Re: compute_query_id and pg_stat_statements

On Wed, May 12, 2021 at 09:51:26AM +0200, Pavel Stehule wrote:
> 
> If I understand well, then computed_query_id does not make sense for
> pg_stat_statemenst, because this extension always requires it.

No, pg_stat_statements requires *a* queryid, not specifially *our* queryid.

> 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.

No, because then you will have a discrepancy between those two.  And if you
want a different queryid approach (say based on object names rather than oid so
it survives logical replication), then you also want that queryid used for
pg_stat_statements.  And that what happen is that you have to fork
pg_stat_statements to only change the queryid implementation, which is one of
the thing that the patch to move the implementation to core solves.

> 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.

Yes, pg_stat_statements cannot work without a queryid, but it CAN work without
core queryid.

> 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?

Yes, but I don't think that it's a good idea.  For instance pg_stat_statements
will behave poorly if you have to regularly evict entry.  For instance: any
query touching a temporary table.  One way to avoid that it to avoid storing
entries that you know are very likely to be eventually evicted.

So to fix this problem, you have 2 ways to go:

1) fix your app and explicitly disable/enable pg_stat_statements around all
  those queries, and hope you won't miss any

2) write your own queryid implementation to not generate a queryid in such case.

2 seems like a reasonable scenario, and if you force pg_stat_statements to
error out in that case then you would be forced to use approach 1.