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

On 2016-06-16 13:53:01 -0500, Kevin Grittner wrote:
> On Thu, Jun 16, 2016 at 1:19 PM, Kevin Grittner <kgrittn(at)gmail(dot)com> wrote:
> > On Thu, Jun 16, 2016 at 11:54 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> >> On 2016-06-16 12:43:34 -0400, Robert Haas wrote:
>
> >>> Maybe it would help if you lay out the whole sequence of events, like:
> >>>
> >>> S1: Does this.
> >>> S2: Does that.
> >>> S1: Now does something else.
> >>
> >> I presume it'd be something like:
> >>
> >> Assuming a 'toasted' table, which contains one row, with a 1GB field.
> >>
> >> S1: BEGIN REPEATABLE READ;
> >> S1: SELECT SUM(length(one_gb_record)) FROM toasted;
> >> S2: DELETE FROM toasted;
> >> AUTOVAC: vacuum toasted's toast table, it's large. skip toasted, it's small
> >> S1: SELECT SUM(length(one_gb_record)) FROM toasted;
> >> <missing chunk error>
> >
> > I'll put together a test like that and post in a bit.
>
> old_snapshot_threshold = '1min'
> autovacuum_vacuum_threshold = 0\
> autovacuum_vacuum_scale_factor = 0.0000000001
>
> test=# CREATE TABLE gb (rec bytea not null);
> CREATE TABLE
> test=# ALTER TABLE gb ALTER COLUMN rec SET STORAGE external;
> ALTER TABLE
> test=# INSERT INTO gb SELECT t FROM (SELECT repeat('x',
> 1000000000)::bytea) x(t);
> INSERT 0 1
> test=# BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
> BEGIN
> test=# SELECT SUM(length(rec)) FROM gb;
> sum
> ------------
> 1000000000
> (1 row)
>
> [wait for autovacuum to run]
>
> test=# SELECT SUM(length(rec)) FROM gb;
> ERROR: snapshot too old

See https://www.postgresql.org/message-id/20160616183207.wygoktoplycdzav7@alap3.anar
for a recipe that reproduce the issue. I presume your example also
vacuums the main table due to the threshold and scale factor you set
(which will pretty much alwasy vacuum a table, no?).

Andres Freund