SQL Cost of using DECODE built-in

Question

I'm trying to tune some SQL and wondering about the cost of using DECODE. I believe that by using this function, it prevents any index on the related column from being used.
Do any "Tuning Gurus" have opinions on the use of DECODE?
TIA,

Answer 1

As a general rule, the Oracle built-in functions are pretty fast to execute. You will not really see any difference in performance between a sql statement using a built-in function in the SELECT clause, and one not using one.
However, a lot depends on where and how you use the DECODE.
SELECT col1,DECODE(col2,1,'YES',2,'NO',3,'MAYBE')
FROM table
will be no slower than
SELECT col1,col2
FROM table
If there is an index on col2, then
SELECT col1,DECODE(col2,1,'YES',2,'NO',3,'MAYBE')
FROM table
WHERE col2 BETWEEN 1 AND 3
will use it.  However
SELECT col1,col2
FROM table
WHERE DECODE(col2,1,'YES',2,'NO',3,'MAYBE') IN ('YES','NO','MAYBE')
will not (unless you have a function based index).  But,
SELECT col1,col3
FROM table
WHERE col2 BETWEEN 1 and 3 and
      col3 = DECODE(col2,1,'YES',2,'NO',3,'MAYBE') IN ('YES','NO','MAYBE')
will.Note that "will" in the above really means can. it is up to the optimizer whether it actually is used or not.
HTH
John