content_image/664795.jpg content_image/664799.jpg content_image/664803.jpg
<img src='content_image/664795.jpg'>

Defines the number of threads that can be used for parallel clustered index reads. Parallel scanning of partitions is supported as of MySQL 8.0.17. Parallel read threads can improve CHECK TABLE performance. InnoDB  reads the clustered index twice during a CHECK TABLE  operation. The second read can be performed in parallel. This feature does not apply to secondary index scans. The innodb_parallel_read_threads  session variable must be set to a value greater than 1 for parallel clustered index reads to occur. The actual number of threads used to perform a parallel clustered index read is determined by the innodb_parallel_read_threads  setting or the number of index subtrees to scan, whichever is smaller. The pages read into the buffer pool during the scan are kept at the tail of the buffer pool LRU list so that they can be discarded quickly when free buffer pool pages are required. 

As of MySQL 8.0.17, the maximum number of parallel read threads (256) is the total number of threads for all client connections. If the thread limit is reached, connections fall back to using a single thread. 

• innodb_print_all_deadlocks 

<img src='content_image/664799.jpg'>

When this option is enabled, information about all deadlocks in InnoDB  user transactions is recorded in the mysqld  error log. Otherwise, you see information about only the last deadlock, using the SHOW ENGINE INNODB STATUS  command. An occasional InnoDB  deadlock is not necessarily an issue, because InnoDB  detects the condition immediately and rolls back one of the transactions automatically. You might use this option to troubleshoot why deadlocks are occurring if an application does not have appropriate error-handling logic to detect the rollback and retry its operation. A large number of deadlocks might indicate the need to restructure transactions that issue DML or SELECT ... FOR UPDATE  statements for multiple tables, so that each transaction accesses the tables in the same order, thus avoiding the deadlock condition. 

For related information, see Section 15.7.5, “Deadlocks in InnoDB” . 

• innodb_print_ddl_logs 

<img src='content_image/664803.jpg'>

