Subject: Re: Strange Create Index behaviour
From: Simon Riggs <simon@2ndquadrant.com>
To: Gary Doades <gpd@gpdnet.co.uk>
Cc: pgsql-performance@postgresql.org
In-Reply-To: <43F38867.6010701@gpdnet.co.uk>
References: <43F38867.6010701@gpdnet.co.uk>
Content-Type: text/plain
Date: Thu, 16 Feb 2006 01:52:09 +0000
Message-Id: <1140054729.12131.225.camel@localhost.localdomain>
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit

On Wed, 2006-02-15 at 20:00 +0000, Gary Doades wrote:

> I have put together a test case that demonstrates the problem (see 
> below). I create a simple table, as close in structure to one of my 
> problem tables and populate an integer column with 100,000 zeros follow 
> by 100,000 random integers between 0 and 100,000. Then create an index 
> on this column. I then drop the table and repeat. The create index 
> should take around 1-2 seconds. A fair proportion of the time it takes 
> 50 seconds!!!
> 
> If I fill the same row with all random data the create index always 
> takes a second or two. If I fill the column with all zeros everything is 
> still OK.

Aside from the importance of investigating sort behaviour, have you
tried to build a partial index WHERE col > 0 ? That way you wouldn't
even be indexing the zeros.

Best Regards, Simon Riggs