Thread: bad execution plan for subselects containing windowing-function
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
whereid = 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=100000loops=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°
Andreas Kretschmer <akretschmer@spamfence.net> writes:
> 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.
Well, yeah: it might change the results of the window functions.
I see no bug here. Your second query asks for a much more complicated
computation, it's not surprising it takes longer.
regards, tom lane
Tom Lane <tgl@sss.pgh.pa.us> wrote: > Andreas Kretschmer <akretschmer@spamfence.net> writes: > > 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. > > Well, yeah: it might change the results of the window functions. > I see no bug here. Your second query asks for a much more complicated > computation, it's not surprising it takes longer. Thank you for the fast answer. But sorry, I disagree. It is the same query with the same result. I can't see how the queries should return different results. What have i overlooked? tia, 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°
Andreas Kretschmer <akretschmer@spamfence.net> writes:
> Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I see no bug here. Your second query asks for a much more complicated
>> computation, it's not surprising it takes longer.
> But sorry, I disagree. It is the same query with the same result. I can't see
> how the queries should return different results.
In the first query
select id, avg(value) over (partition by value) from values where id = 50 order by id;
the avg() calculations are being done over only rows with id = 50. In
the second query
select * from (select id, avg(value) over (partition by value) from values order by id) foo where id = 50;
they are being done over all rows. In this particular example you
happen to get the same result, but that's just because "avg(foo) over
partition by foo" is a dumb example --- it will necessarily just yield
identically foo. In more realistic computations the results would be
different.
regards, tom lane
Tom Lane <tgl@sss.pgh.pa.us> wrote: > Andreas Kretschmer <akretschmer@spamfence.net> writes: > > Tom Lane <tgl@sss.pgh.pa.us> wrote: > >> I see no bug here. Your second query asks for a much more complicated > >> computation, it's not surprising it takes longer. > > > But sorry, I disagree. It is the same query with the same result. I can't see > > how the queries should return different results. > > In the first query > > select id, avg(value) over (partition by value) from values where id = 50 order by id; > > the avg() calculations are being done over only rows with id = 50. In > the second query > > select * from (select id, avg(value) over (partition by value) from values order by id) foo where id = 50; > > they are being done over all rows. In this particular example you > happen to get the same result, but that's just because "avg(foo) over > partition by foo" is a dumb example --- it will necessarily just yield > identically foo. In more realistic computations the results would be > different. Okay, i believe you now ;-) I will try to find a case with different results ... Thx for your fast help! 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°
Andreas Kretschmer <akretschmer@spamfence.net> wrote: > > they are being done over all rows. In this particular example you > > happen to get the same result, but that's just because "avg(foo) over > > partition by foo" is a dumb example --- it will necessarily just yield > > identically foo. In more realistic computations the results would be > > different. > > Okay, i believe you now ;-) > > I will try to find a case with different results ... I have got it! test=# select * from values; id | value ----+------- 1 | 10 2 | 20 3 | 30 4 | 40 5 | 50 6 | 60 7 | 70 8 | 80 9 | 90 (9 rows) Time: 0.240 ms test=*# select id, sum(value) over (order by id) from values where id = 5; id | sum ----+----- 5 | 50 (1 row) Time: 0.352 ms test=*# select * from (select id, sum(value) over (order by id) from values) foo where id = 5; id | sum ----+----- 5 | 150 (1 row) Time: 0.383 ms 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°