Re: compute_query_id and pg_stat_statements

On Thu, May 13, 2021 at 08:52:36AM +0800, Julien Rouhaud wrote:
> On Wed, May 12, 2021 at 08:36:18PM -0400, Bruce Momjian wrote:
> > The problem with compute_query_id=auto is that there is no way to know
> > if the query id is actually enabled, unless you guess from the installed
> > extensions, or we add another variable to report that, and maybe another
> > variable to control the provier, unless we require turning
> > compute_query_id=off if you are using custom query id computation.  What
> > if it is auto, and pg_stat_statments is installed, and you want to use a
> > custom query id computation --- what happens?  As you can see, this is
> > all becoming very complicated.
> 
> Well, as implemented you can get the value of compute_query_id, and if it's
> still "auto" then it's not enabled as calling queryIdWanted() would turn it to
> on.  I agree that it's not ideal but you have a way to know.  We could document
> that auto means that it's set to auto and no one asked to automatically enabled
> it.

Wow, so the extension changes it?  How do we record the "source" of that
change?  Do we have other GUCs that do that?

> Or you can just do e.g.
> 
> SELECT query_id FROM pg_stat_activity WHERE pid = pg_backend_pid();
> 
> and see if you have a query_id or not.

True.

> If you want to use third-party modules, they you have to explicitly disable
> compute_query_id.  If you don't, every query execution will raise an error as
> we documented that third-party modules should error out if they see that a
> query_id is already generated.  Such module could also explicitly check that
> compute_query_id is off and also raise an error if that's not the case.

OK.

> > I think we might be just as well to go with compute_query_id=on/off, and
> > just complain loudly from CREATE EXTENSION, or in the server logs on
> > server start via shared_preload_libraries, or when querying
> > pg_stat_statements system view.  We simply say to change
> > compute_query_id=on or to provide a custom query id implementation.
> 
> I'm not opposed to that, but it was already suggested and apparently people
> didn't like that approach.

Also probably true.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  If only the physical world exists, free will is an illusion.