To: "Kevin Grittner" <Kevin.Grittner@wicourts.gov>
cc: "lars" <lhofhansl@yahoo.com>, "Ivan Voras" <ivoras@freebsd.org>,
	pgsql-performance@postgresql.org
Subject: Re: UPDATEDs slowing SELECTs in a fully cached database 
In-reply-to: <4E1D697C020000250003F29A@gw.wicourts.gov> 
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Comments: In-reply-to "Kevin Grittner" <Kevin.Grittner@wicourts.gov>
	message dated "Wed, 13 Jul 2011 09:46:36 -0500"
Date: Wed, 13 Jul 2011 11:17:41 -0400
Message-ID: <4752.1310570261@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

"Kevin Grittner" <Kevin.Grittner@wicourts.gov> writes:
> ...  Jeff does raise a good point, though -- it seems odd
> that WAL-logging of this pruning would need to be synchronous.

Yeah, we need to get to the bottom of that.  If there's enough
shared_buffer space then it shouldn't be.

> We
> support asynchronous commits -- why not use that feature
> automatically for transactions where the only writes are this sort
> of thing.  Which raises an interesting question -- what happens to
> the timings if your SELECTs are done with synchronous_commit = off?
> I wonder if it would make any sense to implicitly use async commit
> for a transaction which is declared READ ONLY or which never
> acquires and XID?

Huh?  If there was never an XID, there's no commit WAL record, hence
nothing to make asynchronous.

			regards, tom lane