I think I am missing something about how the new CREATE OR REPLACE
FUNCTION ...COST works or I am missing some setting in postgresql conf.
I was hoping I could use it to control the function that is used in
cases where only one needs to be evaluated. Regardless of what I do it
seems to always evaluate the first function in the list. I'm running on
"PostgreSQL 8.3.0, compiled by Visual C++ build 1400"
Here is an example of my test: Functions and tables
CREATE TABLE log_call
(
fn_name character varying(100) NOT NULL,
fn_calltime timestamp with time zone NOT NULL DEFAULT now()
)
WITH (OIDS=FALSE);
CREATE OR REPLACE FUNCTION fn_pg_costlyfunction()
RETURNS integer AS
$$
BEGIN
INSERT INTO log_call(fn_name) VALUES('fn_pg_costlyfunction()');
RETURN 5;
END$$
LANGUAGE 'plpgsql' VOLATILE
COST 1000000;
CREATE OR REPLACE FUNCTION fn_pg_cheapfunction()
RETURNS integer AS
$$
BEGIN
INSERT INTO log_call(fn_name) VALUES('fn_pg_cheapfunction()');
RETURN 5;
END$$
LANGUAGE 'plpgsql' VOLATILE
COST 1;
--- Now for the test -
--Test 1: This shows that fn_pg_costlyfunction() is the only function
that is run -
-- unexpected to me shouldn't no function be evaluated or the cheap one?
--What's the difference between Test 1 and Test 2 that makes Test 2 do
the RIGHT thing?
TRUNCATE TABLE log_call;
SELECT (fn_pg_costlyfunction() > 2 OR fn_pg_cheapfunction() > 2 OR 5 >
2);
--Test 2: This works as I would expect - shows that none of the
functions are run presumably its going straight for 5 > 2
--becuase it recognizes its the cheapest route
TRUNCATE TABLE log_call;
SELECT foo.value
FROM (SELECT (fn_pg_costlyfunction() > 2 OR fn_pg_cheapfunction() > 2 OR
5 > 2 ) as value) as foo
--Test 3: It always runs the first function even though the cost of the
first is higher than the second
(in this case log_call contains fn_pg_costlyfunction())
TRUNCATE TABLE log_call;
SELECT foo.value
FROM (SELECT (fn_pg_costlyfunction() > 2 OR fn_pg_cheapfunction() > 2)
as value) as foo;
TRUNCATE TABLE log_call;
SELECT (fn_pg_costlyfunction() > 2 OR fn_pg_cheapfunction() > 2) as
value;
--Test 4: It always runs the first function even though the cost of the
first is higher than the second
(in this case log_call contains fn_pg_cheapfunction())
TRUNCATE TABLE log_call;
SELECT foo.value
FROM (SELECT (fn_pg_cheapfunction() > 2 OR fn_pg_costlyfunction() > 2 )
as value) as foo;
TRUNCATE TABLE log_call;
SELECT (fn_pg_cheapfunction() > 2 OR fn_pg_costlyfunction() > 2 ) as
value;
Thanks,
Regina
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