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

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Greg Smith <greg(at)2ndquadrant(dot)com>
Cc: Mladen Gogala <mladen(dot)gogala(at)vmsinfo(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: 2011-02-03 00:13:44
Message-ID: 201102030013.p130DiH08690@momjian.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
Greg Smith wrote:
> Mladen Gogala wrote:
> > 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. There must be a relatively simple way of influencing 
> > optimizer decisions. 
> 
> I think that's not quite the right question.  For every person like 
> yourself who is making an informed "the optimizer is really picking the 
> wrong index" request, I think there are more who are asking for that but 
> are not actually right that it will help.  I think you would agree that 
> this area is hard to understand, and easy to make mistakes about, yes?  
> So the right question is "how many questions about queries not using an 
> index would have actually benefitted from the behavior they asked for?"  
> That's a much fuzzier and harder to answer question.

Agreed.  I created an FAQ entry years ago to explain this point and tell
people how to test it:

	http://wiki.postgresql.org/wiki/FAQ#Why_are_my_queries_slow.3F_Why_don.27t_they_use_my_indexes.3F

Once I added that FAQ we had many fewer email questions about index
choice.

> > With all due respect, I consider myself smarter than the optimizer.  
> > I'm 6'4", 235LBS so telling me that you disagree and that I am more 
> > stupid than a computer program,  would not be a smart thing to do. 
> > Please, do not misunderestimate me.
> 
> I remember when I used to only weigh that much.  You are lucky to be 
> such a slim little guy!
> 
> Oh, I guess I should add, :)

Oh, wow, what a great retort.  :-)

-- 
  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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