Re: Feature improvement: can we add queryId for pg_catalog.pg_stat_activity view?

On Sun, Oct 18, 2020 at 4:12 PM Julien Rouhaud <rjuju123@gmail.com> wrote:
>
> On Sun, Oct 18, 2020 at 12:20 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >
> > Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
> > > Wait ... what?  I've been thinking that this GUC is just to enable or
> > > disable the computation of query ID, not to change the algorithm to do
> > > so.  Do we really need to allow different algorithms in different
> > > sessions?
> >
> > We established that some time ago, no?
>
> I thought we established the need for allowing different algorithms,
> but I assumed globally not per session.  Anyway, allowing to enable or
> disable compute_queryid per session would technically allow that,
> assuming that you have another module loaded that computes a queryid
> only if no-one was already computed.  In that case pg_stat_statements
> works as you would expect, you will get a new entry, with a duplicated
> query text.
>
> With a bit more thinking, there's at least one use case where it's
> interesting to disable pg_stat_statements: queries using temporary
> tables.  In that case you're guaranteed to generate an infinity of
> different queryid.  That doesn't really help since you're not
> aggregating anything anymore, and it also makes pg_stat_statements
> virtually unusable as once you have a workload that needs frequent
> eviction, the overhead is so bad that you basically have to disable
> pg_stat_statements.  We could alternatively add a GUC to disable
> queryid computation when one of the tables is a temporary table, but
> that's yet one among many considerations that are probably best
> answered with a custom implementation.
>
> I'm also attaching an updated patch with some attempt to improve the
> documentation.  I mention that in-core algorithm may not suits
> everyone's needs, but we don't actually document what heuristics are.
> Should we give more details on them and what are the most direct
> consequences?

v15 that fixes recent conflicts.