Thread: full outer performance problem
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 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:
If further info like EXPLAIN VERBOSE is useful please say so and I will provide it.
Thanks in advance!
Kim Bisgaard.
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 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 temp_max_60min,temp_dry_at_2m from station s natural join temp_dry_at_2m a full outer join temp_max_60min b using (station_id, timeobs) where s.wmo_id=6065 and timeobs='2004-1-1 0:0:0' and '2004-1-1 0:0:0' between s.startdate and s.enddate;I get the correct results, BUT LOUSY performance, and the following explain:
Nested Loop Left Join (cost=5.84..163484.08 rows=1349 width=12) (actual time=66146.815..119005.381 rows=1 loops=1) Filter: (COALESCE("outer".timeobs, "inner".timeobs) = '2004-01-01 00:00:00'::timestamp without time zone) -> Hash Join (cost=5.84..155420.24 rows=1349 width=16) (actual time=8644.449..110836.038 rows=109826 loops=1) Hash Cond: ("outer".station_id = "inner".station_id) -> Seq Scan on temp_dry_at_2m a (cost=0.00..120615.94 rows=6956994 width=16) (actual time=0.024..104548.515 rows=6956994 loops=1) -> Hash (cost=5.84..5.84 rows=1 width=4) (actual time=0.114..0.114 rows=0 loops=1) -> Index Scan using wmo_idx on station (cost=0.00..5.84 rows=1 width=4) (actual time=0.105..0.108 rows=1 loops=1) Index Cond: ((wmo_id = 6065) AND ('2004-01-01 00:00:00'::timestamp without time zone >= startdate) AND ('2004-01-01 00:00:00'::timestamp without time zone <= enddate)) -> Index Scan using temp_max_60min_idx on temp_max_60min b (cost=0.00..5.96 rows=1 width=20) (actual time=0.071..0.071 rows=0 loops=109826) Index Cond: (("outer".station_id = b.station_id) AND ("outer".timeobs = b.timeobs))Total runtime: 119005.499 ms
(11 rows)
If I change the query to (and thus negates the full outer join):select temp_max_60min,temp_dry_at_2m from station s natural join temp_dry_at_2m a full outer join temp_max_60min b using (station_id, timeobs) where s.wmo_id=6065 and a.timeobs='2004-1-1 0:0:0' and b.timeobs='2004-1-1 0:0:0' and '2004-1-1 0:0:0' between s.startdate and s.enddate;I get wrong results (In the case where one of the records is missing in one of the tables), BUT GOOD performance, and this query plan:
Nested Loop (cost=0.00..17.83 rows=1 width=12) (actual time=79.221..79.236 rows=1 loops=1) -> Nested Loop (cost=0.00..11.82 rows=1 width=24) (actual time=65.517..65.526 rows=1 loops=1) -> Index Scan using wmo_idx on station (cost=0.00..5.83 rows=1 width=4) (actual time=0.022..0.026 rows=1 loops=1) Index Cond: ((wmo_id = 6065) AND ('2004-01-01 00:00:00'::timestamp without time zone >= startdate) AND ('2004-01-01 00:00:00'::timestamp without time zone <= enddate)) -> Index Scan using temp_max_60min_idx on temp_max_60min b (cost=0.00..5.97 rows=1 width=20) (actual time=65.483..65.486 rows=1 loops=1) Index Cond: (("outer".station_id = b.station_id) AND (b.timeobs = '2004-01-01 00:00:00'::timestamp without time zone)) -> Index Scan using temp_dry_at_2m_idx on temp_dry_at_2m a (cost=0.00..6.00 rows=1 width=16) (actual time=13.694..13.698 rows=1 loops=1) Index Cond: (("outer".station_id = a.station_id) AND (a.timeobs = '2004-01-01 00:00:00'::timestamp without time zone))Total runtime: 79.340 ms
(9 rows)
If further info like EXPLAIN VERBOSE is useful please say so and I will provide it.
Thanks in advance!
Kim Bisgaard.
