Re: compute_query_id and pg_stat_statements

st 12. 5. 2021 v 9:13 odesílatel Julien Rouhaud <rjuju123(at)gmail(dot)com> napsal:

> On Wed, May 12, 2021 at 08:58:45AM +0200, Pavel Stehule wrote:
> >
> > I don't like the idea of implicit force enabling any feature flag, but it
> > is better than current design. But it doesn't look like a robust
> solution.
> >
> > Does it mean that if somebody disables computed_query_id, then
> > pg_stat_statements will not work?
>
> It depends, but if you mean "setting up pg_stat_statements, intentionally
> disabling in-core queryid calculation and not configuring an alternative
> source" then yes pg_stat_statements will not work. But I don't see any
> difference from "someone reduce wal_level and complain that replication
> does
> not work" or "someone disable fsync and complain that data got
> corrupted". We
> provide a sensible default configuration, you can mess it up if you don't
> know
> what you're doing.
>

> > Why is there the strong dependency between computed_query_id and
> > pg_stat_statements? Can this dependency be just optional?
>
> Once again no, as it otherwise would mean that postgres unilaterally
> decides
> that pg_stat_statements' approach to compute a query identifier is the one
> and
> only ultimate truth and nothing else could be useful for anyone.
>

ok. Understand.

If I understand well, then computed_query_id does not make sense for
pg_stat_statemenst, because this extension always requires it.

Cannot be better to use queryid inside pg_stat_statements every time
without dependency on computed_query_id? And computed_query_id can be used
only for EXPLAIN and for pg_stat_activity.

pg_stat_statements cannot work without a queryid, so is useless to speak
about configuration. If you use pg_stat_statements, then the queryid will
be computed every time, but the visibility will be only for
pg_stat_statements.

Or a different strategy. I understand so computed_query_id should be
active. But I dislike the empty result of pg_stat_statements when
computed_query_id is off. Is it possible to raise an exception instead of
showing an empty result?

The most correct fix from my perspective is just check in function
pg_stat_statements if query id is computed or not. If not, and there is no
data, then raise an exception with the hint "enable compute_query_id". When
there is data, then show a warning with the mentioned hint and show data.

What do you think about it?

Pavel