Message-ID: <4281C049.4080007@dmi.dk>
Date: Wed, 11 May 2005 10:20:25 +0200
From: Kim Bisgaard <kib@dmi.dk>
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To: John A Meinel <john@arbash-meinel.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: full outer performance problem
References: <428078E6.90007@dmi.dk> <4280C526.3070609@arbash-meinel.com>
In-Reply-To: <4280C526.3070609@arbash-meinel.com>
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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)