Re: Failed Assert in pgstat_assoc_relation

Hi,

On 2022-11-28 13:37:16 -0500, Tom Lane wrote:
> vignesh C <vignesh21(at)gmail(dot)com> writes:
> > I could reproduce this issue with the following steps:
> > create table t1(c int);
> > BEGIN;
> > CREATE TABLE t (c int);
> > SAVEPOINT q;
> > CREATE RULE "_RETURN" AS ON SELECT TO t DO INSTEAD SELECT * FROM t1;
> > select * from t;
> > ROLLBACK TO q;
> > CREATE RULE "_RETURN" AS ON SELECT TO t DO INSTEAD SELECT * FROM t1;
> > ROLLBACK;
>
> Uh-huh. I've not bothered to trace this in detail, but presumably
> what is happening is that the first CREATE RULE converts the table
> to a view, and then the ROLLBACK undoes that so far as the catalogs
> are concerned, but probably doesn't undo related pg_stats state
> changes fully. Then we're in a bad state that will cause problems.
> (It still crashes if you replace the second CREATE RULE with
> "select * from t".)

Yea. I haven't yet fully traced through this, but presumably relcache inval
doesn't fix this because we don't want to loose pending stats after DDL.

Perhaps we need to add a rule about not swapping pgstat* in
RelationClearRelation() when relkind changes?

> As far as HEAD is concerned, maybe it's time to nuke the whole
> convert-table-to-view kluge entirely? Only pg_dump older than
> 9.4 will emit such code, so we're really about out of reasons
> to keep on maintaining it.

Sounds good to me.

> However, I'm not sure that removing that code in v15 will fly,

Agreed, at the very least that'd increase memory usage.

> so maybe we need to make the new pg_stats code a little more
> robust against the possibility of a relkind change.

Possibly via the relcache code.

Greetings,

Andres Freund