Horton

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

Dean Rasheed <dean.a.rasheed@gmail.com>

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From: Dean Rasheed <dean.a.rasheed@gmail.com>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>, Andres Freund <andres@anarazel.de>, Thomas Munro <thomas.munro@enterprisedb.com>, Mark Dilger <hornschnorter@gmail.com>, Adrien Nayrat <adrien.nayrat@dalibo.com>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2018-03-26T16:21:06Z
Lists: pgsql-hackers 
On 26 March 2018 at 14:08, Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:
> On 03/26/2018 12:31 PM, Dean Rasheed wrote:
>> A wider concern I have is that I think this function is trying to be
>> too clever by only resetting selected stats. IMO it should just reset
>> all stats unconditionally when the column type changes, which would
>> be consistent with what we do for regular stats.
>>
> The argument a year ago was that it's more plausible that the semantics
> remains the same. I think the question is how the type change affects
> precision - had the type change in the opposite direction (int to real)
> there would be no problem, because both ndistinct and dependencies would
> produce the same statistics.
>
> In my experience people are far more likely to change data types in a
> way that preserves precision, so I think the current behavior is OK.

Hmm, I don't really buy that argument. Altering a column's type allows
the data in it to be rewritten in arbitrary ways, and I don't think we
should presume that the statistics will still be valid just because
the user *probably* won't do something that changes the data much.

Regards,
Dean

Commits

  1. Convert pre-existing stats_ext tests to new style

  2. Add support for multivariate MCV lists

  3. Improve ANALYZE's strategy for finding MCVs.

  4. Clone extended stats in CREATE TABLE (LIKE INCLUDING ALL)

  5. Try again to fix accumulation of parallel worker instrumentation.

  6. Adjust psql \d query to avoid use of @> operator.

  7. Message style fixes

  8. Add security checks to selectivity estimation functions