Message-ID: <4BB0D63C.90408@2ndquadrant.com>
Date: Mon, 29 Mar 2010 12:33:00 -0400
From: Greg Smith <greg@2ndquadrant.com>
User-Agent: Thunderbird 2.0.0.24 (X11/20100317)
MIME-Version: 1.0
To: Tom Lane <tgl@sss.pgh.pa.us>
CC: Robert Haas <robertmhaas@gmail.com>, Simon Riggs <simon@2ndquadrant.com>,
	pgsql-hackers@postgresql.org
Subject: Re: enable_joinremoval
References: <1269851630.3684.3636.camel@ebony>
	<603c8f071003290637q14d431f7j90afabd6fd2ec952@mail.gmail.com>
	<12679.1269873374@sss.pgh.pa.us>
	<603c8f071003290820h3553d223le3b5228f789608f9@mail.gmail.com>
	<4BB0CF3D.8090701@2ndquadrant.com>
	<603c8f071003290910l310c0e18v6b8986a22c6c4836@mail.gmail.com>
	<14825.1269879474@sss.pgh.pa.us>
In-Reply-To: <14825.1269879474@sss.pgh.pa.us>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

Tom Lane wrote:
> The problem with this line of thought is that it imagines you can look
> at worked-out alternative plans.  You can't, because the planner doesn't
> pursue rejected alternatives that far (and you'd not want to wait long
> enough for it to do so...)
>   

Not on any production system, sure.  I know plenty of people who would 
gladly let a rejected plan enumerator run for *a day* on their 
development box if it let them figure out exactly why the costing on the 
plan they expected ended up higher than the plan they actually get.  
While I know you don't run into this, regular people can easily spend a 
week on one such problem without gaining even that much insight, given 
the current level of instrumentation and diagnostic tools available.  
"Read the source" and "ask Tom" are both effective ways to resolve that 
but have their limits.  (Not because of you, of course--my bigger 
problem are people who just can't share their plans with the lists for 
privacy or security reasons)

-- 
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us