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Re: cheaper snapshots

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Hannu Krosing <hannu(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: cheaper snapshots
Date: 2011-07-28 14:33:11
Message-ID: 8401.1311863591@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Thu, Jul 28, 2011 at 10:17 AM, Hannu Krosing <hannu(at)2ndquadrant(dot)com> wrote:
>> My hope was, that this contention would be the same than simply writing
>> the WAL buffers currently, and thus largely hidden by the current WAL
>> writing sync mechanisma.
>> 
>> It really covers just the part which writes commit records to WAL, as
>> non-commit WAL records dont participate in snapshot updates.

> I'm confused by this, because I don't think any of this can be done
> when we insert the commit record into the WAL stream.  It has to be
> done later, at the time we currently remove ourselves from the
> ProcArray.  Those things need not happen in the same order, as I noted
> in my original post.

But should we rethink that?  Your point that hot standby transactions on
a slave could see snapshots that were impossible on the parent was
disturbing.  Should we look for a way to tie "transaction becomes
visible" to its creation of a commit WAL record?  I think the fact that
they are not an indivisible operation is an implementation artifact, and
not a particularly nice one.

			regards, tom lane

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