The online DDL feature provides support for instant and in-place table alterations and concurrent DML. 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.
For in-place operations, the ability to adjust the balance between performance and concurrency during DDL operations using the LOCK clause. See The LOCK clause.
Less disk space usage and I/O overhead than the table-copy method.
ALGORITHM=INSTANT support is available for ADD COLUMN and other operations in MySQL 8.0.12.
Typically, you do not need to do anything special to enable online DDL. By default, MySQL performs the operation instantly or in place, as permitted, with as little locking as possible.
You can control aspects of a DDL operation using the ALGORITHM and LOCK clauses of the 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 TABLEtbl_nameADD PRIMARY KEY (column), ALGORITHM=INPLACE, LOCK=NONE;
The LOCK clause may be used for operations that are performed in place and is useful for fine-tuning the degree of concurrent access to the table during operations. Only LOCK=DEFAULT is supported for operations that are performed instantly. 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, during an in-place ALTER TABLE operation, specify a clause on the ALTER TABLE statement 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 between algorithms, run a statement with ALGORITHM=INSTANT, ALGORITHM=INPLACE and ALGORITHM=COPY. You can also run a statement with the old_alter_table configuration option enabled to force the use of ALGORITHM=COPY.
To avoid tying up the server with an ALTER TABLE operation that copies the table, include ALGORITHM=INSTANT or ALGORITHM=INPLACE. The statement halts immediately if it cannot use the specified algorithm.