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15.7.5.1 An InnoDB Deadlock Example

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The following example illustrates how an error can occur when a lock request would cause a deadlock. The example involves two clients, A and B.

First, client A creates a table containing one row, and then begins a transaction. Within the transaction, A obtains an S lock on the row by selecting it in share mode:

  1. mysql> CREATE TABLE t (i INT) ENGINE = InnoDB;
  2. Query OK, 0 rows affected (1.07 sec)
  3.  
  4. mysql> INSERT INTO t (i) VALUES(1);
  5. Query OK, 1 row affected (0.09 sec)
  6.  
  7. mysql> START TRANSACTION;
  8. Query OK, 0 rows affected (0.00 sec)
  9.  
  10. mysql> SELECT * FROM t WHERE i = 1 FOR SHARE;
  11. +------+
  12. | i    |
  13. +------+
  14. |    1 |
  15. +------+

Next, client B begins a transaction and attempts to delete the row from the table:

  1. mysql> START TRANSACTION;
  2. Query OK, 0 rows affected (0.00 sec)
  3.  
  4. mysql> DELETE FROM t WHERE i = 1;

The delete operation requires an X lock. The lock cannot be granted because it is incompatible with the S lock that client A holds, so the request goes on the queue of lock requests for the row and client B blocks.

Finally, client A also attempts to delete the row from the table:

  1. mysql> DELETE FROM t WHERE i = 1;
  2. ERROR 1213 (40001): Deadlock found when trying to get lock;
  3. try restarting transaction

Deadlock occurs here because client A needs an X lock to delete the row. However, that lock request cannot be granted because client B already has a request for an X lock and is waiting for client A to release its S lock. Nor can the S lock held by A be upgraded to an X lock because of the prior request by B for an X lock. As a result, InnoDB generates an error for one of the clients and releases its locks. The client returns this error:

  1. ERROR 1213 (40001): Deadlock found when trying to get lock;
  2. try restarting transaction

At that point, the lock request for the other client can be granted and it deletes the row from the table.


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Document created the 26/06/2006, last modified the 26/10/2018
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