Re: Why is query selecting sequential?

Karl Denninger <karl@denninger.net> writes:
> akcs=> explain analyze select forum, (replied > (select lastview from forumlog where forumlog.login='genesis' and
forumlog.forum='General'and number=post.number)) as newflag, * from post where forum = 'General' and toppost = 1 order
bypinned desc, replied desc;                
>                                                          QUERY PLAN
      
>
----------------------------------------------------------------------------------------------------------------------------
>  Sort  (cost=28.41..28.42 rows=6 width=218) (actual time=0.677..0.698 rows=5 loops=1)
>    Sort Key: pinned, replied
>    ->  Index Scan using post_toppost on post  (cost=0.00..28.33 rows=6 width=218) (actual time=0.403..0.606 rows=5
loops=1)
>          Index Cond: ((forum = 'General'::text) AND (toppost = 1))
>          SubPlan
>            ->  Seq Scan on forumlog  (cost=0.00..1.18 rows=1 width=8) (actual time=0.015..0.027 rows=1 loops=5)
>                  Filter: ((login = 'genesis'::text) AND (forum = 'General'::text) AND (number = $0))
>  Total runtime: 0.915 ms
> (8 rows)

As noted elsewhere, the inner subplan will not switch over to an
indexscan until you get some more data in that table.  Note however that
the subplan is only accounting for about 0.13 msec (0.027*5) so it's not
the major cost here anyway.  The slow part seems to be the indexed fetch
from "post", which is taking nearly 0.5 msec to fetch five rows.

            regards, tom lane