Re: compute_query_id and pg_stat_statements

On Wed, May 12, 2021 at 11:33:32PM -0400, Bruce Momjian wrote:
> On Thu, May 13, 2021 at 11:16:13AM +0800, Julien Rouhaud wrote:
> > On Wed, May 12, 2021 at 11:06:52PM -0400, Bruce Momjian wrote:
> > > On Thu, May 13, 2021 at 09:57:00AM +0800, Julien Rouhaud wrote:
> > > 
> > > > source?  What if you have for instance pg_stat_statements, pg_stat_kcache,
> > > > pg_store_plans and pg_wait_sampling installed?  All those extensions need a
> > > > query_id (or at least benefit from it for pg_wait_sampling), is there any value
> > > > to give a full list of all the modules that would enable compute_query_id?
> > > 
> > > Well, we don't have any other cases where the source of the change is
> > > this indeterminate, so I don't really have a suggestion.  I think this
> > > does highlight another case where 'auto' just isn't a good fit for our
> > > API.
> > 
> > It may depends on your next question
> > 
> > > > I'm assuming that anyone wanting to install any of those extensions (or any
> > > > similar one) is fully aware that they aggregate metrics based on at least a
> > > > query_id.  If they don't, well they probably never read any documentation since
> > > > postgres 9.2 which introduced query normalization, and I doubt that they will
> > > > be interested in having access to the information anyway.
> > > 
> > > My point is that we are changing a setting from an extension, and if you
> > > look in pg_setting, it will say "default"?
> > 
> > No, it will say "on".  What the patch I sent implements is:
> 
> I was asking what pg_settings.source will say:
> 
>     SELECT name, soource FROM pg_settings;

Oh sorry.  Here's the full output before and after a dynamic call to
queryIsWanted():

name            | compute_query_id
setting         | auto
unit            | <NULL>
category        | Statistics / Monitoring
short_desc      | Compute query identifiers.
extra_desc      | <NULL>
context         | superuser
vartype         | enum
source          | default
min_val         | <NULL>
max_val         | <NULL>
enumvals        | {auto,on,off}
boot_val        | auto
reset_val       | auto
sourcefile      | <NULL>
sourceline      | <NULL>
pending_restart | f

=# LOAD 'pg_qualstats'; /* will call queryIsWanted() */
WARNING:  01000: Without shared_preload_libraries, only current backend stats will be available.
LOAD

name            | compute_query_id
setting         | on
unit            | <NULL>
category        | Statistics / Monitoring
short_desc      | Compute query identifiers.
extra_desc      | <NULL>
context         | superuser
vartype         | enum
source          | default
min_val         | <NULL>
max_val         | <NULL>
enumvals        | {auto,on,off}
boot_val        | auto
reset_val       | auto
sourcefile      | <NULL>
sourceline      | <NULL>
pending_restart | f

So yes, it says "default" (and it's the same if the change is done during
postmaster init).  It can be fixed if that's the only issue.

> 
> > - compute_query_id = on means it was either explicitly set to on, or
> >   automatically set to on if it was allowed to (so initially set to auto).  It
> >   means you know whether core query_id calculation is enabled or not, you can
> >   know looking at the reset value it it was changed by an extension, you just
> >   don't know which one(s).
> > 
> > - compute_query_id = auto means that it can be set to on, it just wasn't yet,
> >   so it's off, and may change if you have an extension that can be dynamically
> >   loaded and request for core query_id calculation to be enabled
> 
> So, it is 'off', but not set to 'off' in the GUC sense --- just off as
> in not being computed.  You can see the confusion in my just reading
> that sentence.

It's technically not "off" but "not on yet", but that's probably just making it
worse :)

> How do they know to set shared_preload_libraries then?  We change the
> user API all the time, especially in GUCs, and even rename them, but for
> some reason we don't think people using pg_stat_statements can be
> trusted to read the release notes and change their behavior.  I just
> don't get it.

I don't know what to say.  So here is a summary of the complaints that I'm
aware of:

- https://www.postgresql.org/message-id/1953aec168224b95b0c962a622bef0794da6ff40.camel@moonset.ru
That was only a couple of days after the commit just before the feature freeze,
so it may be the less relevant complaint.

- https://www.postgresql.org/message-id/CAOxo6XJEYunL71g0yD-zRzNRRqBG0Ssw-ARygy5pGRdSjK5YLQ%40mail.gmail.com
Did a git bisect to find the commit that changed the behavior and somehow
didn't notice the new setting

- this thread, with Fuji-san saying:

> I'm afraid that users may easily forget to enable compute_query_id when using
> pg_stat_statements (because this setting was not necessary in v13 or before)

- this thread, with Peter E. saying:

> Now there is the additional burden of turning on this weird setting that
> no one understands.  That's a 50% increase in burden.  And almost no one will
> want to use a nondefault setting.  pg_stat_statements is pretty popular.  I
> think leaving in this requirement will lead to widespread confusion and
> complaints.

- this thread, with Pavel saying:

> Until now, the pg_stat_statements was zero-config. So the change is not user
> friendly.

So it's a mix of "it's changing something that didn't change in a long time"
and "it's adding extra footgun and/or burden as it's not doing by default what
the majority of users will want", with an overwhelming majority of people
supporting the "we don't want that extra burden".