MySQL 8.0 Reference Manual Including MySQL NDB Cluster 8.0

17.12.5 Configuring Parallel Threads for Online DDL Operations

The workflow of an online DDL operation that creates or rebuilds a secondary index involves:

The number of parallel threads that can be used to scan clustered index is defined by the innodb_parallel_read_threads variable. The default setting is 4. The maximum setting is 256, which is the maximum number for all sessions. The actual number of threads that scan the clustered index is the number defined by the innodb_parallel_read_threads setting or the number of index subtrees to scan, whichever is smaller. If the thread limit is reached, sessions fall back to using a single thread.

The number of parallel threads that sort and load data is controlled by the innodb_ddl_threads variable, introduced in MySQL 8.0.27. The default setting is 4. Prior to MySQL 8.0.27, sort and load operations are single-threaded.

The following limitations apply: