Re: Sequential scan on FK join

Martin Nickel wrote:
> EXPLAIN SELECT m.mailcode, l.lead_id
>   FROM mailing m
>  INNER JOIN lead l ON m.mailing_id = l.mailing_id
>  WHERE (m.maildate >= '2005-7-01'::date
>          AND m.maildate < '2005-8-01'::date)
>
> Hash Join  (cost=62.13..2001702.55 rows=2711552 width=20)
>   Hash Cond: ("outer".mailing_id = "inner".mailing_id)
>   ->  Seq Scan on lead l  (cost=0.00..1804198.60 rows=34065260 width=8)
>   ->  Hash  (cost=61.22..61.22 rows=362 width=20)
>         ->  Index Scan using mailing_maildate_idx on mailing m  (cost=0.00..61.22 rows=362 width=20)
>               Index Cond: ((maildate >= '2005-07-01'::date) AND (maildate < '2005-08-01'::date))

Well the reason *why* is that the planner expects 2.71 million rows to
be matched. If that was the case, then a seq-scan of 34 million rows
might well make sense. The output from EXPLAIN ANALYSE would show us
whether that estimate is correct - is it?

--
   Richard Huxton
   Archonet Ltd