The online DDL feature permits in-place table alterations or concurrent DML, or both. Benefits of this feature include:
Improved responsiveness and availability in busy production environments, where making a table unavailable for minutes or hours is not practical.
The ability to adjust the balance between performance and concurrency during DDL operations using the
LOCKclause. See The LOCK clause.
Less disk space usage and I/O overhead than the table-copy method.
The online DDL feature builds on the
InnoDB Fast Index Creation feature that is available in
MySQL 5.5, which optimized
CREATE INDEX and
DROP INDEX to avoid table-copying behavior.
The NDB Cluster
NDB storage engine also
supports online table schema changes, but uses its own syntax that
is not compatible with the syntax used for
online DDL operations. For more information, see
Section 18.104.22.168, “ALTER TABLE Online Operations in NDB Cluster”.
Typically, you do not need to do anything special to enable online DDL. By default, MySQL performs the operation in place if permitted, and with as little locking as possible.
You can control aspects of a DDL operation using the
ALGORITHM clauses of
ALTER TABLE statement. These
clauses are placed at the end of the statement, separated from the
table and column specifications by commas. For example:
ALTER TABLE tbl_name ADD PRIMARY KEY (column), ALGORITHM=INPLACE, LOCK=NONE;
LOCK clause is useful for fine-tuning the
degree of concurrent access to the table. The
ALGORITHM clause is primarily intended for
performance comparisons and as a fallback to the older table-copying
behavior in case you encounter any issues. For example:
To avoid accidentally making the table unavailable for reads, writes, or both, specify a clause on the
ALTER TABLEstatement such as
LOCK=NONE(permit reads and writes) or
LOCK=SHARED(permit reads). The operation halts immediately if the requested level of concurrency is not available.
To compare performance, run a statement with
ALGORITHM=COPY. Alternatively, run a statement with the
old_alter_tableconfiguration option disabled and enabled.
To avoid tying up the server with an
ALTER TABLEoperation that copies the table, include
ALGORITHM=INPLACE. The statement halts immediately if it cannot use the in-place mechanism.