On Tue, Jan 5, 2010 at 4:33 PM, Brian Cox <brian.cox@xxxxxx> wrote:
> comparing this to the 1st explain foo output shows some minor differences in
> row estimates -- but nothing, I assume, that could explain the huge time
> difference. Of course, the 1st plan may not (and probably? wasn't) the plan
> that was used to take 124M ms.
>
> Any thoughts on how to avoid this?
The incorrect row estimates can only foul up the plan; they can't
directly make anything slow. Comparing the two plans line by line,
the only difference I see is the fast plan has:
-> Seq Scan on ts_stats_transetgroup_user_weekly b
(cost=0.00..23787.37 rows=89590 width=24) (actual time=0.040..295.414
rows=89758 loops=1)
Filter: ((ts_interval_start_time >= '2009-12-28
00:00:00-08'::timestamp with time zone) AND (ts_interval_start_time <
'2010-01-04 00:00:00-08'::timestamp with time zone))
...while the slow one has:
Index Scan using ts_stats_transetgroup_user_weekly_starttimeindex on
ts_stats_transetgroup_user_weekly b (cost=0.00..1301.21 rows=898
width=24)
Index Cond: ((ts_interval_start_time >=
$3) AND (ts_interval_start_time < $4))
So it looks like using that index to fetch the data is a LOT slower
than just scanning the whole table. In terms of fixing this problem,
I have two ideas:
- If you don't have any queries where this index makes things faster,
then you can just drop the index.
- If you have other queries where this index helps (even though it is
hurting this one), then you're going to have to find a way to execute
the query without using bound parameters - i.e. with the actual values
in there instead of $1 through $4. That will allow the planner to see
that the index scan is a loser because it will see that there are a
lot of rows in the specified range of ts_interval_start_times.
...Robert
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