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MySQL 8.0 Reference Manual  /  ...  /  Online DDL Limitations

15.12.7 Online DDL Limitations

The following limitations apply to online DDL operations:

  • 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 B.3, “Server Error Codes and Messages” for information related to error 1553.

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

  • Before an 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 transaction that holds a metadata lock on the table can cause an online DDL operation to timeout.

  • When running an online DDL operation, the thread that runs the ALTER TABLE 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 for an InnoDB table is mapped to an ALTER TABLE 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 also supports online DDL for rebuilding regular and partitioned InnoDB tables. For additional information, see Section 15.12.1, “Online DDL Overview”.

    Note

    Prior to MySQL 5.6.17 / 5.7.4, there was not online DDL support for this operation.

  • InnoDB tables created before MySQL 5.6 do not support ALTER TABLE ... ALGORITHM=INPLACE for tables that include temporal columns (DATE, DATETIME or TIMESTAMP) and have not been rebuilt using ALTER TABLE ... ALGORITHM=COPY. In this case, an ALTER TABLE ... ALGORITHM=INPLACE operation returns the following error:

    ERROR 1846 (0A000): ALGORITHM=INPLACE is not supported.
    Reason: Cannot change column type INPLACE. Try ALGORITHM=COPY.
  • These limitations are generally applicable to online DDL operations on large tables where table copying is involved:

    • 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 master before it is run on the slave. Also, DML that was processed concurrently on the master is only processed on the slave after the DDL operation on the slave is completed (Bug #73196).

    For additional information related to running online DDL operations on large tables, see Section 15.12.2, “Online DDL Performance, Concurrency, and Space Requirements”.


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