use of FOR UPDATE statement

Question

I am using PL/SQL (Oracle 11g) to update the EMPLOYEES table salary column.

I have used two separate scripts to do the same thing i.e update the salary of employees.

One script uses FOR UPDATE OF statement where as another script doesn't uses it. In both cases I found that oracle holds the row level locks until we execute the ROLLBACK or COMMIT commands.

Then what is the difference in between two scripts?

Which one is better to use?

Here are the two scripts I am talking about:

-- Script 1: Uses FOR UPDATE OF

declare
cursor cur_emp
is
select employee_id,department_id from employees where department_id = 90 for update of salary;
begin
  for rec in cur_emp
  loop
    update Employees
    set salary = salary*10
    where current of cur_emp;
   end loop;
end;


--Script 2: Does the same thing like script 1 but FOR UPDATE OF is not used here

declare
cursor cur_emp
is
select employee_id,department_id from employees where department_id = 90;
begin
  for rec in cur_emp
  loop
    update Employees
    set salary = salary*10
    where Employee_ID = rec.employee_id;
   end loop;
end;

I found that Oracle acquired the row level locks on both cases. So, what is the benefit of using FOR UPDATE OF and Which is the better way of coding?

Answer 1

When you specify FOR UPDATE, the row is locked at the point that you SELECT the data. Without the FOR UPDATE, the row is locked at the point you UPDATE the row. In the second script, another session could potentially lock the row between the time that the SELECT was executed and the point that you tried to UPDATE it.

If you are dealing with a SELECT statement that returns relatively few rows and a tight inner loop, it is unlikely that there will be an appreciable difference between the two. Adding a FOR UPDATE on the SELECT also gives you the opportunity to add a timeout clause if you don't want your script to block indefinitely if some other session happens to have one of the row you're trying to update locked.