Decode in a join condition

Question

Hi all,
I stumbled across a very inefficient query in my product. I think the reason is having "decode" in two join conditions. Before I post the query, execution plan etc, and get the tuned sql statement on a platter, I want to fight it myself. :-)
I want to know, is it true that having a decode in the where clause (as a join condition) is bad for performance? Can you give me some tips on avoiding them (splitting the query into many is not possible here, though I think it is a good solution).
Thanks a lot,
Aditya
P.S. I'm trying to somehow use subqueries to eliminate the decode in the where clause.

Answer 1

Yes. If decode is only join-condition, then Oracle
produces cartesian join and only then filters it
based on decode.+/- 1 :-Q
  select *
    from emp, dept
   where decode(emp.deptno, dept.deptno, 1, 0) = 1
  Statement Id=2   Type=TABLE ACCESS
  Cost=3  TimeStamp=07-05-07::15::49:44
       (1)  SELECT STATEMENT  ALL_ROWS
     Est. Rows: 1  Cost: 9
(4) NESTED LOOPS
     Est. Rows: 1  Cost: 9
           (2)  TABLE TABLE ACCESS FULL SCOTT.DEPT  [Analyzed]
           (2)   Blocks: 5 Est. Rows: 4 of 4  Cost: 3
                Tablespace: USERS
           (3)  TABLE TABLE ACCESS FULL SCOTT.EMP  [Analyzed]
           (3)   Blocks: 5 Est. Rows: 1 of 14  Cost: 2
                Tablespace: USERS
  select *
    from emp, dept
  -- where decode(emp.deptno, dept.deptno, 1, 0) = 1
[pre] Statement Id=2 Type=TABLE ACCESS
Cost=3 TimeStamp=07-05-07::15::50:49
(1) SELECT STATEMENT ALL_ROWS
Est. Rows: 56 Cost: 9
(5) MERGE JOIN CARTESIAN
Est. Rows: 56 Cost: 9
(2) TABLE TABLE ACCESS FULL SCOTT.DEPT [Analyzed]
(2) Blocks: 5 Est. Rows: 4 of 4 Cost: 3
Tablespace: USERS
(4) BUFFER SORT
Est. Rows: 14 Cost: 6
(3) TABLE TABLE ACCESS FULL SCOTT.EMP [Analyzed]
(3) Blocks: 5 Est. Rows: 14 of 14 Cost: 2
Tablespace: USERS