14.7.5.1 An InnoDB Deadlock Example
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:
mysql> CREATE TABLE t (i INT) ENGINE = InnoDB;
Query OK, 0 rows affected (1.07 sec)
mysql> INSERT INTO t (i) VALUES(1);
Query OK, 1 row affected (0.09 sec)
mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT * FROM t WHERE i = 1 LOCK IN SHARE MODE;
+------+
| i |
+------+
| 1 |
+------+
Next, client B begins a transaction and attempts to delete the row from the table:
mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)
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:
mysql> DELETE FROM t WHERE i = 1;
ERROR 1213 (40001): Deadlock found when trying to get lock;
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:
ERROR 1213 (40001): Deadlock found when trying to get lock;
try restarting transaction
At that point, the lock request for the other client can be granted and it deletes the row from the table.