Kim Bisgaard 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 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 temp_max_60min,temp_dry_at_2m
> from station s natural join
> temp_dry_at_2m a full outer join temp_max_60min b using (station_id, timeobs)
> where s.wmo_id=6065
> and timeobs='2004-1-1 0:0:0'
> and '2004-1-1 0:0:0' between s.startdate and s.enddate;
>
> I get the correct results, BUT LOUSY performance, and the following explain:
>
> Nested Loop Left Join (cost=5.84..163484.08 rows=1349 width=12) (actual time=66146.815..119005.381 rows=1 loops=1)
> Filter: (COALESCE("outer".timeobs, "inner".timeobs) = '2004-01-01 00:00:00'::timestamp without time zone)
> -> Hash Join (cost=5.84..155420.24 rows=1349 width=16) (actual time=8644.449..110836.038 rows=109826 loops=1)
Well, the estimate here is quite a bit off. It thinks you will be
getting 1349 (which is probably why it picked a nested loop plan), but
then it is getting 109826 rows.
I'm guessing it is misunderstanding the selectivity of the timeobs column.
> Hash Cond: ("outer".station_id = "inner".station_id)
> -> Seq Scan on temp_dry_at_2m a (cost=0.00..120615.94 rows=6956994 width=16) (actual
time=0.024..104548.515rows=6956994 loops=1)
> -> Hash (cost=5.84..5.84 rows=1 width=4) (actual time=0.114..0.114 rows=0 loops=1)
> -> Index Scan using wmo_idx on station (cost=0.00..5.84 rows=1 width=4) (actual time=0.105..0.108
rows=1loops=1)
> Index Cond: ((wmo_id = 6065) AND ('2004-01-01 00:00:00'::timestamp without time zone >=
startdate)AND ('2004-01-01 00:00:00'::timestamp without time zone <= enddate))
> -> Index Scan using temp_max_60min_idx on temp_max_60min b (cost=0.00..5.96 rows=1 width=20) (actual
time=0.071..0.071rows=0 loops=109826)
> Index Cond: (("outer".station_id = b.station_id) AND ("outer".timeobs = b.timeobs))
> Total runtime: 119005.499 ms
> (11 rows)
I think the bigger problem is that a full outer join says grab all rows,
even if they are null.
What about this query:
SELECT temp_max_60min,temp_dry_at_2m
FROM (station s LEFT JOIN temp_dry_at_2m a USING (station_id, timeobs)
LEFT JOIN temp_max_60min b USING (station_id, timeobs)
where s.wmo_id=6065
and timeobs='2004-1-1 0:0:0'
and '2004-1-1 0:0:0' between s.startdate and s.enddate;
After that, you should probably have a multi-column index on
(station_id, timeobs), which lets postgres use just that index for the
lookup, rather than using an index and then a filter. (Looking at your
next query you might already have that index).
>
> If I change the query to (and thus negates the full outer join):
This is the same query, I think you messed up your copy and paste.
>
> select temp_max_60min,temp_dry_at_2m
> from station s natural join
> temp_dry_at_2m a full outer join temp_max_60min b using (station_id, timeobs)
> where s.wmo_id=6065
> and _a.timeobs='2004-1-1 0:0:0' and b._timeobs='2004-1-1 0:0:0'
> and '2004-1-1 0:0:0' between s.startdate and s.enddate;
>
>
> I get wrong results (In the case where one of the records is missing in
> one of the tables), BUT GOOD performance, and this query plan:
>
> Nested Loop (cost=0.00..17.83 rows=1 width=12) (actual time=79.221..79.236 rows=1 loops=1)
> -> Nested Loop (cost=0.00..11.82 rows=1 width=24) (actual time=65.517..65.526 rows=1 loops=1)
> -> Index Scan using wmo_idx on station (cost=0.00..5.83 rows=1 width=4) (actual time=0.022..0.026 rows=1
loops=1)
> Index Cond: ((wmo_id = 6065) AND ('2004-01-01 00:00:00'::timestamp without time zone >= startdate) AND
('2004-01-0100:00:00'::timestamp without time zone <= enddate))
> -> Index Scan using temp_max_60min_idx on temp_max_60min b (cost=0.00..5.97 rows=1 width=20) (actual
time=65.483..65.486rows=1 loops=1)
> Index Cond: (("outer".station_id = b.station_id) AND (b.timeobs = '2004-01-01 00:00:00'::timestamp
withouttime zone))
> -> Index Scan using temp_dry_at_2m_idx on temp_dry_at_2m a (cost=0.00..6.00 rows=1 width=16) (actual
time=13.694..13.698rows=1 loops=1)
> Index Cond: (("outer".station_id = a.station_id) AND (a.timeobs = '2004-01-01 00:00:00'::timestamp without
timezone))
> Total runtime: 79.340 ms
> (9 rows)
>
>
> If further info like EXPLAIN VERBOSE is useful please say so and I will
> provide it.
