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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 15:57:18
Message-ID: 25470.1311868638@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 10:33 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> 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.

> Well, I agree with you that it isn't especially nice, but it seems
> like a fairly intractable problem.  Currently, the standby has no way
> of knowing in what order the transactions became visible on the
> master.

Right, but if the visibility order were *defined* as the order in which
commit records appear in WAL, that problem neatly goes away.  It's only
because we have the implementation artifact that "set my xid to 0 in the
ProcArray" is decoupled from inserting the commit record that there's
any difference.

			regards, tom lane

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