> -----Mensaje original-----
> De: Scott Marlowe [mailto:scott.marlowe@[EMAIL PROTECTED]
> Enviado el: Mi=E9rcoles, 18 de Junio de 2008 17:47
> Para: Fernando Hevia
> >
> > For complex calculations I have obtained better performance using=20
> > nested queries. For example:
> >
> > select a, b, c select
> > ( select a, b, a*b as c from ta) subquery1 where c =3D 2;
> >
> > This nesting is probably overhead in such a simple case as=20
> this, but=20
> > in more complex ones and specially with volatile functions it will=20
> > provide an improvement.
>=20
> I was under the impresion from previous discussions that the=20
> query planner flattened these out to be the same query. Do=20
> you get different query plans when you re-arrange this way?
>=20
Take a look at this example (tried on 8.2.7 & 8.1.11):
create or replace function test(p1 integer, p2 integer) returns integer[]
as
$BODY$
declare
retval integer[];
begin
raise info 'called test(%, %)', p1, p2;
retval[0] =3D p1 + p2;
retval[1] =3D p1 * p2;
retval[2] =3D p1 - p2;
return retval;
end;
$BODY$
LANGUAGE 'plpgsql' IMMUTABLE;
-- In this case function test is called three times:
pg=3D# select (test(1, 2))[0] as sum, (test(1, 2))[1] as prod, (test(1,
2))=
[2]
as dif;
INFO: called test(1, 2)
INFO: called test(1, 2)
INFO: called test(1, 2)
sum | prod | dif
-----+------+-----
3 | 2 | -1
(1 row)
-- In this case function test is called only once:
pg=3D# select res[0] as sum, res[1] as prod, res[2] as dif from
pg-# (select (test(1, 2))::integer[] as res) t ;
INFO: called test(1, 2)
sum | prod | dif
-----+------+-----
3 | 2 | -1
(1 row)
I assume the second form will perform better since test is being called
only
once.
I might be missing something in this assumption but at first glance it
seems
pretty straightforward.
Regards,
Fernando.
--=20
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