>
> Thanks in advance!
> Kim Bisgaard.
I still feel like you will have a problem with an outer join in this
circumstance, because it will have to scan all of both tables.
I think what you are wanting is "give me everything where station_id =
X, and there is a row in either a or b".
I think my LEFT JOIN example does that, but I also think there would be
a subselect form which would work, which might do better. Something like:
SELECT temp_max_60min,temp_dry_at_2m
FROM (SELECT station_id, timeobs FROM station s
WHERE s.wmo_id=6065
AND timeobs = '2004-1-1 0:0:0'
AND '2004-1-1 0:0:0' BETWEEN s.startdate AND s.enddate
) AS s
JOIN (SELECT temp_max_60min, temp_dry_at_2m
FROM temp_dry_at_2m a
FULL OUTER JOIN temp_max_60min b
USING (station_id, timeobs)
WHERE station_id = s.station_id
AND timeobs = '2004-1-1 0:0:0'
)
;
If I did this correctly, you should have a very restrictive scan done on
station, which only returns a few rows based on timeobs & station_id.
But it might be better to turn that final FULL OUTER JOIN into 2 LEFT
JOINs like I did the first time:
SELECT temp_max_60min,temp_dry_at_2m
FROM (SELECT station_id, timeobs FROM station s
WHERE s.wmo_id=6065
AND timeobs = '2004-1-1 0:0:0'
AND '2004-1-1 0:0:0' BETWEEN s.startdate AND s.enddate
) AS s
LEFT JOIN temp_dry_at_2m a USING (station_id, timeobs)
LEFT JOIN temp_max_60min b USING (station_id, timeobs)
;
I would hope postgres could do this from just my earlier plan. And I
hope I understand what you want, such that 2 LEFT JOINS work better than
your FULL OUTER JOIN. If you only want rows where one of both temp_dry
or temp_max exist, you probably could just add the line:
WHERE (temp_max_60_min IS NOT NULL OR temp_dry_at_2m IS NOT NULL)
John
=:->
Attachment
Kim Bisgaard <kib+pg@dmi.dk> writes:
> I have two BIG tables (virtually identical) with 3 NOT NULL columns
> Station_id, TimeObs, Temp_XXXX, with indexes on (Station_id, TimeObs)
> and valid ANALYSE (set statistics=100). I want to join the two tables
> with a FULL OUTER JOIN.
I'm confused. If the columns are NOT NULL, why isn't this a valid
transformation of your original query?
> select temp_max_60min,temp_dry_at_2m
> from station s natural join
> temp_dry_at_2m a full outer join temp_max_60min b using (station_id, timeobs)
> where s.wmo_id=6065
> and _a.timeobs='2004-1-1 0:0:0' and b._timeobs='2004-1-1 0:0:0'
> and '2004-1-1 0:0:0' between s.startdate and s.enddate;
Seems like it's not eliminating any rows that would otherwise succeed.
regards, tom lane
Hi,
Look for my comments further down...
John A Meinel wrote:
> Kim Bisgaard 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 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 temp_max_60min,temp_dry_at_2m
>> from station s natural join
>> temp_dry_at_2m a full outer join temp_max_60min b using (station_id,
>> timeobs)
>> where s.wmo_id=6065
>> and timeobs='2004-1-1 0:0:0'
>> and '2004-1-1 0:0:0' between s.startdate and s.enddate;
>>
>> I get the correct results, BUT LOUSY performance, and the following
>> explain:
>>
>> Nested Loop Left Join (cost=5.84..163484.08 rows=1349 width=12)
>> (actual time=66146.815..119005.381 rows=1 loops=1)
>> Filter: (COALESCE("outer".timeobs, "inner".timeobs) = '2004-01-01
>> 00:00:00'::timestamp without time zone)
>> -> Hash Join (cost=5.84..155420.24 rows=1349 width=16) (actual
>> time=8644.449..110836.038 rows=109826 loops=1)
>
>
> Well, the estimate here is quite a bit off. It thinks you will be
> getting 1349 (which is probably why it picked a nested loop plan), but
> then it is getting 109826 rows.
> I'm guessing it is misunderstanding the selectivity of the timeobs
> column.
I think you are right..
