How about this?
select item, very_expensive_function(item) as exp, cheap
from ( Select item, cheap_function(item) as cheap
From atable where cheap_function(item) > 0 ) sub
where very_expensive_function(item) > 0
-----Original Message-----
From: pgsql-general-owner@postgresql.org
[mailto:pgsql-general-owner@postgresql.org] On Behalf Of
han.holl@informationslogik.nl
Sent: Thursday, January 11, 2007 4:59 AM
To: pgsql-general@postgresql.org
Subject: [GENERAL] Optimize expresiions.
Hello,
Given a table:
create table atable (
item integer;
);
and a view:
create view aview as select
item,
very_expensive_function(item) as exp,
cheap_function(item) as cheap
from atable;
Now the query:
select item from aview where exp > 0 and cheap > 0;
will lead to a sequential scan on atable with filter:
very_expensive_function(item) > 0 and cheap_function(item) > 0
The query would run much faster with the filter reordered.
Is there a way to tell the planner/optimizer that certain functions are
more
expensive than others, and should be postponed in lazy evaluation ?
Or is there a hook in the system that would allow me too look at and
maybe
reorder expressions before they are executed ?
Cheers,
Han Holl
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