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Re: query planner does not canonicalize infix operators

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Dimitri Fontaine <dimitri(at)2ndQuadrant(dot)fr>
Cc: Daniel Farina <daniel(at)heroku(dot)com>, Will Leinweber <will(at)heroku(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: query planner does not canonicalize infix operators
Date: 2012-04-15 15:21:22
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Dimitri Fontaine <dimitri(at)2ndQuadrant(dot)fr> writes:
> Looks like we're missing out some operator properties, like the neutral
> element and if the operator is transitive, commutative or associative. I
> think I remember us talking about how knowing about operators being
> associative would also help optimize a class of join problems.

We do understand, and use, transitivity for btree equality operators
(cf mergejoin planning, EquivalenceClasses, etc).  I have limited
enthusiasm for introducing a more general concept, because it seems like
doing anything with it would add a great deal more planning effort for
(typically) little reward.  I can imagine cases where, say, deducing
"a < c" from "a < b and b < c" would be helpful ... but they don't come
up in common queries.

			regards, tom lane

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