5.5.1. Overview of Online DDL

Historically, many DDL operations on InnoDB tables were expensive. Many ALTER TABLE operations worked by creating a new, empty table defined with the requested table options and indexes, then copying the existing rows to the new table one-by-one, updating the indexes as the rows were inserted. After all rows from the original table were copied, the old table was dropped and the copy was renamed with the name of the original table.

MySQL 5.5, and MySQL 5.1 with the InnoDB Plugin, optimized CREATE INDEX and DROP INDEX to avoid the table-copying behavior. That feature was known as Fast Index Creation. MySQL 5.6 enhances many other types of ALTER TABLE operations to avoid copying the table. Another enhancement allows SELECT queries and INSERT, UPDATE, and DELETE (DML) statements to proceed while the table is being altered. This combination of features is now known as online DDL.

This new mechanism also means that you can generally speed the overall process of creating and loading a table and associated indexes by creating the table with without any secondary indexes, then adding the secondary indexes after the data is loaded.

Although no syntax changes are required in the CREATE INDEX or DROP INDEX commands, some factors affect the performance, space usage, and semantics of this operation (see Section 5.5.9, "Limitations of Online DDL").

The online DDL enhancements in MySQL 5.6 improve many DDL operations that formerly required a table copy, blocked DML operations on the table, or both. Table 5.9, "Summary of Online Status for DDL Operations" shows the variations of the ALTER TABLE statement and shows how the online DDL feature applies to each one.

The "In-Place?" column shows which operations allow the ALGORITHM=INPLACE clause; the preferred value is "Yes".
The "Copies Table?" column shows which operations are able to avoid the expensive table-copying operation; the preferred value is "No". This column is mostly the reverse of the "In-Place?" column, except that a few operations allow ALGORITHM=INPLACE but still involve some amount of table copying.
The "Allows Concurrent DML?" column shows which operations can be performed fully online; the preferred value is "Yes". You can specify LOCK=NONE to assert that full concurrency is allowed during the DDL, but MySQL automatically allows this level of concurrency when possible. When concurrent DML is allowed, concurrent queries are also always allowed.
The "Allows Concurrent Queries?" column shows which DDL operations allow queries on the table while the operation is in progress; the preferred value is "Yes". Concurrent query is allowed during all online DDL operations. It is shown with "Yes" listed for all cells, for reference purposes. You can specify LOCK=SHARED to assert that concurrent queries are allowed during the DDL, but MySQL automatically allows this level of concurrency when possible.
The "Notes" column explains any exceptions to the "yes/no" values of the other columns, such as when the answer depends on the setting of a configuration option or some other clause in the DDL statement. The values "Yes*" and "No*" indicate that an answer depends on these additional notes.

