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:
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/
