2010/1/12 Matteo Beccati <php@beccati.com>:
> Il 12/01/2010 08:55, Pavel Stehule ha scritto:
>>
>> I checked query and I was surprised with very strange plan:
>>
>> postgres=# explain select a, b from a,b,c;
>> QUERY PLAN
>> -----------------------------------------------------------------------
>> Nested Loop (cost=0.00..276595350.00 rows=13824000000 width=8)
>> -> Nested Loop (cost=0.00..115292.00 rows=5760000 width=8)
>> -> Seq Scan on a (cost=0.00..34.00 rows=2400 width=4)
>> -> Materialize (cost=0.00..82.00 rows=2400 width=4)
>> -> Seq Scan on b (cost=0.00..34.00 rows=2400 width=4)
>> -> Materialize (cost=0.00..82.00 rows=2400 width=0)
>> -> Seq Scan on c (cost=0.00..34.00 rows=2400 width=0)
>> (7 rows)
>
> It doesn't surprise me. Tables are empty, thus get a default non-0 row
> estimate, which happens to be 2400:
>
I though so default estimate is used only when table wasn't analysed.
But you have a true. I am verifying it on 8.3 and the behave is same.
so all is ok.
Regards
Pavel Stehule
> test=# create table a (a int);
> CREATE TABLE
> test=# ANALYZE a;
> ANALYZE
> test=# EXPLAIN SELECT * from a;
> QUERY PLAN
> -----------------------------------------------------
> Seq Scan on a (cost=0.00..14.80 rows=2400 width=4)
> (1 row)
>
>
> That said, 2400^3 (cross join of 3 tables) == 13824000000
>
>
> Cheers
> --
> Matteo Beccati
>
> Development & Consulting - http://www.beccati.com/
>