Oracle PL/SQL - Are NO_DATA_FOUND Exceptions bad for stored procedure performance?

Question

I\'m writing a stored procedure that needs to have a lot of conditioning in it. With the general knowledge from C#.NET coding that exceptions can hurt performance, I\'ve always

Answer 1

If it's important you really need to benchmark both options!

Having said that, I have always used the exception method, the reasoning being it's better to only hit the database once.

Answer 2

The count(*) will never raise exception because it always returns actual count or 0 - zero, no matter what. I'd use count.

Answer 3

The first (excellent) answer stated -

The method with count() is unsafe. If another session deletes the row that met the condition after the line with the count(*), and before the line with the select ... into, the code will throw an exception that will not get handled.

Not so. Within a given logical Unit of Work Oracle is totally consistent. Even if someone commits the delete of the row between a count and a select Oracle will, for the active session, obtain the data from the logs. If it cannot, you will get a "snapshot too old" error.

Answer 4

An alternative to @Steve's code.

DECLARE
  CURSOR foo_cur IS 
    SELECT NEEDED_FIELD WHERE condition ;
BEGIN
  FOR foo_rec IN foo_cur LOOP
     ...
  END LOOP;
EXCEPTION
  WHEN OTHERS THEN
    RAISE;
END ;

The loop is not executed if there is no data. Cursor FOR loops are the way to go - they help avoid a lot of housekeeping. An even more compact solution:

DECLARE
BEGIN
  FOR foo_rec IN (SELECT NEEDED_FIELD WHERE condition) LOOP
     ...
  END LOOP;
EXCEPTION
  WHEN OTHERS THEN
    RAISE;
END ;

Which works if you know the complete select statement at compile time.

Answer 5

May be beating a dead horse here, but I bench-marked the cursor for loop, and that performed about as well as the no_data_found method:

declare
  otherVar  number;
begin
  for i in 1 .. 5000 loop
     begin
       for foo_rec in (select NEEDED_FIELD from t where cond = 0) loop
         otherVar := foo_rec.NEEDED_FIELD;
       end loop;
       otherVar := 0;
     end;
   end loop;
end;

PL/SQL procedure successfully completed.

Elapsed: 00:00:02.18

Answer 6

Since SELECT INTO assumes that a single row will be returned, you can use a statement of the form:

SELECT MAX(column)
  INTO var
  FROM table
 WHERE conditions;

IF var IS NOT NULL
THEN ...

The SELECT will give you the value if one is available, and a value of NULL instead of a NO_DATA_FOUND exception. The overhead introduced by MAX() will be minimal-to-zero since the result set contains a single row. It also has the advantage of being compact relative to a cursor-based solution, and not being vulnerable to concurrency issues like the two-step solution in the original post.