From: =?ISO-8859-1?Q?Gunnlaugur_=DE=F3r_Briem?= <gunnlaugur@gmail.com>
Subject: Re: Constraint exclusion on UNION ALL subqueries with WHERE
 conditions
Date: Tue, 20 Sep 2011 02:15:30 -0700 (PDT)
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To: pgsql-performance@postgresql.org

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;

                                        QUERY PLAN                                        
--------------------------------------------------------------------------
 Result  (cost=0.00..70.04 rows=4 width=4)
   ->  Append  (cost=0.00..70.04 rows=4 width=4)
         ->  Subquery Scan v_heavy_view  (cost=0.00..35.00 rows=2 width=4)
               Filter: ((v_heavy_view.col < 3) AND (v_heavy_view.col = 2))
               ->  Function Scan on generate_series  (cost=0.00..20.00 rows=1000 width=0)
         ->  Subquery Scan v_heavy_view  (cost=0.00..35.00 rows=2 width=4)
               Filter: ((v_heavy_view.col >= 3) AND (v_heavy_view.col = 2))
               ->  Function Scan on generate_series  (cost=0.00..20.00 rows=1000 width=0)

I want the planner to notice that (v_heavy_view.col >= 3) AND (v_heavy_view.col = 2) can never be satisfied, and skip that subquery.

Regards,

- Gulli