Avoid multiple function calls in query

Question

I have a query as follows:
select
col1,
substr(test_func(col2,test_func(col3)) , 1 , length(test_func(col2,test_func(col3))),
col3,
substr(test_func(col2,test_func(col3)) , 1 , length(test_func(col2,test_func(col3)) - 5),
col4
from table1
where pk_column = pi_param;
The test_func has couple of select queries inside and returns a string which i need to split in a particular way and return as two different column values.
To optimize performance, I need to reduce the multiple calls to test_func function.
I tried query with query like below:
select
col1,
substr(func_ret_value,1,length(func_ret_value)) value1,
col3
substr(func_ret_value,1, length(func_ret_value) - 5 ) value2,
col4
from(
select
col1,
col3,
test_func(col2,test_func(col3)) func_ret_value
col4
from table1
where pk_column = pi_param);
Above approach works but when i check the trace of above query , it still shows multiple executions( of the select queries inside the function.
Also there is no performance gain.
How do i avoid these multiple calls to the function and improve performance.
Thanks.

Answer 1

Hi,
You can call the function once, in a sub-query, then use it (by referencing the column alias tf in the example below) as often as you want to in a super-query, like this:
WITH     got_tf     AS
     SELECT  table1.*     -- or list columns needed
     ,     test_func ( col2
                 , test_func (col3)
                 )     AS tf
     FROM    table1
SELECT  col1
,     tf          AS value1     -- = SUBSTR (tf, 1, LENGTH (tf))
,     col3
,     SUBSTR ( tf
            , 1
            , LENGTH (tf) - 5
            )      AS value2
,     col4
FROM      got_tf
WHERE      pk_column     = pi_param;If possible, create the function as DETERMINISTIC.