Re: [HACKERS] Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

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.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company