To: "Kevin Grittner" <Kevin.Grittner@wicourts.gov>
cc: "Ivan Voras" <ivoras@freebsd.org>, pgsql-performance@postgresql.org,
	"lars" <lhofhansl@yahoo.com>
Subject: Re: UPDATEDs slowing SELECTs in a fully cached database 
In-reply-to: <4E1EB165020000250003F2FC@gw.wicourts.gov> 
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	<4752.1310570261@sss.pgh.pa.us>
	<4E1DCE81020000250003F2DC@gw.wicourts.gov><4E1DCE81020000250003F2DC@gw.wicourts.gov>
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Comments: In-reply-to "Kevin Grittner" <Kevin.Grittner@wicourts.gov>
	message dated "Thu, 14 Jul 2011 09:05:41 -0500"
Date: Thu, 14 Jul 2011 11:47:00 -0400
Message-ID: <464.1310658420@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

"Kevin Grittner" <Kevin.Grittner@wicourts.gov> writes:
> Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> It seems like we ought to distinguish heap cleanup activities from
>> user-visible semantics (IOW, users shouldn't care if a HOT cleanup
>> has to be done over after restart, so if the transaction only
>> wrote such records there's no need to flush).  This'd require more
>> process-global state than we keep now, I'm afraid.
 
> That makes sense, and seems like the right long-term fix.  It seems
> like a boolean might do it; the trick would be setting it (or not)
> in all the right places.

The implementation I was imagining was to define another bit in the info
parameter for XLogInsert, say XLOG_NON_TRANSACTIONAL.  This could be a
high-order bit that would not go to disk.  Anytime it was *not* set,
XLogInsert would set a global boolean that would remember that the
current transaction wrote a transactional WAL record.  This is the
right default since the vast majority of call sites are writing records
that we would want to have flushed at commit.  There are just a couple
of places that would need to be changed to add this flag to their calls.

			regards, tom lane