Table 5.9. Summary of Online Status for DDL Operations

Operation	In-Place?	Copies Table?	Allows Concurrent DML?	Allows Concurrent Query?	Notes
`CREATE INDEX`, `ADD INDEX`	Yes*	No*	Yes	Yes	Some restrictions for `FULLTEXT` index; see next row. Currently, the operation is not in-place (that is, it copies the table) if the same index being created was also dropped by an earlier clause in the same `ALTER TABLE` statement.
`ADD FULLTEXT INDEX`	Yes	No*	No	Yes	Creating the first `FULLTEXT` index for a table involves a table copy, unless there is a user-supplied `FTS_DOC_ID` column. Subsequent `FULLTEXT` indexes on the same table can be created in-place.
`DROP INDEX`	Yes	No	Yes	Yes
Set default value for a column	Yes	No	Yes	Yes	Modifies `.frm` file only, not the data file.
Change auto-increment value for a column	Yes	No	Yes	Yes	Modifies a value stored in memory, not the data file.
Add a foreign key constraint	Yes*	No*	Yes	Yes	To avoid copying the table, disable `foreign_key_checks` during constraint creation.
Drop a foreign key constraint	Yes	No	Yes	Yes	The `foreign_key_checks` option can be enabled or disabled.
Rename a column	Yes*	No*	Yes*	Yes	To allow concurrent DML, keep the same data type and only change the column name.
Add a column	Yes	Yes	Yes*	Yes	Concurrent DML is not allowed when adding an auto-increment column. Although `ALGORITHM=INPLACE` is allowed, the data is reorganized substantially, so it is still an expensive operation.
Drop a column	Yes	Yes	Yes	Yes	Although `ALGORITHM=INPLACE` is allowed, the data is reorganized substantially, so it is still an expensive operation.
Reorder columns	Yes	Yes	Yes	Yes	Although `ALGORITHM=INPLACE` is allowed, the data is reorganized substantially, so it is still an expensive operation.
Change `ROW_FORMAT` property	Yes	Yes	Yes	Yes	Although `ALGORITHM=INPLACE` is allowed, the data is reorganized substantially, so it is still an expensive operation.
Change `KEY_BLOCK_SIZE` property	Yes	Yes	Yes	Yes	Although `ALGORITHM=INPLACE` is allowed, the data is reorganized substantially, so it is still an expensive operation.
Make column `NULL`	Yes	Yes	Yes	Yes	Although `ALGORITHM=INPLACE` is allowed, the data is reorganized substantially, so it is still an expensive operation.
Make column `NOT NULL`	Yes*	Yes	Yes	Yes	When `SQL_MODE` includes `strict_all_tables` or `strict_all_tables`, the operation fails if the column contains any nulls. Although `ALGORITHM=INPLACE` is allowed, the data is reorganized substantially, so it is still an expensive operation.
Change data type of column	No	Yes	No	Yes
Add primary key	Yes*	Yes	Yes	Yes	Although `ALGORITHM=INPLACE` is allowed, the data is reorganized substantially, so it is still an expensive operation. `ALGORITHM=INPLACE` is not allowed under certain conditions if columns have to be converted to `NOT NULL`. See Example 5.9, "Creating and Dropping the Primary Key".
Drop primary key and add another	Yes	Yes	Yes	Yes	`ALGORITHM=INPLACE` is only allowed when you add a new primary key in the same `ALTER TABLE`; the data is reorganized substantially, so it is still an expensive operation.
Drop primary key	No	Yes	No	Yes	Restrictions apply when you drop a primary key primary key without adding a new one in the same `ALTER TABLE` statement.
Convert character set	No	Yes	No	Yes	Rebuilds the table if the new character encoding is different.
Specify character set	No	Yes	No	Yes	Rebuilds the table if the new character encoding is different.
Rebuild with `FORCE` option	No	Yes	No	Yes	Acts like the `ALGORITHM=COPY` clause or the setting`old_alter_table=1`.

The following sections shows the basic syntax, and usage notes related to online DDL, for each of the major operations that can be performed with concurrent DML, in-place, or both:

Secondary Indexes

Create secondary indexes: CREATE INDEX name ON table (col_list) or ALTER TABLE table ADD INDEX name (col_list). (Creating a a FULLTEXT index still requires locking the table.)
Drop secondary indexes: DROP INDEX name ON table; or ALTER TABLE table DROP INDEX name

Creating and dropping secondary indexes on InnoDB tables skips the table-copying behavior, the same as in MySQL 5.5 and MySQL 5.1 with the InnoDB Plugin.

In MySQL 5.6 and higher, the table remains available for read and write operations while the index is being created or dropped. The CREATE INDEX or DROP INDEX statement only finishes after all transactions that are accessing the table are completed, so that the initial state of the index reflects the most recent contents of the table. Previously, modifying the table while an index was being created or dropped typically resulted in a deadlock that cancelled the INSERT, UPDATE, or DELETE statement on the table.

