Re: full outer performance problem

On Wed, Jun 08, 2005 at 11:37:40 +0200,
Kim Bisgaard <kib+pg(at)dmi(dot)dk> wrote:
> Hi,
>
> I'm having problems with the query optimizer and FULL OUTER JOIN on
> PostgreSQL 7.4. I cannot get it to use my indexes with full outer joins.
> I might be naive, but I think that it should be possible?
>
> I have two BIG tables (virtually identical) with 3 NOT NULL columns
> Station_id, TimeObs, Temp_XXXX, with unique indexes on (Station_id,
> TimeObs) and valid ANALYSE (set statistics=100). I want to join the two
> tables with a FULL OUTER JOIN.
>
> When I specify the query as:
>
> SELECT station_id, timeobs,temp_grass, temp_dry_at_2m
> FROM temp_dry_at_2m a
> FULL OUTER JOIN temp_grass b
> USING (station_id, timeobs)
> WHERE station_id = 52981
> AND timeobs = '2004-1-1 0:0:0'
>
> I get the correct results
>
> station_id | timeobs | temp_grass | temp_dry_at_2m
> ------------+---------------------+------------+----------------
> 52944 | 2004-01-01 00:10:00 | | -1.1
> (1 row)
>
> BUT LOUSY performance, and the following EXPLAIN:
>
> QUERY PLAN
> ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> Merge Full Join (cost=1542369.83..1618958.58 rows=6956994 width=32)
> (actual time=187176.408..201436.264 rows=1 loops=1)
> Merge Cond: (("outer".station_id = "inner".station_id) AND
> ("outer".timeobs = "inner".timeobs))
> Filter: ((COALESCE("outer".station_id, "inner".station_id) = 52981) AND
> (COALESCE("outer".timeobs, "inner".timeobs) = '2004-01-01
> 00:00:00'::timestamp without time zone))
> -> Sort (cost=1207913.44..1225305.93 rows=6956994 width=16) (actual
> time=145748.253..153851.607 rows=6956994 loops=1)
> Sort Key: a.station_id, a.timeobs
> -> Seq Scan on temp_dry_at_2m a (cost=0.00..117549.94
> rows=6956994 width=16) (actual time=0.049..54226.770 rows=6956994
> loops=1)
> -> Sort (cost=334456.38..340472.11 rows=2406292 width=16) (actual
> time=31668.876..34491.123 rows=2406292 loops=1)
> Sort Key: b.station_id, b.timeobs
> -> Seq Scan on temp_grass b (cost=0.00..40658.92 rows=2406292
> width=16) (actual time=0.052..5484.489 rows=2406292 loops=1)
> Total runtime: 201795.989 ms
> (10 rows)

Someone else will need to comment on why Postgres can't use a more
efficient plan. What I think will work for you is to restrict
the station_id and timeobs on each side and then do a full join.
You can try something like the sample query below (which hasn't been tested):
SELECT station_id, timeobs, temp_grass, temp_dry_at_2m
FROM
(SELECT station_id, timeobs, temp_dry_at_2m
FROM temp_dry_at_2m
WHERE
station_id = 52981
AND
timeobs = '2004-1-1 0:0:0') a
FULL OUTER JOIN
(SELECT station_id, timeobs, temp_grass
FROM temp_grass
WHERE
station_id = 52981
AND
timeobs = '2004-1-1 0:0:0') b
USING (station_id, timeobs)