computer, but by memory that'll trigger the snapshot export routine to include it. Import in turn requires the xid to check if the source is still alive. But there's better ways, e.g. using the virtual xactid.
>
It does, and while that's unfortunate the logical replication does not
actually export the snapshots. It uses the USE_SNAPSHOT option where the
snapshot is just installed into current transaction but not exported. So
not calling the GetTopTransactionId() would solve it at least for that
in-core use-case. I don't see any bad side effects from doing so yet, so
it might be good enough solution for PG10.
--
Petr Jelinek http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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