Does sub program reduces the performance

Question

hi all,
suppose that i have two solutions to increase the salaries of the employees for a table
method 1:
create or replace function inc_sal (pempno emp.empno%type) return integer
is
vsal emp.sal%type;
begin
select sal into vsal from emp where empno=pempno;
vsal:=vsal+vsal*0.30;
return(vsal);
end;
sql> select sal, inc_sal(empno) from emp;
and
method 2:
sql> select sal, sal*0.30 as inc_sal from emp;
now in the first method, for each record of the table the function is called and collect the returned value...
but for the second method there is nothing like that
so now which method is faster in performance, does the number of function calls in first method reduce the performance......?
thanks and regards,
sri ram.

Answer 1

Oracle has a separate SQL and PL/SQL engine. When you go from one to the other, this is called a 'context switch'.
A context switch is a costly operation.
In variant 1 you seem to embed the function you wrote in SQL.
This means
- for each record you will have a context switch
- in the function you call SQL: another context switch
- The SQL in your function will show as recursive SQL and will never get merged into the main query.
So actually variant 1 is a textbook example of 'slow by slow programming' or 'How to write unscalable applications'
Remember Tom Kytes mantra:
When you can do it in SQL, you should do it in SQL
When you can't do it in SQL, you should do it in PL/SQL
When you can't do it in PL/SQL, you should do it in Java (in the database)
When you can't do it in Java, you should do it in any language outside the database.
Sybrand Bakker
Senior Oracle DBA