From: Andreas Kretschmer <akretschmer@spamfence.net>
To: pgsql-performance@postgresql.org
Subject: bad execution plan for subselects containing windowing-function
Date: Thu, 14 Jan 2010 18:03:18 +0100
Message-ID: <20100114170318.GA7634@tux> (raw)
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)
Kaufbach, Saxony, Germany, Europe. N 51.05082°, E 13.56889°
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To: pgsql-performance@postgresql.org
Cc: akretschmer@spamfence.net
Subject: Re: bad execution plan for subselects containing windowing-function
In-Reply-To: <20100114170318.GA7634@tux>
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