To: pgsql-hackers@postgreSQL.org
cc: pgsql-performance@postgreSQL.org
Subject: Re: [PERFORM] Constraint exclusion on UNION ALL subqueries with WHERE
	conditions
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Comments: In-reply-to =?ISO-8859-1?Q?Gunnlaugur_=DE=F3r_Briem?=
	<gunnlaugur@gmail.com>
	message dated "Tue, 20 Sep 2011 02:15:30 -0700"
Date: Wed, 21 Sep 2011 13:11:00 -0400
Message-ID: <25530.1316625060@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

=?ISO-8859-1?Q?Gunnlaugur_=DE=F3r_Briem?= <gunnlaugur@gmail.com> writes:
> On Monday, September 19, 2011 3:59:30 AM UTC, Tom Lane wrote:
>> Works for me in 8.4.8.  Do you have constraint_exclusion set to ON?

> I did try with constraint_exclusion set to on, though the docs suggest partition should be enough ("examine constraints only for ... UNION ALL subqueries")

> Here's a minimal test case (which I should have supplied in the original post, sorry), tried just now in 8.4.8:

> CREATE OR REPLACE VIEW v_heavy_view
> AS SELECT (random()*1e5)::integer col
> FROM generate_series(1, 1e6::integer);

> CREATE OR REPLACE VIEW v_test_constraint_exclusion AS
> SELECT col FROM v_heavy_view WHERE col < 3
> UNION ALL SELECT col FROM v_heavy_view WHERE col >= 3;

> EXPLAIN SELECT * FROM v_test_constraint_exclusion WHERE col=2;

Hmm.  The reason this particular case doesn't work is that we don't
apply relation_excluded_by_constraints() to functions-in-FROM.
It's only used for plain-table RTEs, not subqueries, functions,
etc.  I suspect the complainant's real case involved an unflattenable
subquery.

Probably the rationale for that coding was that only plain tables
could have CHECK constraints; but the portion of the logic that looks
for mutually contradictory scan constraints could apply to non-table
relations.

Should we change the code to make such checks in these cases?
The default behavior (with constraint_exclusion = partition) would
still be to do nothing extra, but it would add planning expense when
constraint_exclusion = on.

			regards, tom lane