You should be aware of the following considerations when creating or dropping indexes using the InnoDB storage engine:

If any of the indexed columns use the UTF-8 character encoding, MySQL copies the table instead of using Fast Index Creation. This has been reported as MySQL Bug#33650.

Due to a limitation of MySQL, the table is copied, rather than using Fast Index Creation when you create an index on a TEMPORARY TABLE. This has been reported as MySQL Bug#39833.

The command ALTER IGNORE TABLE t ADD UNIQUE INDEX does not delete duplicate rows. This has been reported as MySQL Bug#40344. The IGNORE keyword is ignored, and duplicates cause failure of the operation with the following error message:

ERROR 23000: Duplicate entry '347' for key 'pl'

As noted above, a newly-created index contains only information about data current at the time the index was created. Therefore, you should not run queries in a transaction that might use a secondary index that did not exist at the beginning of the transaction. There is no way for InnoDB to access “old” data that is consistent with the rest of the data read by the transaction. See the discussion of locking in Section 2.4, “Concurrency Considerations for Fast Index Creation”.

Prior to InnoDB storage engine 1.0.4, unexpected results could occur if a query attempts to use an index created after the start of the transaction containing the query. If an old transaction attempts to access a “too new” index, InnoDB storage engine 1.0.4 and later reports an error:

ERROR HY000: Table definition has changed, please retry transaction

As the error message suggests, committing (or rolling back) the transaction, and restarting it, cures the problem.

InnoDB storage engine 1.0.2 introduces some improvements in error handling when users attempt to drop indexes. See section Section 8.7, “Better Error Handling When Dropping Indexes” for details.

MySQL 5.5 does not support efficient creation or dropping of FOREIGN KEY constraints. Therefore, if you use ALTER TABLE to add or remove a REFERENCES constraint, the child table is copied, rather than using Fast Index Creation.