Message-ID: <4DED4A87.7000901@agliodbs.com>
Date: Mon, 06 Jun 2011 14:45:43 -0700
From: Josh Berkus <josh@agliodbs.com>
Organization: PostgreSQL Experts Inc.
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To: pgsql-performance@postgresql.org
Subject: 8.4/9.0 simple query performance regression
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All,

Just got this simple case off IRC today:

8.4.4
This plan completes in 100ms:

old_prod=# explain analyze select email from u_contact where id not in
(select contact_id from u_user);
                                                     QUERY PLAN

--------------------------------------------------------------------------------------------------------------------
 Seq Scan on u_contact  (cost=2217.72..4759.74 rows=35560 width=22)
(actual time=61.283..107.169 rows=4521 loops=1)
   Filter: (NOT (hashed SubPlan 1))
   SubPlan 1
     ->  Seq Scan on u_user  (cost=0.00..2051.38 rows=66538 width=8)
(actual time=0.034..33.303 rows=66978 loops=1)
 Total runtime: 108.001 ms


9.0.2
This plan does not complete in 15 minutes or more:

new_prod=# explain select email from u_contact where id not in (select
contact_id from u_user);
                                QUERY PLAN
---------------------------------------------------------------------------
 Seq Scan on u_contact  (cost=0.00..100542356.74 rows=36878 width=22)
   Filter: (NOT (SubPlan 1))
   SubPlan 1
     ->  Materialize  (cost=0.00..2552.56 rows=69504 width=8)
           ->  Seq Scan on u_user  (cost=0.00..1933.04 rows=69504 width=8)
(5 rows)

I'm at a bit of a loss as to what's happening here.  I'd guess another
failure of a bail-out-early plan, but I can't see how that would work
with this query.

-- 
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com