Re: compute_query_id and pg_stat_statements

On Sun, Apr 25, 2021 at 01:17:03PM -0400, Tom Lane wrote:
> Julien Rouhaud <rjuju123@gmail.com> writes:
> > On Sun, Apr 25, 2021 at 11:39:55AM -0400, Tom Lane wrote:
> >> I agree repeated warnings would be bad news.  I was wondering if we could
> >> arrange a single warning at the time pg_stat_statements is preloaded into
> >> the postmaster.  In this way, if you tried to use a configuration file
> >> that used to work, you'd hopefully get some notice about why it no longer
> >> does what you want.  Also, if you are preloading pg_stat_statements, it
> >> seems reasonable to assume that you'd like the global value of the flag
> >> to be "on", even if there are use-cases for transiently disabling it.
> 
> > What about people who wants to use pg_stat_statements but are not ok with our
> > query_id heuristics and use a third-party plugin for that?
> 
> They're still going to want the GUC set to something other than "off",
> no?

They will want compute_query_id to be off.  And they actually will *need* that,
as we recommend third-party plugins computing alternative query_id to error out
if they see a that a query_id has already been generated, to avoid any problem
if compute_query_id is being temporarily toggled.  That's what I did in the POC
plugin for external query_id at [1].

> In any case it's just a one-time log message, so it's not likely
> to be *that* annoying.

In that case it should be phrased in a way that makes it clear that
pg_stat_statements can work without enabling compute_query_id, something like:

"compute_query_id is disabled.  This module won't track any activity unless you
configured a third-party extension that computes query identifiers"

[1] https://github.com/rjuju/pg_queryid/blob/master/pg_queryid.c#L172