Thread: view columns and performance

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

Eric Schwarzenbach <subscriber@blackbrook.org> writes:
> 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.

If the view gets "flattened" into the calling query then unreferenced
columns will be optimized away, otherwise probably not.  You haven't
given enough details about your intended view definition to be sure
whether it can be optimized, but if it's just a join it's fine.

            regards, tom lane