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: 2018-03-26T10:31:01Z
Lists: pgsql-hackers 
On 18 March 2018 at 23:57, Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:
> Attached is an updated version of the patch series, addressing issues
> pointed out by Alvaro.

I'm just starting to look at this now, and I think I'll post
individual comments/questions as I get to them rather than trying to
review the whole thing, because it's quite a large patch. Apologies if
some of this has already been discussed.

Looking at the changes to UpdateStatisticsForTypeChange():

+   memset(nulls, 1, Natts_pg_statistic_ext * sizeof(bool));

why the "1" there -- is it just a typo?

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.

Consider, for example, what would happen if a column was changed from
real to int -- all the data values will be coerced to integers, losing
precision, and any ndistinct and dependency stats would likely be
completely wrong afterwards. IMO that's a bug, and should be
back-patched independently of these new types of extended stats.

Thoughts?

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