PL/SQL User's Guide and Reference

Parameter Aliasing

To optimize execution, the PL/SQL compiler can choose different methods of parameter passing (copy or reference) for different parameters in the same subprogram call. When the compiler chooses the copy method, the value of an actual parameter is copied into the subprogram. When the compiler chooses the reference method, the address of an actual parameter is passed to the subprogram.

The easy-to-avoid problem of aliasing occurs when a global variable appears as an actual parameter in a subprogram call and then is referenced within the subprogram. The result is indeterminate because it depends on the method of parameter passing chosen by the compiler. Consider the following example:

DECLARE
   rent REAL;
   PROCEDURE raise_rent (increase IN OUT REAL) IS
   BEGIN
      rent := rent + increase;
      /* At this point, if the compiler passed the address
         of the actual parameter to the subprogram, the same 
         variable has two names. Thus, the term 'aliasing'. */
      ...
   END raise_rent;
   ...
BEGIN
   ...
   raise_rent(rent);  -- indeterminate

Aliasing also occurs when the same actual parameter appears twice in a subprogram call. Unless both formal parameters are IN parameters, the result is indeterminate, as the following example shows:

DECLARE
   str VARCHAR2(10);
   PROCEDURE reverse (in_str VARCHAR2, out_str OUT VARCHAR2) IS
   BEGIN
      /* Reverse order of characters in string here. */
      ...
      /* At this point, whether the value of in_str 
         is 'abcd' or 'dcba' depends on the methods of 
         parameter passing chosen by the compiler. */
   END reverse;
   ...
BEGIN
   str := 'abcd';
   reverse(str, str);  -- indeterminate