Question

I have used MERGE command in my prepared statement,and when i was executed it in a single threaded env,its working fine,But in multi threaded environment,it causes some problem.That is data is duplicated,that is if i have 5 threads,each record will duplicate 5 times.I think there is no lock in db to help the thread.
My code:

//db:oracle
sb.append("MERGE INTO EMP_BONUS EB USING (SELECT 1 FROM DUAL) on (EB.EMP_id = ?) WHEN MATCHED  THEN  UPDATE SET TA =?,DA=?,TOTAL=?,MOTH=?  WHEN NOT MATCHED THEN "+ "INSERT (EMP_ID, TA, DA, TOTAL, MOTH, NAME)VALUES(?,?,?,?,?,?) ");

//sql operation,calling from run() method
public void executeMerge(String threadName) throws Exception {              
        ConnectionPro cPro = new ConnectionPro();
        Connection connE = cPro.getConection();
        connE.setAutoCommit(false);
        System.out.println(sb.toString());
        System.out.println("Threadname="+threadName);
        PreparedStatement   pStmt= connE.prepareStatement(sb.toString());
        try {
            count = count + 1;

            for (Employee employeeObj : employee) {//datalist of employee

                pStmt.setInt(1, employeeObj.getEmp_id());
                pStmt.setDouble(2, employeeObj.getSalary() * .10);
                pStmt.setDouble(3, employeeObj.getSalary() * .05);
                pStmt.setDouble(4, employeeObj.getSalary()
                        + (employeeObj.getSalary() * .05)
                        + (employeeObj.getSalary() * .10));
                pStmt.setInt(5, count);
                pStmt.setDouble(6, employeeObj.getEmp_id());
                pStmt.setDouble(7, employeeObj.getSalary() * .10);
                pStmt.setDouble(8, employeeObj.getSalary() * .05);
                pStmt.setDouble(9, employeeObj.getSalary()
                        + (employeeObj.getSalary() * .05)
                        + (employeeObj.getSalary() * .10));
                pStmt.setInt(10, count);
                pStmt.setString(11, threadName);
                // pStmt.executeUpdate();

                pStmt.addBatch();
            }

            pStmt.executeBatch();
            connE.commit();
        } catch (Exception e) {
            connE.rollback();
            throw e;

        } finally {

            pStmt.close();
            connE.close();

        }
    }

if employee.size=5, thread count =5,after execution i would get 25 records instead of 5

Answer 1

If there is no constraint (i.e. a primary key or a unique key constraint on the emp_id column in emp_bonus), there would be nothing to prevent the database from allowing each thread to insert 5 rows. Since each database session cannot see uncommitted changes made by other sessions, each thread would see that there was no row in emp_bonus with the emp_id the thread is looking for (I’m assuming that employeeObj.getEmp_id() returns the same 5 emp_id values in each thread) so each thread would insert all 5 rows leaving you with a total of 25 rows if there are 5 threads. If you have a unique constraint that prevents the duplicate rows from being inserted, Oracle will allow the other 4 threads to block until the first thread commits allowing the subsequent threads to do updates rather than inserts. Of course, this will cause the threads to be serialized defeating any performance gains you would get from running multiple threads.