Re: Oracle v. Postgres 9.0 query performance

pg_dw=# set enable_nestloop =0;
SET
Time: 0.165 ms
pg_dw=# explain CREATE TABLE ecr_opens with (FILLFACTOR=100)
pg_dw-# as
pg_dw-# select o.emailcampaignid, count(memberid) opencnt
pg_dw-# from openactivity o,ecr_sents s
pg_dw-# where s.emailcampaignid = o.emailcampaignid
pg_dw-# group by o.emailcampaignid;
QUERY
PLAN
-----------------------------------------------------------------------------------------
HashAggregate (cost=4391163.81..4391288.05 rows=9939 width=12)
-> Hash Join (cost=14.78..4344767.23 rows=9279316 width=12)
Hash Cond: (o.emailcampaignid = s.emailcampaignid)
-> Seq Scan on openactivity o (cost=0.00..3529930.67
rows=192540967 width=12)
-> Hash (cost=8.79..8.79 rows=479 width=4)
-> Seq Scan on ecr_sents s (cost=0.00..8.79 rows=479
width=4)

Yikes. Two sequential scans.

On Wed, 2011-06-08 at 11:33 -0400, Tom Lane wrote:
> Tony Capobianco <tcapobianco(at)prospectiv(dot)com> writes:
> > pg_dw=# explain CREATE TABLE ecr_opens with (FILLFACTOR=100)
> > pg_dw-# as
> > pg_dw-# select o.emailcampaignid, count(memberid) opencnt
> > pg_dw-# from openactivity o,ecr_sents s
> > pg_dw-# where s.emailcampaignid = o.emailcampaignid
> > pg_dw-# group by o.emailcampaignid;
> > QUERY
> > PLAN
> > -------------------------------------------------------------------------------------------------------------
> > GroupAggregate (cost=0.00..1788988.05 rows=9939 width=12)
> > -> Nested Loop (cost=0.00..1742467.24 rows=9279316 width=12)
> > -> Index Scan using ecr_sents_ecid_idx on ecr_sents s
> > (cost=0.00..38.59 rows=479 width=4)
> > -> Index Scan using openact_emcamp_idx on openactivity o
> > (cost=0.00..3395.49 rows=19372 width=12)
> > Index Cond: (o.emailcampaignid = s.emailcampaignid)
> > (5 rows)
>
> > Should this query be hashing the smaller table on Postgres rather than
> > using nested loops?
>
> Yeah, seems like it. Just for testing purposes, do "set enable_nestloop
> = 0" and see what plan you get then.
>
> regards, tom lane
>