What is table metadata lock?

4 Metadata Locking. MySQL uses metadata locking to manage concurrent access to database objects and to ensure data consistency. Metadata locking does involve some overhead, which increases as query volume increases. Metadata contention increases the more that multiple queries attempt to access the same objects.

What does locking data mean?

Record locking is the technique of preventing simultaneous access to data in a database, to prevent inconsistent results. This prevents records from being overwritten incorrectly, but allows only one record to be processed at a time, locking out other users who need to edit records at the same time.

What causes database locking?

What is a database lock in the context of SQL? When two sessions or users of database try to update or delete the same data in a table, then there will be a concurrent update problem. In order to avoid this problem, database locks the data for the first user and allows him to update/delete the data.

What is Lock_wait_timeout?

From the MySQL 5.5 docs for lock_wait_timeout: This variable specifies the timeout in seconds for attempts to acquire metadata locks. The permissible values range from 1 to 31536000 (1 year). The default for innodb_lock_wait_timeout, by comparison, is 50 seconds.

What causes waiting for table metadata lock?

What does metadata lock wait mean? When there is an active transaction (explicit or implicit) on the table, MySQL does not allow writing of data to metadata. It does this to maintain metadata consistency in the table in a concurrent environment.

How does InnoDB check engine status?

SHOW ENGINE INNODB STATUS is a specific form of the SHOW ENGINE statement that displays the InnoDB Monitor output, which is extensive InnoDB information which can be useful in diagnosing problems.

How long does a data lock take?

The Clinical Data Manager is responsible for steering the data management process to ensure that the database is locked on time, and correctly. In this blog we lay out the 6 steps to database lock success.

What happens after database lock in clinical trial?

A database is locked after review, query resolution and determination that it is ready for analysis. Once the database is complete and clean the database is locked to prevent further changes to the data and the analysis may be performed.

How do I stop my database from locking?

When an object is being accessed concurrently by multiple programs or users, consider increasing free space, causing fewer rows to be stored on a single page, at least until data is added. The fewer rows per page, the less intrusive page locking will be because fewer rows will be impacted by a page lock.

What Causes lock wait timeout MySQL?

The common causes are: The offensive transaction is not fast enough to commit or rollback the transaction within innodb_lock_wait_timeout duration. The offensive transaction is waiting for row lock to be released by another transaction.

What does waiting for table flush mean?

If a query is running that accesses that table at the time the the FLUSH TABLE is issued, the FLUSH TABLE will be blocked until the query completes. While the FLUSH TABLE is blocked, no new queries accessing the affected tables can start running and their state in the process list will show “Waiting for table flush“.