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Fast index creation was implemented in MySQL as a way to speed up the process of adding or dropping indexes on tables with many rows. However, cases have been found in which fast index creation creates an inconsistency between MySQL and InnoDB data dictionaries.
This feature implements a session variable that disables fast index creation. This causes indexes to be created in the way they were created before fast index creation was implemented. While this is slower, it avoids the problem of data dictionary inconsistency between MySQL and InnoDB.
Internally, OPTIMIZE TABLE is mapped to ALTER TABLE ... ENGINE=innodb for InnoDB tables. As a consequence, it now also benefits from fast index creation, with the same restrictions as for ALTER TABLE.