Skip site navigation (1) Skip section navigation (2)

Re: [HACKERS] Slow count(*) again...

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Mladen Gogala <mladen(dot)gogala(at)vmsinfo(dot)com>
Cc: Andrew Dunstan <andrew(at)dunslane(dot)net>, Bruce Momjian <bruce(at)momjian(dot)us>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "david(at)lang(dot)hm" <david(at)lang(dot)hm>, Craig Ringer <craig(at)postnewspapers(dot)com(dot)au>, Vitalii Tymchyshyn <tivv00(at)gmail(dot)com>, "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: [HACKERS] Slow count(*) again...
Date: 2011-02-02 18:20:59
Message-ID: AANLkTinqLbFettmezJP9iecGncxb2pp+OqfmZiqf0tCW@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
On Wed, Feb 2, 2011 at 1:11 PM, Mladen Gogala <mladen(dot)gogala(at)vmsinfo(dot)com> wrote:
> Not necessarily autoanalyze, some default rules for the situations when
> stats is not there should be put in place,
> like the following:
> 1) If there is a usable index on the temp table - use it.
> 2) It there isn't a usable index on the temp table and there is a join, make
> the temp table the first table
>   in the nested loop join.

The default selectivity estimates ought to make this happen already.

create temporary table foo (a integer, b text);
CREATE TABLE
insert into foo select g, random()::text||random()::text from
generate_series(1, 10000) g;
INSERT 0 10000
alter table foo add primary key (a);
NOTICE:  ALTER TABLE / ADD PRIMARY KEY will create implicit index
"foo_pkey" for table "foo"
ALTER TABLE
explain select * from foo where a = 1;
                             QUERY PLAN
---------------------------------------------------------------------
 Index Scan using foo_pkey on foo  (cost=0.00..8.27 rows=1 width=36)
   Index Cond: (a = 1)
(2 rows)

You're going to need to come up with actual examples of situations
that you think can be improved upon if you want to get anywhere here.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

In response to

pgsql-performance by date

Next:From: Greg SmithDate: 2011-02-02 18:22:55
Subject: Re: Configuration for a new server.
Previous:From: Jon NelsonDate: 2011-02-02 18:19:20
Subject: Re: [HACKERS] Slow count(*) again...

pgsql-hackers by date

Next:From: Dimitri FontaineDate: 2011-02-02 18:22:33
Subject: Re: ALTER EXTENSION UPGRADE, v3
Previous:From: Nicolas GrillyDate: 2011-02-02 18:20:15
Subject: Why "copy ... from stdio" does not return immediately when reading invalid data?

Privacy Policy | About PostgreSQL
Copyright © 1996-2014 The PostgreSQL Global Development Group