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

From: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
To: Mark Dilger <hornschnorter(at)gmail(dot)com>
Cc: Adrien Nayrat <adrien(dot)nayrat(at)dalibo(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [HACKERS] PATCH: multivariate histograms and MCV lists
Date: 2017-11-25 17:56:03
Message-ID: ea799763-9729-826a-d1a2-0c5210e26482@2ndquadrant.com
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Hi,

On 11/25/2017 05:15 PM, Mark Dilger wrote:
>
>> On Nov 18, 2017, at 12:28 PM, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>>
>> Hi,
>>
>> Attached is an updated version of the patch, adopting the psql describe
>> changes introduced by 471d55859c11b.
>>
>> regards
>>
>> --
>> Tomas Vondra http://www.2ndQuadrant.com
>> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
>> <0001-multivariate-MCV-lists.patch.gz><0002-multivariate-histograms.patch.gz>
>
> Thanks, Tomas, again for your work on this feature.
>
> Applying just the 0001-multivariate-MCV-lists.patch to the current master, and
> then extending the stats_ext.sql test as follows, I am able to trigger an error,
> "ERROR: operator 4294934272 is not a valid ordering operator".
>

Ah, that's a silly bug ...

The code assumes that VacAttrStats->extra_data is always StdAnalyzeData,
and attempts to extract the ltopr from that. But for arrays that's of
course not true (array_typanalyze uses ArrayAnalyzeExtraData instead).

The reason why this only fails after the second INSERT is that we need
at least two occurrences of a value before considering it eligible for
MCV list. So after the first INSERT we don't even call the serialize.

Attached is a fix that should resolve this in MCV lists by looking up
the operator using lookup_type_cache() when serializing the MCV.

FWIW histograms have the same issue, but on more places (not just in
serialize, but also when building the histogram).

I'll send a properly updated patch series shortly, with tests checking
correct behavior with arrays.

Thanks for the report.

regards

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

Attachment Content-Type Size
0001-MCV-fix.patch text/x-patch 1.4 KB

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