Have an interesting issue with a complex query, where apparently I need to
twist the query planner's arm, and am looking for advice on how to do so.
The situation: I have a table, events, with about 300,000 records.
It does an outer join to a second table, cases, with about 150,000 records.
A very simplified version query would be like
FROM events LEFT OUTER JOIN cases ON events.case_id = cases.case_id
WHERE events.event_date BETWEEN 'date1' AND 'date2'
This join is very expensive, as you can imagine. Yet I can't seem to force
the query planner to apply the filter conditions to the events table *before*
attempting to join it to cases. Here's the crucial explain lines:
-> Merge Left Join (cost=0.00..11880.82
rows=15879 width=213) (actual time=5.777..901.899 rows=648 loops=1)
Merge Cond: ("outer".case_id =
Join Filter: (("outer".link_type)::text
-> Index Scan using idx_event_ends on
events (cost=0.00..4546.15 rows=15879 width=80
) (actual time=4.144..333.769 rows=648 loops=1)
Filter: ((status <> 0) AND
((event_date + duration) >= '2004-02-18 00:00:00'::timestamp without time
zone) AND (event_date <= '2004-03-05 23:59:00'::timestamp without time zone))
-> Index Scan using cases_pkey on
cases (cost=0.00..6802.78 rows=117478 width=137) (
actual time=0.139..402.363 rows=116835 loops=1)
As you can see, part of the problem is a pretty drastic (20x) mis-estimation
of the selectivity of the date limits on events -- and results in 90% of the
execution time of my query on this one join. I've tried raising the
statistics on event_date, duration, and case_id (to 1000), but this doesn't
seem to affect the estimate or the query plan.
In the above test, idx_event_ends indexes (case_id, status, event_date,
(event_date + duration)), but as you can see the planner uses only the first
column. This was an attempt to circumvent the planner's tendency to
completely ignoring any index on (event_date, (event_date + duration)) --
even though that index is the most selective combination on the events table.
Is there anything I can do to force the query planner to filter on events
before joining cases, other than waiting for version 7.5?
Aglio Database Solutions
pgsql-performance by date
|Next:||From: Josh Berkus||Date: 2004-02-19 00:30:43|
|Subject: Re: Forcing filter/join order?|
|Previous:||From: Andrew Lazarus||Date: 2004-02-18 22:51:52|
|Subject: JOIN order, 15K, 15K, 7MM rows|