Re: snapshot too old, configured by time

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Kevin Grittner <kgrittn(at)gmail(dot)com>
Cc: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Thom Brown <thom(at)linux(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Steve Singer <steve(at)ssinger(dot)info>, Kevin Grittner <kgrittn(at)ymail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: snapshot too old, configured by time
Date: 2016-05-03 05:28:38
Message-ID: 20160503052838.GB27541@momjian.us
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On Mon, May 2, 2016 at 03:50:36PM -0500, Kevin Grittner wrote:
> >> Also, it seems we have similar behavior already in applying WAL on the
> >> standby --- we delay WAL replay when there is a long-running
> >> transaction. Once the time expires, we apply the WAL. Do we cancel the
> >> long-running transaction at that time, or wait for the long-running
> >> transaction to touch some WAL we just applied? If the former, does
> >> Kevin's new code allow us to do the later?
> >
> > Is this a TODO item?
>
> I'm not aware of any TODO items existing or needed here. The
> feature operates by adjusting the xmin used by vacuum and pruning,
> and leaving all the other mechanisms functioning as they were.
> That looked to me like it should interact with replication streams
> correctly. If someone sees something that needs adjustment please
> speak up Real Soon Now.

My question is whether this method could also be used to avoid read-only
query cancel when we force replay of a conflicting wal record. Could we
wait for the read-only query to try to _access_ some old data before
cancelling it?

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ As you are, so once was I. As I am, so you will be. +
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