Re: Strange planner decision on quite simple select

Markus Wollny wrote:
> Hello!
>
> I've got a table BOARD_MESSAGE (message_id int8, thread_id int8, ...)
> with pk on message_id and and a non_unique not_null index on thread_id.
> A count(*) on BOARD_MESSAGE currently yields a total of 1231171 rows,
> the planner estimated a total of 1232530 rows in this table. I've got
> pg_autovacuum running on the database and run an additional nightly
> VACUUM ANALYZE over it every night.
>
> I've got a few queries of the following type:
>
> select          *
>                                 from PUBLIC.BOARD_MESSAGE
>                                 where THREAD_ID = 3354253
>                                 order by        MESSAGE_ID asc
>                                 limit           20
>                                 offset          0;
>
>
> There are currently roughly 4500 rows with this thread_id in
> BOARD_MESSAGE. Explain-output is like so:
>
>                                                       QUERY PLAN
>
> ------------------------------------------------------------------------
> ----------------------------------------------
>  Limit  (cost=0.00..3927.22 rows=20 width=1148)
>    ->  Index Scan using pk_board_message on board_message
> (cost=0.00..1100800.55 rows=5606 width=1148)
>          Filter: (thread_id = 3354253)
> (3 rows)
>
> I didn't have the patience to actually complete an explain analyze on
> that one - I cancelled the query on several attempts after more than 40
> minutes runtime. Now I fiddled a little with this statement and tried
> nudging the planner in the right direction like so:

Hmm - it shouldn't take that long. If I'm reading this right, it's
expecting to have to fetch 5606 rows to match thread_id=3354253 the 20
times you've asked for. Now, what it probably doesn't know is that
thread_id is correlated with message_id quite highly (actually, I don't
know that, I'm guessing). So - it starts at message_id=1 and works
along, but I'm figuring that it needs to reach message_id's in the 3-4
million range to see any of the required thread.

Suggestions:
1. Try "ORDER BY thread_id,message_id" and see if that nudges things
your way.
2. Keep #1 and try replacing the index on (thread_id) with
(thread_id,message_id)

--
   Richard Huxton
   Archonet Ltd