SQL statements commonly used in MySQL to view locks and transactions
When we are using the MySQL database, it is very important to understand how to view the status of locks and transactions. This information can help us debug and optimize database performance, as well as solve concurrent access problems. In this blog, I will introduce some commonly used MySQL queries to view the status of locks and transactions.
1. View the current lock status
To view the lock status in the current database, you can use the following SQL statement:
SHOW OPEN TABLES WHERE In_use > 0;
This statement will display the table currently being used, including the table name and thread ID using the lock.
2. View the current transaction status
To view the transaction status in the current database, you can use the following SQL statement:
SHOW ENGINE INNODB STATUS;
This statement will display the status information of the InnoDB engine, which includes the currently executing transaction information, lock waiting information, and other related statistics.
3. View the current transaction list
To see a list of currently executing transactions, you can use the following SQL statement:
SELECT * FROM INFORMATION_SCHEMA.INNODB_TRX;
This statement will return a result set containing all currently active transactions, including information such as transaction ID, start time, and status of waiting locks.
4. Check the status of the current lock
To view the current lock status, you can use the following SQL statement:
SELECT * FROM INFORMATION_SCHEMA.INNODB_LOCKS;
This statement will return a result set containing the locks currently being locked, including information such as the type of lock, the object locked, and the transaction ID holding the lock.
5. Check the status of the current lock waiting
To view the current lock waiting status, you can use the following SQL statement:
SELECT * FROM INFORMATION_SCHEMA.INNODB_LOCK_WAITS;
This statement will return a result set that contains the transaction that is currently waiting for the lock, including the transaction ID waiting for the lock, the type of the lock that is waiting, and the type of the lock that is being waited for.
A deadlock occurs to resolve the deadlock SQL statement
1. Find deadlock information:
SHOW ENGINE INNODB STATUS;
This statement will return InnoDB engine status information, which contains a detailed description of the deadlock, including information about the transactions and locks involved.
2. Kill the process that caused the deadlock:
KILL <thread_id>;
Using the deadlock information provided in the previous step, determine the thread ID causing the deadlock and terminate that thread with this command. This will forcibly terminate the running transaction, releasing the deadlock.
3. Adjust transaction isolation level
Sometimes deadlocks may occur because the transaction isolation level is not set properly. You can try to adjust the transaction isolation level to a higher level, such as changing the isolation level from REPEATABLE READ to SERIALIZABLE. Doing so may reduce concurrency performance, but it reduces the probability of deadlocks.
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
4. Restart the database:
If the deadlock problem persists, consider restarting the MySQL database. Restart will terminate all connections and transactions and clear existing lock state.
5. Optimize queries and transactions:
One of the causes of deadlocks can be poorly designed queries or transactions. The risk of deadlock can be reduced by optimizing query statements, adding indexes, and reducing the scope or holding time of locks.
6. Solve the deadlock by setting the timeout period:
SET innodb_lock_wait_timeout = <timeout_value>;
This statement can set the lock waiting timeout of InnoDB. If a transaction cannot acquire the required lock resource within the timeout period, it will automatically abandon the request and return an error. By setting an appropriate timeout, the deadlock duration can be reduced.
7. Refactor the business logic:
If the deadlock problem occurs frequently, the logic of the transaction may need to be redesigned. Consider changing the order of transactions, detaching transactions, or introducing a finer-grained locking strategy.