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Re: foreign key locks, 2nd attempt

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: foreign key locks, 2nd attempt
Date: 2012-03-15 21:54:32
Message-ID: 1331848117-sup-4005@alvh.no-ip.org (view raw or flat)
Thread:
Lists: pgsql-hackers
Excerpts from Simon Riggs's message of jue mar 15 18:38:53 -0300 2012:
> On Thu, Mar 15, 2012 at 2:26 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> > But that would only make sense if
> > we thought that getting rid of the fsyncs would be more valuable than
> > avoiding the blocking here, and I don't.
> 
> You're right that the existing code could use some optimisation.
> 
> I'm a little tired, but I can't see a reason to fsync this except at checkpoint.

Hang on.  What fsyncs are we talking about?  I don't see that the
multixact code calls any fsync except that checkpoint and shutdown.

> Also seeing that we issue 2 WAL records for each RI check. We issue
> one during MultiXactIdCreate/MultiXactIdExpand and then immediately
> afterwards issue a XLOG_HEAP_LOCK record. The comments on both show
> that each thinks it is doing it for the same reason and is the only
> place its being done. Alvaro, any ideas why that is.

AFAIR the XLOG_HEAP_LOCK log entry only records the fact that the row is
being locked by a multixact -- it doesn't record the contents (member
xids) of said multixact, which is what the other log entry records.

-- 
Álvaro Herrera <alvherre(at)commandprompt(dot)com>
The PostgreSQL Company - Command Prompt, Inc.
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

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