### 15.12.8 Online DDL Limitations

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 The following limitations apply to online DDL operations:

* The table is copied when creating an index on a `TEMPORARY TABLE`.

* The [`ALTER TABLE`](alter-table.html) clause `LOCK=NONE` is not permitted if there are `ON...CASCADE` or `ON...SET NULL` constraints on the table.

* Before an in-place online DDL operation can finish, it must wait for transactions that hold metadata locks on the table to commit or roll back. An online DDL operation may briefly require an exclusive metadata lock on the table during its execution phase, and always requires one in the final phase of the operation when updating the table definition. Consequently, transactions holding metadata locks on the table can cause an online DDL operation to block. The transactions that hold metadata locks on the table may have been started before or during the online DDL operation. A long running or inactive transaction that holds a metadata lock on the table can cause an online DDL operation to timeout.

* When running an in-place online DDL operation, the thread that runs the [`ALTER TABLE`](alter-table.html) statement applies an online log of DML operations that were run concurrently on the same table from other connection threads. When the DML operations are applied, it is possible to encounter a duplicate key entry error (ERROR 1062 (23000): Duplicate entry), even if the duplicate entry is only temporary and would be reverted by a later entry in the online log. This is similar to the idea of a foreign key constraint check in `InnoDB` in which constraints must hold during a transaction.

* [`OPTIMIZE TABLE`](optimize-table.html) for an `InnoDB` table is mapped to an [`ALTER TABLE`](alter-table.html) operation to rebuild the table and update index statistics and free unused space in the clustered index. Secondary indexes are not created as efficiently because keys are inserted in the order they appeared in the primary key. [`OPTIMIZE TABLE`](optimize-table.html) is supported with the addition of online DDL support for rebuilding regular and partitioned `InnoDB` tables.

* Tables created before MySQL 5.6 that include temporal columns ([`DATE`](datetime.html), [`DATETIME`](datetime.html) or [`TIMESTAMP`](datetime.html)) and have not been rebuilt using ` ALGORITHM=COPY` do not support `ALGORITHM=INPLACE`. In this case, an [`ALTER TABLE ... ALGORITHM=INPLACE`](alter-table.html) operation returns the following error:

  ```
  ERROR 1846 (0A000): ALGORITHM=INPLACE is not supported.
  Reason: Cannot change column type INPLACE. Try ALGORITHM=COPY.
  ```

* The following limitations are generally applicable to online DDL operations on large tables that involve rebuilding the table:

  * There is no mechanism to pause an online DDL operation or to throttle I/O or CPU usage for an online DDL operation.

  * Rollback of an online DDL operation can be expensive should the operation fail.

  * Long running online DDL operations can cause replication lag. An online DDL operation must finish running on the source before it is run on the replica. Also, DML that was processed concurrently on the source is only processed on the replica after the DDL operation on the replica is completed.

   For additional information related to running online DDL operations on large tables, see [Section 15.12.2, “Online DDL Performance and Concurrency”](innodb-online-ddl-performance.html).