>
>> Hash Cond: ("outer".station_id = "inner".station_id)
>> -> Seq Scan on temp_dry_at_2m a (cost=0.00..120615.94
>> rows=6956994 width=16) (actual time=0.024..104548.515 rows=6956994
>> loops=1)
>> -> Hash (cost=5.84..5.84 rows=1 width=4) (actual
>> time=0.114..0.114 rows=0 loops=1)
>> -> Index Scan using wmo_idx on station
>> (cost=0.00..5.84 rows=1 width=4) (actual time=0.105..0.108 rows=1
>> loops=1)
>> Index Cond: ((wmo_id = 6065) AND ('2004-01-01
>> 00:00:00'::timestamp without time zone >= startdate) AND ('2004-01-01
>> 00:00:00'::timestamp without time zone <= enddate))
>> -> Index Scan using temp_max_60min_idx on temp_max_60min b
>> (cost=0.00..5.96 rows=1 width=20) (actual time=0.071..0.071 rows=0
>> loops=109826)
>> Index Cond: (("outer".station_id = b.station_id) AND
>> ("outer".timeobs = b.timeobs))
>> Total runtime: 119005.499 ms
>> (11 rows)
>
>
> I think the bigger problem is that a full outer join says grab all rows,
> even if they are null.
>
> What about this query:
> SELECT temp_max_60min,temp_dry_at_2m
> FROM (station s LEFT JOIN temp_dry_at_2m a USING (station_id, timeobs)
> LEFT JOIN temp_max_60min b USING (station_id, timeobs)
> where s.wmo_id=6065
> and timeobs='2004-1-1 0:0:0'
> and '2004-1-1 0:0:0' between s.startdate and s.enddate;
This works very well, and gives the correct result - thanks!!
>
> After that, you should probably have a multi-column index on
> (station_id, timeobs), which lets postgres use just that index for the
> lookup, rather than using an index and then a filter. (Looking at your
> next query you might already have that index).
Yes I have.
>
>>
>> If I change the query to (and thus negates the full outer join):
>
>
> This is the same query, I think you messed up your copy and paste.
Nope. Changed "and timeobs='2004-1-1 0:0:0' " to "and
a.timeobs='2004-1-1 0:0:0' and b.timeobs='2004-1-1 0:0:0' "
>
>>
>> select temp_max_60min,temp_dry_at_2m
>> from station s natural join
>> temp_dry_at_2m a full outer join temp_max_60min b using (station_id,
>> timeobs)
>> where s.wmo_id=6065
>> and _a.timeobs='2004-1-1 0:0:0' and b._timeobs='2004-1-1 0:0:0'
>> and '2004-1-1 0:0:0' between s.startdate and s.enddate;
>>
>>
>> I get wrong results (In the case where one of the records is missing in
>> one of the tables), BUT GOOD performance, and this query plan:
>>
>> Nested Loop (cost=0.00..17.83 rows=1 width=12) (actual
>> time=79.221..79.236 rows=1 loops=1)
>> -> Nested Loop (cost=0.00..11.82 rows=1 width=24) (actual
>> time=65.517..65.526 rows=1 loops=1)
>> -> Index Scan using wmo_idx on station (cost=0.00..5.83
>> rows=1 width=4) (actual time=0.022..0.026 rows=1 loops=1)
>> Index Cond: ((wmo_id = 6065) AND ('2004-01-01
>> 00:00:00'::timestamp without time zone >= startdate) AND ('2004-01-01
>> 00:00:00'::timestamp without time zone <= enddate))
>> -> Index Scan using temp_max_60min_idx on temp_max_60min b
>> (cost=0.00..5.97 rows=1 width=20) (actual time=65.483..65.486 rows=1
>> loops=1)
>> Index Cond: (("outer".station_id = b.station_id) AND
>> (b.timeobs = '2004-01-01 00:00:00'::timestamp without time zone))
>> -> Index Scan using temp_dry_at_2m_idx on temp_dry_at_2m a
>> (cost=0.00..6.00 rows=1 width=16) (actual time=13.694..13.698 rows=1
>> loops=1)
>> Index Cond: (("outer".station_id = a.station_id) AND
>> (a.timeobs = '2004-01-01 00:00:00'::timestamp without time zone))
>> Total runtime: 79.340 ms
>> (9 rows)
>>
>>
>> If further info like EXPLAIN VERBOSE is useful please say so and I will
>> provide it.
