Re: Should Oracle outperform PostgreSQL on a complex - Mailing list pgsql-performance

From Tom Lane
Subject Re: Should Oracle outperform PostgreSQL on a complex
Date
Msg-id 9010.1134845227@sss.pgh.pa.us
Whole thread Raw
In response to Re: Should Oracle outperform PostgreSQL on a complex  (Tom Lane <tgl@sss.pgh.pa.us>)
Responses Re: Should Oracle outperform PostgreSQL on a complex  ("Luke Lonergan" <llonergan@greenplum.com>)
List pgsql-performance
I wrote:
> However, I submit that it wouldn't pick such a plan anyway, and should
> not, because the idea is utterly stupid.

BTW, some experimentation suggests that in fact a star join is already
slower than the "regular" plan in 8.1.  You can force a star-join plan
to be generated like this:

regression=# set join_collapse_limit TO 1;
SET
regression=# explain select * from fact,d1 cross join d2 where fact.f1=d1.f1 and fact.f2=d2.f1;
                                QUERY PLAN
---------------------------------------------------------------------------
 Hash Join  (cost=4.71..8238.71 rows=102400 width=16)
   Hash Cond: (("outer".f1 = "inner".f1) AND ("outer".f2 = "inner".f1))
   ->  Seq Scan on fact  (cost=0.00..1578.00 rows=102400 width=8)
   ->  Hash  (cost=4.21..4.21 rows=100 width=8)
         ->  Nested Loop  (cost=1.11..4.21 rows=100 width=8)
               ->  Seq Scan on d1  (cost=0.00..1.10 rows=10 width=4)
               ->  Materialize  (cost=1.11..1.21 rows=10 width=4)
                     ->  Seq Scan on d2  (cost=0.00..1.10 rows=10 width=4)
(8 rows)

and at least in the one test case I tried, this runs slower than the
nested-hash plan.  EXPLAIN ANALYZE misleadingly makes it look faster,
but that's just because of the excessive per-plan-node ANALYZE
overhead.  Try doing something like

    \timing
    select count(*) from fact, ...

to get realistic numbers.

            regards, tom lane

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