Expanded fast index creation¶
This feature is a tech preview. Before using this feature in production, we recommend that you test restoring production from physical backups in your environment, and also use the alternative backup method for redundancy.
Percona has implemented several changes related to MySQL’s fast index creation feature. 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.
This feature implements a session variable that enables extended fast index creation. Besides optimizing DDL directly, expand_fast_index_creation may also optimize index access for subsequent DML statements because using it results in much less fragmented indexes.
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:
UNIQUE KEY, and
CONSTRAINTclauses are omitted from
CREATE TABLEstatements corresponding to InnoDB tables.
ALTER TABLEis issued after dumping the data, in order to create the previously omitted keys.
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 TABLEstatement, the optimization is disabled for all keys.
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
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:
ALTER TABLEare ignored to enforce uniqueness where necessary when copying the data to a temporary table;
OPTIMIZE TABLEalways 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 TABLEafter restoring the data from a dump would actually make the restore slower;
mysqldump –innodb-optimize-keys ignores indexes on
AUTO_INCREMENTcolumns, 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 KEYdefined, because in this case InnoDB picks such an index as the clustered one.