To: Yeb Havinga <yebhavinga@gmail.com>
cc: =?UTF-8?B?0JLRltGC0LDQu9GW0Lkg0KLQuNC80YfQuNGI0LjQvQ==?=
	<tivv00@gmail.com>, pgsql-performance@postgresql.org
Subject: Re: Planner not using column limit specified for one column for
	another column equal to first
In-reply-to: <4BC88364.9050404@gmail.com> 
References: <m2y331e40661004160102tff445f9eg165a04efe3b409ed@mail.gmail.com>
	<8534.1271427587@sss.pgh.pa.us> <4BC88364.9050404@gmail.com>
Comments: In-reply-to Yeb Havinga <yebhavinga@gmail.com>
	message dated "Fri, 16 Apr 2010 17:33:56 +0200"
Date: Fri, 16 Apr 2010 11:45:48 -0400
Message-ID: <10157.1271432748@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Yeb Havinga <yebhavinga@gmail.com> writes:
> New expensive planner infrastructure to support from a>b and b>c infer 
> a>c, yes.

> But I wonder if something like Leibniz's principle of identity holds for 
> members of the same equivalence class, e.g. like if x,y are both members 
> of the same EC, then for every predicate P, P(x) iff P(y).

This could only be assumed to apply for predicates constructed from
operators that are in the equivalence operator's btree opfamily.
Now, that would certainly be a large enough set of cases to sometimes
give useful results --- but I stand by the opinion that it wouldn't
win often enough to justify the added planner overhead.

			regards, tom lane