Re: Multivariate MCV list vs. statistics target

From: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
To: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
Cc: PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Multivariate MCV list vs. statistics target
Date: 2019-06-29 10:41:21
Message-ID: 20190629104121.xw4qmpzlhy4e2r4g@development
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On Fri, Jun 21, 2019 at 08:09:18AM +0100, Dean Rasheed wrote:
>On Thu, 20 Jun 2019 at 23:12, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>> On Thu, Jun 20, 2019 at 08:08:44AM +0100, Dean Rasheed wrote:
>> >On Tue, 18 Jun 2019 at 22:34, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>> >>
>> >> So I'm thinking we should allow tweaking the statistics for extended
>> >> stats, and serialize it in the pg_statistic_ext catalog. Any opinions
>> >> why that would be a bad idea?
>> >
>> >Seems reasonable to me. This might not be the only option we'll ever
>> >want to add though, so perhaps a "stxoptions text[]" column along the
>> >lines of a relation's reloptions would be the way to go.
>> I don't know - I kinda dislike the idea of stashing stuff like this into
>> text[] arrays unless there's a clear need for such flexibility (i.e.
>> vision to have more such options). Which I'm not sure is the case here.
>> And we kinda have a precedent in pg_attribute.attstattarget, so I'd use
>> the same approach here.
>Hmm, maybe. I can certainly understand your dislike of using text[].
>I'm not sure that we can confidently say that multivariate statistics
>won't ever need additional tuning knobs, but I have no idea at the
>moment what they might be, and nothing else has come up so far in all
>the time spent considering MCV lists and histograms, so maybe this is

OK, attached is a patch implementing this - it adds


modifying a new stxstattarget column in pg_statistic_ext catalog,
following the same logic as pg_attribute.attstattarget.

During analyze, the per-ext-statistic value is determined like this:

1) When pg_statistic_ext.stxstattarget != (-1), we just use this value
and we're done.

2) Otherwise we inspect per-column attstattarget values, and use the
largest value. This is what we do now, so it's backwards-compatible

3) If the value is still (-1), we use default_statistics_target.


Tomas Vondra
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Attachment Content-Type Size
alter-statistics-v1.patch text/plain 22.3 KB

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