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Re: RES: RES: select on 1milion register = 6s

From: Decibel! <decibel(at)decibel(dot)org>
To: Craig James <craig_james(at)emolecules(dot)com>
Cc: pgsql-perform(at)postgresql(dot)org
Subject: Re: RES: RES: select on 1milion register = 6s
Date: 2007-07-30 06:52:41
Message-ID: 20070730065241.GR25704@decibel.org (view raw or flat)
Thread:
Lists: pgsql-performance
Please reply-all so others can learn and contribute.

On Sun, Jul 29, 2007 at 09:38:12PM -0700, Craig James wrote:
> Decibel! wrote:
> >It's unlikely that it's going to be faster to index scan 2.3M rows than
> >to sequential scan them. Try setting enable_seqscan=false and see if it
> >is or not.
> 
> Out of curiosity ... Doesn't that depend on the table?  Are all of the data 
> for one row stored contiguously, or are the data stored column-wise?  If 
> it's the former, and the table has hundreds of columns, or a few columns 
> with large text strings, then wouldn't the time for a sequential scan 
> depend not on the number of rows, but rather the total amount of data?

Yes, the time for a seqscan is mostly dependent on table size and not
the number of rows. But the number of rows plays a very large role in
the cost of an indexscan.
-- 
Decibel!, aka Jim C. Nasby, Database Architect  decibel(at)decibel(dot)org 
Give your computer some brain candy! www.distributed.net Team #1828

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