Message-ID: <51C4AEE5.8040704@agliodbs.com>
Date: Fri, 21 Jun 2013 12:52:05 -0700
From: Josh Berkus <josh@agliodbs.com>
Organization: PostgreSQL Experts Inc.
User-Agent: Mozilla/5.0 (X11; Linux x86_64;
 rv:17.0) Gecko/20130510 Thunderbird/17.0.6
MIME-Version: 1.0
To: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Weird, bad 0.5% selectivity estimate for a column equal to itself
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk
Sender: pgsql-performance-owner@postgresql.org

Folks,

I'm getting something really odd in 9.2.4, where the planner estimates
that the selectivity of a column equal to itself is always exactly 0.5%
(i.e. 0.005X).  I can't figure out where this constant is coming from,
or why it's being applied.

Test case:

create table esttest (
    id int not null primary key,
    state1 int not null default 0,
    state2 int not null default 0,
    state3 int not null default 0
);

insert into esttest (id, state1, state2, state3)
select i,
    (random()*3)::int,
    (random())::int,
    (random()*100)::int
from generate_series (1, 20000)
    as gs(i);

vacuum analyze esttest;

explain analyze
select * from esttest
where state1 = state1;

explain analyze
select * from esttest
where state2 = state2;

explain analyze
select * from esttest
where state3 = state3;

Results of test case:

badestimate=# explain analyze
badestimate-# select * from esttest
badestimate-# where state1 = state1;
                                                QUERY PLAN
----------------------------------------------------------------------------------------------------------
 Seq Scan on esttest  (cost=0.00..359.00 rows=100 width=16) (actual
time=0.009..4.145 rows=20000 loops=1)
   Filter: (state1 = state1)
 Total runtime: 5.572 ms
(3 rows)

badestimate=#
badestimate=# explain analyze
badestimate-# select * from esttest
badestimate-# where state2 = state2;
                                                QUERY PLAN
----------------------------------------------------------------------------------------------------------
 Seq Scan on esttest  (cost=0.00..359.00 rows=100 width=16) (actual
time=0.006..4.166 rows=20000 loops=1)
   Filter: (state2 = state2)
 Total runtime: 5.595 ms
(3 rows)

badestimate=#
badestimate=# explain analyze
badestimate-# select * from esttest
badestimate-# where state3 = state3;
                                                QUERY PLAN
----------------------------------------------------------------------------------------------------------
 Seq Scan on esttest  (cost=0.00..359.00 rows=100 width=16) (actual
time=0.005..4.298 rows=20000 loops=1)
   Filter: (state3 = state3)
 Total runtime: 5.716 ms
(3 rows)


-- 
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com


-- 
Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance