Date: Fri, 26 Aug 2005 02:27:09 +0200
From: Tobias Brox <tobias@nordicbet.com>
To: pgsql-performance@postgresql.org
Subject: Limit + group + join
Message-ID: <20050826002709.GK10328@tobias.lan>
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Organization: Group Nordicbet
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Consider this setup - which is a gross simplification of parts of our
production system ;-)

  create table c (id integer primary key);
  create table b (id integer primary key, c_id integer);
  create index b_on_c on b(c_id)

  insert into c (select ... lots of IDs ...);
  insert into b (select id, id from c); /* keep it simple :-) */
  
Now, I'm just interessted in some few rows.  

All those gives good plans:

explain select c.id from c order by c.id limit 1;
explain select c.id from c group by c.id order by c.id limit 1;
explain select c.id from c join b on c_id=c.id order by c.id limit 1;

... BUT ... combining join, group and limit makes havoc:

explain select c.id from c join b on c_id=c.id  group by c.id order by c.id
desc limit 5;
                                     QUERY PLAN                                      
-------------------------------------------------------------------------------------
 Limit  (cost=3809.65..3809.67 rows=5 width=4)
   ->  Group  (cost=3809.65..3940.59 rows=26187 width=4)
         ->  Sort  (cost=3809.65..3875.12 rows=26188 width=4)
               Sort Key: c.id
               ->  Hash Join  (cost=559.34..1887.89 rows=26188 width=4)
                     Hash Cond: ("outer".id = "inner".c_id)
                     ->  Seq Scan on c  (cost=0.00..403.87 rows=26187 width=4)
                     ->  Hash  (cost=403.87..403.87 rows=26187 width=4)
                           ->  Seq Scan on b  (cost=0.00..403.87 rows=26187 width=4)
(9 rows)

I get the same behaviour on pg 7.4.7 and pg 8.0.2.  Of course, I can
probably use subqueries instead of join - though, I would have wished the
planner could do better ;-)

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