From: | Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com> |
---|---|
To: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
Cc: | Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Michael Paquier <michael(at)paquier(dot)xyz>, Andres Freund <andres(at)anarazel(dot)de>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, Mark Dilger <hornschnorter(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: [HACKERS] PATCH: multivariate histograms and MCV lists |
Date: | 2019-03-17 12:14:15 |
Message-ID: | CAEZATCWK9j=tfzysBAgpU6KDZE8JyU8G_09r_GDUwhz42iqn6Q@mail.gmail.com |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-hackers |
On Sat, 16 Mar 2019 at 23:44, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)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
From | Date | Subject | |
---|---|---|---|
Next Message | Michael Paquier | 2019-03-17 12:34:36 | Re: Offline enabling/disabling of data checksums |
Previous Message | Dean Rasheed | 2019-03-17 11:47:47 | Re: [HACKERS] PATCH: multivariate histograms and MCV lists |