Date: Wed, 20 Jun 2007 09:49:02 +0200
To: "Tom Lane" <tgl@sss.pgh.pa.us>, "Greg Smith" <gsmith@gregsmith.com>
Subject: Re: Volunteer to build a configuration tool
From: PFC <lists@peufeu.com>
Cc: pgsql-performance@postgresql.org
Content-Type: text/plain; format=flowed; delsp=yes; charset=utf-8
MIME-Version: 1.0
References: <A3AC4FA47DC0B1458C3E5396E685E63302395E53@SAB-DC1.sab.uiuc.edu>
	<1182298719.4482.133.camel@ipso.snappymail.ca>
	<Pine.GSO.4.64.0706200040001.7594@westnet.com>
	<7621.1182318230@sss.pgh.pa.us>
	<Pine.GSO.4.64.0706200224430.16657@westnet.com>
	<9001.1182323187@sss.pgh.pa.us>
Content-Transfer-Encoding: 7bit
Message-ID: <op.tt7mn0m9cigqcu@apollo13>
In-Reply-To: <9001.1182323187@sss.pgh.pa.us>
User-Agent: Opera Mail/9.10 (Linux)


> To me, the worst catch-22 we face in this area is that we'd like the
> optimizer's choices of plan to be stable and understandable, but the
> real-world costs of queries depend enormously on short-term conditions
> such as how much of the table has been sucked into RAM recently by
> other queries.  I have no good answer to that one.

	Yeah, there is currently no way to tell the optimizer things like :

	- this table/portion of a table is not frequently accessed, so it won't  
be in the cache, so please use low-seek plans (like bitmap index scan)
	- this table/portion of a table is used all the time so high-seek-count  
plans can be used like index scan or nested loops since everything is in  
RAM

	Except planner hints (argh) I see no way to give this information to the  
machine... since it's mostly in the mind of the DBA. Maybe a per-table  
"cache temperature" param (hot, warm, cold), but what about the log table,  
the end of which is cached, but not the old records ? It's messy.

	Still PG does a pretty excellent job most of the time.