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

On Wed, Oct 14, 2020 at 5:43 PM Julien Rouhaud <rjuju123@gmail.com> wrote:
>
> On Tue, Oct 13, 2020 at 4:53 AM Bruce Momjian <bruce@momjian.us> wrote:
> >
> > On Mon, Oct 12, 2020 at 04:07:30PM -0400, Tom Lane wrote:
> > > Bruce Momjian <bruce@momjian.us> writes:
> > > > On Mon, Oct 12, 2020 at 02:26:15PM -0400, Tom Lane wrote:
> > > >> Yeah, I agree --- a version number is the wrong way to think about this.
> > >
> > > > The version number was to invalidate _all_ query hashes if the
> > > > algorithm is slightly modified, rather than invalidating just some of
> > > > them, which could lead to confusion.
> > >
> > > Color me skeptical as to the use-case for that.  From users' standpoints,
> > > the hash is mainly going to change when we change the set of parse node
> > > fields that get hashed.  Which is going to happen at every major release
> > > and no (or at least epsilon) minor releases.  So I do not see a point in
> > > tracking an algorithm version number as such.  Seems like make-work.
> >
> > OK, I came up with the hash idea only to address one of your concerns
> > about mismatched hashes for algorithm improvements/changes.  Seems we
> > might as well just document that cross-version hashes are different.
>
> Ok, so I tried to implement what seems to be the consensus.  First
> attached patch moves the current pgss queryid computation in core,
> with a new compute_queryid GUC (on/off).  One thing I don't really
> like about this patch is that the JumbleState that pgss needs in order
> to normalize the query string (the constants location and such) has to
> be done by the core while computing the queryid and provided to pgss
> in post_parse_analyse hook.  That isn't ideal as it looks very
> specific to pgss needs.  On the other hand it means that you can now
> use pgss with custom queryid heuristics by disabling compute_queryid
> and having your module doing only that in post_parse_analyse_hook.
> You'll however need to be careful to configure
> shared_preload_libraries such that your custom module's
> post_parse_analyse_hook is called first, so pgss' one can be called
> with the needed JumbleState.  Note that if no JumbleState is provided
> pgss will store non normalized queries, but will otherwise behave as
> intended.
>
> The 2nd patch is the rebased original queryid exposure patch.  No big
> changes, except that it now handles utility statements queryid
> generated during post_parse_analysis, same as regular queries.  This
> should simplify the work needed for custom queryid third party
> modules.
>
> The 3rd patch changes explain (verbose) to display the queryid if one
> has been generated, whether by core or a third-party module.  For
> instance:
>
> rjuju=# set compute_queryid = on;
> SET
> rjuju=# explain (verbose) select relname from pg_class;
>                               QUERY PLAN
> -----------------------------------------------------------------------
>  Seq Scan on pg_catalog.pg_class  (cost=0.00..16.90 rows=390 width=64)
>    Output: relname
>  Query Identifier: -5494854185674379299
> (3 rows)

There was a possibly uninitialized var issue in the previous patches
(thanks cfbot), v13 fixes that.