Re: [HACKERS] PATCH: multivariate histograms and MCV lists

On Sat, 16 Mar 2019 at 23:44, Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:
> >
> > 16). This regression test fails for me:
> >
> > @@ -654,11 +654,11 @@
> >  -- check change of unrelated column type does not reset the MCV statistics
> >  ALTER TABLE mcv_lists ALTER COLUMN d TYPE VARCHAR(64);
> >  SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a =
> > 1 AND b = ''1''');
> >   estimated | actual
> >  -----------+--------
> > -        50 |     50
> > +        11 |     50
> >  (1 row)
> >
> > Maybe that's platform-dependent, given what you said about
> > reltuples/relpages being reset. An easy workaround for this would be
> > to modify this test (and perhaps the one that follows) to just query
> > pg_statistic_ext to see if the MCV statistics have been reset.
> >
>
> Ah, sorry for not explaining this bit - the failure is expected, due to
> the reset of relpages/reltuples I mentioned. We do keep the extended
> stats, but the relsize estimate changes a bit. It surprised me a bit,
> and this test made the behavior apparent. The last patchset included a
> piece that changes that - if we decide not to change this, I think we
> can simply accept the actual output.
>

I don't think changing the way reltuples is reset ought to be within
the scope of this patch. There might be good reasons for it being the
way it is. Perhaps open a discussion on a separate thread?

As far as this test goes, how about just doing this:

-- check change of unrelated column type does not reset the MCV statistics
ALTER TABLE mcv_lists ALTER COLUMN d TYPE VARCHAR(64);
SELECT stxmcv IS NOT NULL AS has_mcv
  FROM pg_statistic_ext WHERE stxrelid = 'mcv_lists'::regclass;

-- check change of column type resets the MCV statistics
ALTER TABLE mcv_lists ALTER COLUMN c TYPE numeric;
SELECT stxmcv IS NOT NULL AS has_mcv
  FROM pg_statistic_ext WHERE stxrelid = 'mcv_lists'::regclass;

Regards,
Dean