From: | Matteo Beccati <php(at)beccati(dot)com> |
---|---|
To: | Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> |
Cc: | PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Subject: | Re: planner or statistical bug on 8.5 |
Date: | 2010-01-12 08:03:18 |
Message-ID: | 4B4C2CC6.4060902@beccati.com |
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Lists: | pgsql-hackers |
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/
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