Why does SQL execute inner selected functions again in the outer select?

Question

Hi,
Why does SQL execute inner selected functions again in the outer select?
Given:
CREATE OR REPLACE FUNCTION K_TEST
RETURN NUMBER IS
BEGIN
RETURN 1;
END;
SELECT K_TEST, K_TEST FROM DUAL;Will (logically) execute the function twice.
SELECT intest, intest
FROM (SELECT K_TEST intest FROM DUAL);Will execute the function twice too!
Why can't SQL buffer the inner result?
Does anyone have an idea on how to achieve executing the function only once? My original called function is quite heavy and returning a user defined type. Adding predicates, the function is executed 3 or 4 times!
Thanks for any tips,
K.

Answer 1

Hello
Depending on your version of Oracle, Sub query caching could help....
XXXX> create sequence seq_1
  2  /
Sequence created.
Elapsed: 00:00:00.07
XXXX> CREATE OR REPLACE FUNCTION K_TEST
  2  RETURN NUMBER IS
  3
  4      ln_Ret  NUMBER;
  5
  6  BEGIN
  7      SELECT
  8          seq_1.NEXTVAL
  9      INTO
10          ln_Ret
11      FROM
12          dual;
13
14      RETURN ln_Ret;
15  END;
16  /
Function created.
Elapsed: 00:00:00.60
XXXX>
XXXX> SELECT K_TEST, K_TEST FROM DUAL;
    K_TEST     K_TEST
         1          2
1 row selected.
Elapsed: 00:00:00.06
XXXX> SELECT K_TEST, K_TEST FROM DUAL;
    K_TEST     K_TEST
         3          4
1 row selected.
Elapsed: 00:00:00.01
XXXX> SELECT intest, intest
  2  FROM (SELECT K_TEST intest FROM DUAL);
    INTEST     INTEST
         5          6
1 row selected.
Elapsed: 00:00:00.10
XXXX> SELECT
  2      intest,intest
  3  FROM
  4      (
  5          SELECT (SELECT K_TEST FROM dual) intest
  6          FROM
  7          dual
  8
  9      )
10  /
    INTEST     INTEST
         7          7
1 row selected.
Elapsed: 00:00:00.03
XXXX> /
    INTEST     INTEST
         8          8
1 row selected.
Elapsed: 00:00:00.01THis last example takes advantage of a specific optimisation for calling functions in a subquery. Not sure if it would suit your circumstance though...
Alternatively - again depending on your version - you might be able to look at function result caching...
HTH
David
Edited by: Bravid on Feb 1, 2012 12:32 PM