Date: Thu, 14 Jan 2010 18:03:18 +0100
From: Andreas Kretschmer <akretschmer@spamfence.net>
To: pgsql-performance@postgresql.org
Subject: bad execution plan for subselects containing windowing-function
Message-ID: <20100114170318.GA7634@tux>
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Hi,

version: 8.4.2


I have a table called values:

test=*# \d values
    Table "public.values"
 Column |  Type   | Modifiers
--------+---------+-----------
 id     | integer |
 value  | real    |
Indexes:
    "idx_id" btree (id)

The table contains 100000 random rows and is analysed.


And i have 2 queries, both returns the same result:

test=*# explain analyse select id, avg(value) over (partition by value) from values where id = 50 order by id;
                                                           QUERY PLAN                                                            
---------------------------------------------------------------------------------------------------------------------------------
 WindowAgg  (cost=531.12..549.02 rows=1023 width=8) (actual time=2.032..4.165 rows=942 loops=1)                                  
   ->  Sort  (cost=531.12..533.68 rows=1023 width=8) (actual time=2.021..2.270 rows=942 loops=1)                                 
         Sort Key: value                                                                                                         
         Sort Method:  quicksort  Memory: 53kB                                                                                   
         ->  Bitmap Heap Scan on "values"  (cost=24.19..479.98 rows=1023 width=8) (actual time=0.269..1.167 rows=942 loops=1)    
               Recheck Cond: (id = 50)                                                                                           
               ->  Bitmap Index Scan on idx_id  (cost=0.00..23.93 rows=1023 width=0) (actual time=0.202..0.202 rows=942 loops=1) 
                     Index Cond: (id = 50)                                                                                       
 Total runtime: 4.454 ms                                                                                                         
(9 rows)                                                                                                                         

Time: 4.859 ms
test=*# explain analyse select * from (select id, avg(value) over (partition by value) from values  order by id) foo where id = 50;
                                                               QUERY PLAN                                                               
----------------------------------------------------------------------------------------------------------------------------------------
 Subquery Scan foo  (cost=22539.64..24039.64 rows=500 width=12) (actual time=677.196..722.975 rows=942 loops=1)                         
   Filter: (foo.id = 50)                                                                                                                
   ->  Sort  (cost=22539.64..22789.64 rows=100000 width=8) (actual time=631.991..690.411 rows=100000 loops=1)                           
         Sort Key: "values".id                                                                                                          
         Sort Method:  external merge  Disk: 2528kB                                                                                     
         ->  WindowAgg  (cost=11116.32..12866.32 rows=100000 width=8) (actual time=207.462..479.330 rows=100000 loops=1)                
               ->  Sort  (cost=11116.32..11366.32 rows=100000 width=8) (actual time=207.442..281.546 rows=100000 loops=1)               
                     Sort Key: "values".value                                                                                           
                     Sort Method:  external merge  Disk: 1752kB                                                                         
                     ->  Seq Scan on "values"  (cost=0.00..1443.00 rows=100000 width=8) (actual time=0.010..29.742 rows=100000 loops=1) 
 Total runtime: 725.362 ms                                                                                                              
(11 rows)                                                                                                                               


No question, this is a silly query, but the problem is the 2nd query: it
is obviously not possible for the planner to put the where-condition
into the subquery. That's bad if i want to create a view:

test=*# create view view_values as select id, avg(value) over (partition by value) from values  order by id;
CREATE VIEW
Time: 41.280 ms
test=*# commit;
COMMIT
Time: 0.514 ms
test=# explain analyse select * from view_values where id=50;

It is the same bad plan with the Seq Scan on "values".


Is this a bug or PEBKAC or something else?


Andreas
-- 
Really, I'm not out to destroy Microsoft. That will just be a completely
unintentional side effect.                              (Linus Torvalds)
"If I was god, I would recompile penguin with --enable-fly."   (unknown)
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