How To Manage Transaction Isolation Level? - Oracle DBA FAQ - Introduction to PL/SQL

Transaction isolation level can be managed in a procedure by using the SET TRANSACTION and COMMIT statements. Here is a sample script on how to manage transaction isolation level:

SQL> CREATE OR REPLACE PROCEDURE HR.TOTAL_SALARY AS
  2    total_salary NUMBER(12,2);
  3  BEGIN
  4    SET TRANSACTION READ ONLY;
  5    SELECT SUM (salary) INTO total_salary FROM employees;
  6    DBMS_OUTPUT.PUT_LINE('Total salary 1: ' 
  7       || total_salary); 
  8    -- Other sessions may change salaries of some records
  9    SELECT SUM (salary) INTO total_salary FROM employees;
 10    DBMS_OUTPUT.PUT_LINE('Total salary 2: ' 
 11       || total_salary);
 12    COMMIT;
 13  END;
 14  /

SQL> EXECUTE TOTAL_SALARY;
Total salary 1: 691400
Total salary 2: 691400

"READ ONLY" transaction level takes a read only snapshot of the database. This allows other sessions to update the database without any locks. All queries in the session will produces identical results. So both SELECT statements in this script will return the same value guaranteed.