Message-ID: <4C52E7E4.4020204@blackbrook.org>
Date: Fri, 30 Jul 2010 10:55:32 -0400
From: Eric Schwarzenbach <subscriber@blackbrook.org>
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To: pgsql-performance@postgresql.org
Subject: view columns and performance
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I'm wondering whether columns, in the select list of a view that is used 
in a join, which are not used either as join criteria or in the select 
list of the overall query, effect the performance of the query.

In other words supposed I define a view something like

CREATE view MyView AS SELECT a,b, c, d, e, f, g FROM  (several tables 
joined together)

Assume for the sake of simplicity there are no aggregates or such, we're 
just joining tables, and getting a bunch of columns back each of the 
various tables.

I then then perform a query something like

SELECT v.a, x.h, y.i FROM MyView as v JOIN otherTable as x on (x.m = 
v.a) JOIN yetAnotherTable as y on (y.n=v.a)

Does all the extra clutter of b,c,d,e,f in MyView affect the performance 
of the query by taking up extra space in memory during the joins or is 
the optimizer smart enough to realize that they aren't needed and evoke 
the query as if MyView were really defined as

CREATE view MyView AS SELECT a FROM  (several tables joined together)?

Thanks,

Eric