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MySQL Tutorial - Experiment with Data Locks

By: FYIcenter.com

(Continued from previous topic...)

How To Experiment Data Locks?

If you want to have some experience with data locks, you can create two windows running two mysql transactions in two sessions. In session 1, you can run a UPDATE statement with REPEATABLE READ transaction isolation level to create an exclusive lock. Before committing session 1, switch to session 2, and run a UPDATE statements on the same row as session 1. The UPDATE statement will be put into wait status because of the data lock. Follow the tutorial exercise below to experience yourself:

(session 1)

>\mysql\bin\mysql -u dev -piyf fyi

mysql> SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
Query OK, 0 rows affected (0.00 sec)

mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT * FROM fyi_links;
+-----+---------------+-------+--------+--------------------
| id  | url           | notes | counts | created           
+-----+---------------+-------+--------+--------------------
| 101 | fyicenter.com | Good  |    999 | 2006-07-01 20:34:10
| 110 | centerfyi.com | Wrong |      0 | 2006-07-01 20:34:12
+-----+---------------+-------+--------+--------------------
2 rows in set (0.00 sec)

mysql> UPDATE fyi_links SET url='FYICENTER.COM' 
   WHERE id=101;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

(Exclusive lock (X) placed on row id=101)

Switch to session 2:

(session 2)

>\mysql\bin\mysql -u dev -piyf fyi

mysql> SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
Query OK, 0 rows affected (0.00 sec)

mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)

mysql> UPDATE fyi_links SET notes='Best' where id=101;

(Blocked to wait for exclusive lock (X) on row id=101)

Switch to back session 1:

(session 1)

mysql> COMMIT;
Query OK, 0 rows affected (0.05 sec)

(Exclusive lock (X) on row id=101 released)

Switch to session 2:

(session 2)

Query OK, 1 row affected (11.80 sec)
Rows matched: 1  Changed: 1  Warnings: 0

(Got permission to update row id=101)

(Continued on next topic...)

  1. What Is a Transaction?
  2. How To Start a New Transaction?
  3. How To End the Current Transaction?
  4. How To Create a Table for Transaction Testing?
  5. How To Switch between Autocommit-On and Autocommit-Off Modes?
  6. How To Find Out the Current Transaction Mode?
  7. How To Start a New Transaction Explicitly?
  8. How To Commit the Current Transaction?
  9. How To Rollback the Current Transaction?
  10. What Happens to the Current Transaction If a START TRANSACTION Is Executed?
  11. What Happens to the Current Transaction If a DDL Statement Is Executed?
  12. What Happens to the Current Transaction If the Session Is Ended?
  13. What Happens to the Current Transaction If the Session Is Killed?
  14. How Does MySQL Handle Read Consistency?
  15. What Are Transaction Isolation Levels?
  16. How To View and Change the Current Transaction Isolation Level?
  17. What Is a Data Lock?
  18. How To Experiment Data Locks?
  19. How Long a Transaction Will Wait for a Data Lock?
  20. What Happens to Your Transactions When ERROR 1205 Occurred?
  21. What Is a Dead Lock?
  22. How To Experiment Dead Locks?
  23. What Happens to Your Transactions When ERROR 1213 Occurred?
  24. What Are Impacts on Applications from Locks, Timeouts, and DeadLocks?

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