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InnoDB uses the Contention-Aware Transaction Scheduling (CATS) algorithm to prioritize transactions that are waiting for locks. When multiple transactions are waiting for a lock on the same object, the CATS algorithm determines which transaction receives the lock first.

The CATS algorithm prioritizes waiting transactions by assigning a scheduling weight, which is computed based on the number of transactions that a transaction blocks. For example, if two transactions are waiting for a lock on the same object, the transaction that blocks the most transactions is assigned a greater scheduling weight. If weights are equal, priority is given to the longest waiting transaction.

You can view transaction scheduling weights by querying theTRX_SCHEDULE_WEIGHT column in the Information SchemaINNODB_TRX table. Weights are computed for waiting transactions only. Waiting transactions are those in aLOCK WAIT transaction execution state, as reported by theTRX_STATE column. A transaction that is not waiting for a lock reports a NULLTRX_SCHEDULE_WEIGHT value.

INNODB_METRICS counters are provided for monitoring of code-level transaction scheduling events. For information about usingINNODB_METRICS counters, seeSection 17.15.6, “InnoDB INFORMATION_SCHEMA Metrics Table”.