From: Richard Huxton <dev@archonet.com>
To: pgsql-sql@postgresql.org
Subject: Expression transformation curiosity
Date: Sun, 17 Aug 2003 11:32:58 +0100
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I just had a look at EXPLAIN ANALYSE output for an SQL function I'm trying to 
write. The WHERE clause wasn't too complex, but the explain output took up a 
dozen lines.

Boiling the problem down, I've looked at a clause of the form:
  a OR (b AND c)
which PG converts to:
  (a OR b) AND (a OR c)

Now these two are equivalent, but it would take me forever to demonstrate that 
with the full query. I'm happy the planner is going to get it right, but I'm 
confused as to why the transformation occurs.

Is it an artefact of displaying the EXPLAIN, or is it actually processed that 
way? You could see how testing "a" twice could be expensive in some 
situations.

Oh, btw - 7.3.2 on this box, I'll be looking at 7.3.3/4 later today.

-- 
  Richard Huxton
  Archonet Ltd