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.
A new option,
--innodb-optimize-keys, was implemented in mysqldump. It changes the way InnoDB tables are dumped, so that secondary 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.
- An additional
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.
- This optimization won’t work in case the index is dropped and added in the same
ALTER TABLEstatement because in that case MySQL copies the table.
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.
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_creationis OFF to avoid dropping keys that are part of a FOREIGN KEY constraint;
ALTER TABLEafter restoring the data from a dump would actually make the restore slower;
AUTO_INCREMENTcolumns, because they must be indexed, so it is impossible to temporarily drop the corresponding index;
PRIMARY KEYdefined, because in this case InnoDB picks such an index as the clustered one.
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