Group by on Function return value

Question

Dear All
I'm facing a problem with when I have a group by on the return value of a function.
Following is the scenario.
Im having the following select query which executes in less than millisec for more than 20k records.
SELECT T1.FIELD1,T1.FIELD2, MY_FUNCTION(T1.FIELD2) FROM TABLE T1 ;
But when I modify the above query in the following way for the same number of records, the performance is going down to more than 4 mts.
SELECT T1.FIELD1,T1.FIELD2, MY_FUNCTION(T1.FIELD2)FIELD3 FROM TABLE T1 GROUP BY T1.FIELD1,T1.FIELD3;
can any one please suggest me whats the best solution in this regard.
Thanks in advance
Narayanan

Answer 1

SQL> create or replace function get_sub(a varchar2)
  2  return varchar2
  3  is
  4  begin
  5   return substr(a,1,1);
  6  end;
  7  /
Function created.
SQL> select deptno, get_sub(ename) sub from emp;
    DEPTNO S
        20 S
        30 A
        30 W
        20 J
        30 M
        30 B
        10 C
        20 S
        10 K
        30 T
        20 A
        30 J
        20 F
        10 M
14 rows selected.
SQL> select deptno, get_sub(ename) sub, count(1)
  2  from emp
  3  group by deptno, get_sub(ename)
  4  /
    DEPTNO S   COUNT(1)
        10 C          1
        10 K          1
        10 M          1
        20 A          1
        20 F          1
        20 J          1
        20 S          2
        30 A          1
        30 B          1
        30 J          1
        30 M          1
        30 T          1
        30 W          1
13 rows selected.Rgds.
Everything which is not affected by aggregate functions
should be included into GROUP BY. And GROUP BY can't use
aliases but expressions only.
You have to think what exactly do you want to get
for FIELD02 - do you want get max/min/avg/count ?
Message was edited by:
dnikiforov
(I'm asking because your query probably should look like
SELECT T1.FIELD1,T1.FIELD2, MY_FUNCTION(T1.FIELD2) FIELD3 FROM TABLE T1 GROUP BY T1.FIELD1,T1.FIELD2
Message was edited by:
dnikiforov