The mysqldump Command¶

A new option, --innodb-optimize-keys, was implemented in mysqldump. It changes the way InnoDB tables are dumped, so that secondary and foreign keys are created after loading the data, thus taking advantage of fast index creation. More specifically:

• KEY, UNIQUE KEY, and CONSTRAINT clauses are omitted from CREATE TABLE statements corresponding to InnoDB tables.
• An additional ALTER TABLE is issued after dumping the data, in order to create the previously omitted keys.

ALTER TABLE¶

When ALTER TABLE requires a table copy, secondary keys are now dropped and recreated later, after copying the data. The following restrictions apply:

• Only non-unique keys can be involved in this optimization.
• If the table contains foreign keys, or a foreign key is being added as a part of the current ALTER TABLE statement, the optimization is disabled for all keys.

OPTIMIZE TABLE¶

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.

Caveats¶

InnoDB fast index creation uses temporary files in tmpdir for all indexes being created. So make sure you have enough tmpdir space when using expand_fast_index_creation. It is a session variable, so you can temporarily switch it off if you are short on tmpdir space and/or don’t want this optimization to be used for a specific table.

There’s also a number of cases when this optimization is not applicable:

• UNIQUE indexes in ALTER TABLE are ignored to enforce uniqueness where necessary when copying the data to a temporary table;
• ALTER TABLE and OPTIMIZE TABLE always process tables containing foreign keys as if expand_fast_index_creation is OFF to avoid dropping keys that are part of a FOREIGN KEY constraint;
• mysqldump --innodb-optimize-keys ignores foreign keys because InnoDB requires a full table rebuild on foreign key changes. So adding them back with a separate ALTER TABLE after restoring the data from a dump would actually make the restore slower;
• mysqldump --innodb-optimize-keys ignores indexes on AUTO_INCREMENT columns, because they must be indexed, so it is impossible to temporarily drop the corresponding index;
• mysqldump --innodb-optimize-keys ignores the first UNIQUE index on non-nullable columns when the table has no PRIMARY KEY defined, because in this case InnoDB picks such an index as the clustered one.

System Variables¶

variable expand_fast_index_creation¶

Command Line:	Yes
Config File:	No
Scope:	Local/Global
Dynamic:	Yes
Variable Type:	Boolean
Default Value:	OFF
Range:	ON/OFF