HOW does 8.3 CREATE REPLACe .. FUNCTION ..COST work

["Obe, Regina" <robe.dnd@xxxxxxxxxxxxxxxx>]

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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