Re: count * performance issue

Hi,

I have been reading this conversation for a few days now and I just wanted
to ask this. From the release notes, one of the new additions in 8.3 is
(Allow col IS NULL to use an index (Teodor)).

Sorry, if I am missing something here, but shouldn't something like this
allow us to get a (fast) accurate count ?

SELECT COUNT(*) from table WHERE indexed_field IS NULL
+
SELECT COUNT(*) from table WHERE indexed_field IS NOT NULL

*Robins Tharakan*

---------- Forwarded message ----------
From: Greg Smith <gsmith(at)gregsmith(dot)com>
Date: Tue, Mar 11, 2008 at 4:31 AM
Subject: Re: [PERFORM] count * performance issue
To: Joe Mirabal <jmmirabal(at)gmail(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org

On Mon, 10 Mar 2008, Joe Mirabal wrote:

> I run queries on the data nad get immediate max and min as well as other
> aggrgate functions very quickly, however a select count(*) of the table
> takes forever usually nearly an hour or more.

Are you sure the form of "select count(*)" you're using is actually
utilizing the index to find a useful subset? What do you get out of
EXPLAIN ANALZYE on the query?

In order for indexes to be helpful a couple of things need to happen:
1) They have to be structured correctly to be useful
2) There needs to be large enough settings for shared_buffes and
effective_cache_size that the database things it can use them efficiently
3) The tables involved need to be ANALYZEd to keep their statistics up to
date.

The parameters to run a 400GB *table* are very different from the
defaults; if you want tuning suggestions you should post the non-default
entries in your postgresql.conf file from what you've already adjusted
along with basic information about your server (PostgreSQL version, OS,
memory, disk setup).

> We in our warehouse use the count(*) as our verification of counts by
> day/month's etc

If you've got a database that size and you're doing that sort of thing on
it, you really should be considering partitioning as well.

--
* Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD

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