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

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Mladen Gogala <mladen(dot)gogala(at)vmsinfo(dot)com>
Cc: Greg Smith <greg(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Andrew Dunstan <andrew(at)dunslane(dot)net>, 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 21:11:25
Message-ID: 201102022111.p12LBP800917@momjian.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
Mladen Gogala wrote:
> > 2) The sort of random I/O done by index lookups can be as much as 50X as 
> > expensive on standard hard drives as sequential, if every block goes to 
> > physical hardware.
> >   
> 
> Greg, how many questions about queries not using an index have you seen? 
> There is a reason why people keep asking that. The sheer number of 
> questions like that on this group should tell you that there is a 
> problem there. 

Very few of those reports found that an index scan was indeed faster ---
they just assumed so but when they actually tested it, they understood.

-- 
  Bruce Momjian  <bruce(at)momjian(dot)us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +

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