Normally, adding a column to a table requires the full table to be rebuilt. The complexity of the operation is proportional to the size of the table, or O(n·m) where n is the number of rows in the table and m is the number of indexes.
Online ALTER TABLE for InnoDB tables (available in MariaDB 10.0+) enables one to modify the table while it is being rebuilt, but can still significantly increase the I/O and memory consumption and cause a replication lag.
With instant ADD COLUMN, all that is needed is an O(log n) operation to insert a special hidden record into the table, and an update of the data dictionary. For a large table, instead of taking several hours, the operation would be completed in the blink of an eye. The ADD COLUMN operation is only slightly more expensive than a regular INSERT, due to locking constraints.
In the past, some developers may have implemented a kind of "instant add column" in the application by encoding multiple columns in a single TEXT or BLOB column. MariaDB Dynamic Columns was an early example of that. A more recent example is JSON and related string manipulation functions.
Adding real columns has the following advantages over encoding columns into a single "expandable" column:
- Efficient storage in a native binary format
- Data type safety
- Indexes can be built natively
- Constraints are available: UNIQUE, CHECK, FOREIGN KEY
- DEFAULT values can be specified
- Triggers can be written more easily
With instant ADD COLUMN, you can enjoy all the benefits of structured storage without the drawback of having to rebuild the table.
Instant ADD COLUMN is available for both old and new InnoDB tables. Basically you can just upgrade from MySQL 5.x or MariaDB and start adding columns instantly.
- Instant ADD COLUMN only applies when the added columns appear last in the table. The place specifier LAST is the default. If AFTER col is specified, then col must be the last column, or the operation will require the table to be rebuilt.
- If the table contains a hidden FTS_DOC_ID column due to a FULLTEXT INDEX, then instant ADD COLUMN will not be possible.
- If the table becomes empty (either via TRUNCATE or DELETE), the table will be converted to the old "non-instant" format.
- Naturally, the operation is crash-safe. If the server is killed while instant ADD COLUMN was executing, the operation will be rolled back. If it was the very first instant ADD COLUMN operation, the table will be restored to the old "non-instant" format.
- InnoDB data files after instant ADD COLUMN cannot be imported to older versions of MariaDB or MySQL. Any table-rebuilding operation such as ALTER TABLE…FORCE will convert the table to the old format.
- Instant ADD COLUMN is not available for ROW_FORMAT=COMPRESSED.
- ALTER TABLE…DROP COLUMN requires the table to be rebuilt.
CREATE TABLE t(id INT PRIMARY KEY, u INT UNSIGNED NOT NULL UNIQUE) ENGINE=InnoDB; INSERT INTO t(id,u) VALUES(1,1),(2,2),(3,3); ALTER TABLE t ADD COLUMN (d DATETIME DEFAULT current_timestamp(), p POINT NOT NULL DEFAULT ST_GeomFromText('POINT(0 0)'), t TEXT CHARSET utf8 DEFAULT 'The quick brown fox jumps over the lazy dog'); UPDATE t SET t=NULL WHERE id=3; SELECT id,u,d,ST_AsText(p),t FROM t; SELECT variable_value FROM information_schema.global_status WHERE variable_name = 'innodb_instant_alter_column';
The above example illustrates that when the added columns are declared NOT NULL, a DEFAULT value must be available, either implied by the data type or set explicitly by the user. The expression need not be constant, but it must not refer to the columns of the table, such as DEFAULT u+1 (a MariaDB extension). The DEFAULT current_timestamp() would be evaluated at the time of the ALTER TABLE and apply to each row, like it does for non-instant ALTER TABLE. If a subsequent ALTER TABLE changes the DEFAULT value for subsequent INSERT, the values of the columns in existing records will naturally be unaffected.
The design was brainstormed in April by engineers from MariaDB Corporation, Alibaba and Tencent. A prototype was developed by Vin Chen (陈福荣) from the Tencent Game DBA Team.