Re: multivariate statistics v14

From: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
To: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
Cc: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: multivariate statistics v14
Date: 2016-03-09 18:18:10
Message-ID: 1457547490.24545.90.camel@2ndquadrant.com
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On Wed, 2016-03-09 at 18:21 +0100, Tomas Vondra wrote:
> Hi,
>
> On Wed, 2016-03-09 at 08:45 -0800, Jeff Janes wrote:
> > On Wed, Mar 9, 2016 at 7:02 AM, Tomas Vondra
> > <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
> > > Hi,
> > >
> > > thanks for the feedback. Attached is v14 of the patch series, fixing
> > > most of the points you've raised.
> >
> >
> > Hi Tomas,
> >
> > Applied to aa09cd242fa7e3a694a31f, I still get the seg faults in make
> > check if I configure without --enable-cassert.
>
> Ah, after disabling asserts I can reproduce it too. And the reason why
> it fails is quite simple - clauselist_selectivity modifies the original
> list of clauses, which then confuses cost_qual_eval.

More precisely, it gets confused because the first clause in the list
gets deleted but cost_qual_eval never learns about that, and follows
stale pointer to the next cell, thus a segfault.

>
> Can you try if the attached patch fixes the issue? I'll need to rework a
> bit more of the code, but let's see if this fixes the issue on your
> machine too.
>
> > With --enable-cassert, it passes the regression test.
>
> I wonder how can it work with casserts and fail without them. That's
> kinda exactly the opposite to what I'd expect ...

FWIW it seems to be somehow related to this assert in clausesel.c:

Assert(count_mv_attnums(list_union(stat_clauses, stat_conditions),
relid, MV_CLAUSE_TYPE_MCV | MV_CLAUSE_TYPE_HIST) >= 2);

With the assert in place, the code passes without a failure. After
removing the assert (commenting it out), or even just changing it to

Assert(count_mv_attnums(stat_clauses, relid,
MV_CLAUSE_TYPE_MCV | MV_CLAUSE_TYPE_HIST)
+ count_mv_attnums(stat_conditions, relid,
MV_CLAUSE_TYPE_MCV | MV_CLAUSE_TYPE_HIST) >= 2);

i.e. removing the list_union, it fails as expected.

The only thing that I can think of is that list_union happens to place
the right stuff at the right position in memory - pure luck.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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