>>
>> Thanks in advance!
>> Kim Bisgaard.
>
>
> I still feel like you will have a problem with an outer join in this
> circumstance, because it will have to scan all of both tables.
Maybe I misunderstand outer joins but since there are no rows with
NULLs, I think it is a matter of finding the rows that are there or
makeing one up one if there are no rows?
>
> I think what you are wanting is "give me everything where station_id =
> X, and there is a row in either a or b".
> I think my LEFT JOIN example does that, but I also think there would be
> a subselect form which would work, which might do better. Something like:
>
> SELECT temp_max_60min,temp_dry_at_2m
> FROM (SELECT station_id, timeobs FROM station s
> WHERE s.wmo_id=6065
> AND timeobs = '2004-1-1 0:0:0'
> AND '2004-1-1 0:0:0' BETWEEN s.startdate AND s.enddate
> ) AS s
> JOIN (SELECT temp_max_60min, temp_dry_at_2m
> FROM temp_dry_at_2m a
> FULL OUTER JOIN temp_max_60min b
> USING (station_id, timeobs)
> WHERE station_id = s.station_id
> AND timeobs = '2004-1-1 0:0:0'
> )
> ;
>
> If I did this correctly, you should have a very restrictive scan done on
> station, which only returns a few rows based on timeobs & station_id.
> But it might be better to turn that final FULL OUTER JOIN into 2 LEFT
> JOINs like I did the first time:
>
> SELECT temp_max_60min,temp_dry_at_2m
> FROM (SELECT station_id, timeobs FROM station s
> WHERE s.wmo_id=6065
> AND timeobs = '2004-1-1 0:0:0'
> AND '2004-1-1 0:0:0' BETWEEN s.startdate AND s.enddate
> ) AS s
> LEFT JOIN temp_dry_at_2m a USING (station_id, timeobs)
> LEFT JOIN temp_max_60min b USING (station_id, timeobs)
> ;
>
> I would hope postgres could do this from just my earlier plan. And I
> hope I understand what you want, such that 2 LEFT JOINS work better than
> your FULL OUTER JOIN. If you only want rows where one of both temp_dry
> or temp_max exist, you probably could just add the line:
>
> WHERE (temp_max_60_min IS NOT NULL OR temp_dry_at_2m IS NOT NULL)
Thanks John. You have opened my eyes for a new way to formulate my queries!
>
>
> John
> =:->
--
Kim Bisgaard
Computer Department Phone: +45 3915 7562 (direct)
Danish Meteorological Institute Fax: +45 3915 7460 (division)
Sorry for not listing the exact layout of temp_XXXX:
Regards,
Kim Bisgaard.
Tom Lane wrote:
obsdb=> \d temp_dry_at_2m Table "public.temp_dry_at_2m" Column | Type | Modifiers ----------------+-----------------------------+----------- obshist_id | integer | not null station_id | integer | not null timeobs | timestamp without time zone | not null temp_dry_at_2m | real | not null Indexes: "temp_dry_at_2m_pkey" primary key, btree (obshist_id) "temp_dry_at_2m_idx" btree (station_id, timeobs)The difference between the two queries is if a (station_id,timeobs) row is missing in one table, then the first returns one record(null,9.3) while the second return no records.
Regards,
Kim Bisgaard.
Tom Lane wrote:
Kim Bisgaard <kib+pg@dmi.dk> writes:I have two BIG tables (virtually identical) with 3 NOT NULL columns Station_id, TimeObs, Temp_XXXX, with indexes on (Station_id, TimeObs) and valid ANALYSE (set statistics=100). I want to join the two tables with a FULL OUTER JOIN.I'm confused. If the columns are NOT NULL, why isn't this a valid transformation of your original query?select temp_max_60min,temp_dry_at_2m from station s natural join temp_dry_at_2m a full outer join temp_max_60min b using (station_id, timeobs) where s.wmo_id=6065 and _a.timeobs='2004-1-1 0:0:0' and b._timeobs='2004-1-1 0:0:0' and '2004-1-1 0:0:0' between s.startdate and s.enddate;Seems like it's not eliminating any rows that would otherwise succeed. regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to majordomo@postgresql.org so that your message can get through to the mailing list cleanly
-- Kim Bisgaard Computer Department Phone: +45 3915 7562 (direct) Danish Meteorological Institute Fax: +45 3915 7460 (division)