Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

On 2016-06-30 23:51:18 -0400, Noah Misch wrote:
> On Thu, Jun 16, 2016 at 01:56:44PM -0500, Kevin Grittner wrote:
> > On Thu, Jun 16, 2016 at 1:32 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> >
> > > With old_snapshot_threshold=1 I indeed can reproduce the issue. I
> > > disabled autovacuum, to make the scheduling more predictable. But it
> > > should "work" just as well with autovacuum.
> > >
> > > S1: CREATE TABLE toasted(largecol text);
> > > INSERT INTO toasted SELECT string_agg(random()::text, '-') FROM generate_series(1, 10000000);
> > > BEGIN;
> > > DELETE FROM toasted;
> > > S2: BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
> > > S2: SELECT hashtext(largecol), length(largecol) FROM toasted;
> > >> ...
> > > S1: COMMIT;
> > > S2: SELECT hashtext(largecol), length(largecol) FROM toasted;
> > >> ...
> > > S1: /* wait for snapshot threshold to be passed */
> > > S1: VACUUM pg_toast.pg_toast_16437;
> > >> INFO: 00000: "pg_toast_16437": found 61942 removable, 0 nonremovable row versions in 15486 out of 15486 pages
> > >> DETAIL: 0 dead row versions cannot be removed yet.
> > > S2: SELECT hashtext(largecol), length(largecol) FROM toasted;
> > > ERROR: XX000: missing chunk number 0 for toast value 16455 in pg_toast_16437
> > > LOCATION: toast_fetch_datum, tuptoaster.c:1945
> >
> > Thanks! That's something I should be able to work with.
> > Unfortunately, I am going to be on vacation, so I won't have any
> > results until sometime after 28 June.
>
> This PostgreSQL 9.6 open item is past due for your status update. Kindly send
> a status update within 24 hours, and include a date for your subsequent status
> update. Refer to the policy on open item ownership:
> http://www.postgresql.org/message-id/20160527025039.GA447393@tornado.leadboat.com

IIRC Kevin is out of the office this week, so this'll have to wait till
next week.

Andres