During an online DDL operation that copies the table, files are written to the temporary directory ($TMPDIR on Unix, %TEMP% on Windows, or the directory specified by the --tmpdir configuration variable). Each temporary file is large enough to hold one column in the new table or index, and each one is removed as soon as it is merged into the final table or index.

An ALTER TABLE statement that contains DROP INDEX and ADD INDEX clauses that both name the same index uses a table copy, not Fast Index Creation.

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.

InnoDB handles error cases when users attempt to drop indexes needed for foreign keys. See section Section 14.2.5.9, “Better Error Handling when Dropping Indexes” for details.

The ALTER TABLE clause LOCK=NONE is not allowed if there are ON...CASCADE or ON...SET NULL constraints on the table.

During each online DDL ALTER TABLE statement, regardless of the LOCK clause, there are brief periods at the beginning and end requiring an exclusive lock on the table (the same kind of lock specified by the LOCK=EXCLUSIVE clause). Thus, an online DDL operation might wait before starting if there is a long-running transaction performing inserts, updates, deletes, or SELECT ... FOR UPDATE on that table; and an online DDL operation might wait before finishing if a similar long-running transaction was started while the ALTER TABLE was in progress.