Date: Sat, 25 Sep 2021 17:31:52 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: pgsql-hackers@postgresql.org
Subject: Re: extended stats on partitioned tables
Message-ID: <20210925223152.GA7877@telsasoft.com>
References: <20210923212624.GI831@telsasoft.com>
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(Resending with -hackers)

It seems like your patch should also check "inh" in examine_variable and
statext_expressions_load.

Which leads to another issue in stable branches:

ANALYZE builds only non-inherited stats, but they're incorrectly used for
inherited queries - the rowcount estimate is worse on inheritence parents with
extended stats than without.

 CREATE TABLE p(i int, j int);
 CREATE TABLE p1() INHERITS(p);
 INSERT INTO p SELECT a, a/10 FROM generate_series(1,9)a;
 INSERT INTO p1 SELECT a, a FROM generate_series(1,999)a;
 CREATE STATISTICS ps ON i,j FROM p;
 VACUUM ANALYZE p,p1;

postgres=# explain analyze SELECT * FROM p GROUP BY 1,2;
 HashAggregate  (cost=26.16..26.25 rows=9 width=8) (actual time=2.571..3.282 rows=1008 loops=1)

postgres=# begin; DROP STATISTICS ps; explain analyze SELECT * FROM p GROUP BY 1,2; rollback;
 HashAggregate  (cost=26.16..36.16 rows=1000 width=8) (actual time=2.167..2.872 rows=1008 loops=1)

I guess examine_variable() should have corresponding logic to the hardcoded
!inh in analyze.c.

-- 
Justin