[official MySQL document] deadlock

A deadlock is a situation where different transactions are unable to proceed because each holds a lock that the other needs. Because both transactions are waiting for a resource to become available, neither ever release the locks it holds.

Deadlock is generated in this way ： Different transactions hold the locks needed by each other , And wait for the other party to release the lock .

One select ··· lock in share mode And delete Deadlock instance of

Create a table , And insert data ：

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)

Business A	Business B	result
select * from t where i = 1 lock in share mode;		Get S lock
	delete from t where i = 1;	Try to get X lock , For business A Hold the read lock of the row , So enter the lock waiting queue .
delete from t where i = 1;		Deadlock occurs , Here is the transaction A Trying to get a write lock , But the business B In front of the waiting queue , Business A Cannot upgrade a read lock to a write lock .A wait for B Release the write lock ,B wait for A Release read lock .

When update、select for update Different tables are locked in different order , It's prone to deadlock .

  In the same table , Two transactions lock different records or gaps in different order , It's also deadlocked .

InnoDB Use automatic row level locking . Even in only insert or delete In a row of transactions , There can also be deadlocks . This is because these operations are not real “ atom ”; They will automatically insert or delete lines （ There may be more than one ） Index record setting lock .

 

How to avoid deadlock

1. Business is better than LOCK TABLE It will be better

2. The business is as small as possible , Avoid reading and writing large amounts of data

3. As far as possible, different transactions acquire locks in the same order （select for update）

4. stay select for update and update where Create index on

5. The possibility of deadlock has nothing to do with the isolation level of transactions , Deadlocks are related to write operations , The isolation level solves the visibility of reading

6. SHOW ENGINE INNODB STATUS You can check the cause of the deadlock .

7. When deadlocks occur frequently , Can make innodb_print_all_deadlocks

8. Use select for update , select lock in share mode, The recommended isolation level is RC

9. Operate multiple tables , Or different rows of the same table , It is recommended to use the same sequence , Deadlock can be avoided

10. Set the appropriate index , Avoid scanning a large number of data rows

11. Use as common as possible select instead of for update, Avoid locking

12. Serialization is the final solution