Column Properties

Set a default value for a column: ALTER TABLE tbl ALTER COLUMN col SET DEFAULT literal or ALTER TABLE tbl ALTER COLUMN col DROP DEFAULT

The default values for columns are stored in the .frm file for the table, not the InnoDB data dictionary.
Changing the auto-increment value for a column: ALTER TABLE table AUTO_INCREMENT=next_value;

Especially in a distributed system using replication or sharding, you sometimes reset the auto-increment counter for a table to a specific value. The next row inserted into the table uses the specified value for its auto-increment column. You might also use this technique in a data warehousing environment where you periodically empty all the tables and reload them, and you can restart the auto-increment sequence from 1.
Renaming a column: ALTER TABLE tbl CHANGE old_col_name new_col_name datatype

When you keep the same data type and [NOT] NULL attribute, only changing the column name, this operation can always be performed online.

As part of this enhancement, you can now rename a column that is part of a foreign key constraint, which was not allowed before. The foreign key definition is automatically updated to use the new column name. Renaming a column participating in a foreign key only works with the in-place mode of ALTER TABLE. If you use the ALGORITHM=COPY clause, or some other condition causes the command to use ALGORITHM=COPY behind the scenes, the ALTER TABLE statement will fail.

Foreign Keys

Adding or dropping a foreign key constraint:
```
ALTER TABLE tbl1 ADD CONSTRAINT fk_name FOREIGN KEY index (col1) REFERENCES tbl2(col2) referential_actions;ALTER TABLE tbl DROP FOREIGN KEY fk_name;
```
Dropping a foreign key can be performed online with the foreign_key_checks option enabled or disabled. Creating a foreign key online requires foreign_key_checks to be disabled.

If you do not know the names of the foreign key constraints on a particular table, issue the following statement and find the constraint name in the CONSTRAINT clause for each foreign key:
```
show create table table\G
```
Or, query the information_schema.table_constraints table and use the constraint_name and constraint_type columns to identify the foreign key names.

As a consequence of this enhancement, you can now also drop a foreign key and its associated index in a single statement, which previously required separate statements in a strict order:
```
ALTER TABLE table DROP FOREIGN KEY constraint, DROP INDEX index;
```

If foreign keys are already present in the table being altered (that is, it is a child table containing any FOREIGN KEY ... REFERENCE clauses), additional restrictions apply to online DDL operations, even those not directly involving the foreign key columns:

Concurrent DML is disallowed during online DDL operations on such child tables. (This restriction is being evaluated as a bug and might be lifted.)
An ALTER TABLE on the child table could also wait for another transaction to commit, if a change to the parent table caused associated changes in the child table through an ON UPDATE or ON DELETE clause using the CASCADE or SET NULL parameters.

In the same way, if a table is the parent table in a foreign key relationship, even though it does not contain any FOREIGN KEY clauses, it could wait for the ALTER TABLE to complete if an INSERT, UPDATE, or DELETE statement caused an ON UPDATE or ON DELETE action in the child table.

Notes on `ALGORITHM=COPY`

Any ALTER TABLE operation run with the ALGORITHM=COPY clause prevents concurrent DML operations. Concurrent queries are still allowed. That is, a table-copying operation always includes at least the concurrency restrictions of LOCK=SHARED (allow queries but not DML). You can further restrict concurrency for such operations by specifying LOCK=EXCLUSIVE (prevent DML and queries).

Concurrent DML but Table Copy Still Required

Some other ALTER TABLE operations allow concurrent DML, and are faster than MySQL 5.5 and prior: the table-copying operation is optimized, even though a table copy is still required:

Adding, dropping, or reordering columns.
Adding or dropping a primary key.
Changing the ROW_FORMAT or KEY_BLOCK_SIZE properties for a table.
Changing the nullable status for a column.

Note

As your database schema evolves with new columns, data types, constraints, indexes, and so on, keep your CREATE TABLE statements up to date with the latest table definitions. Even with the performance improvements of online DDL, it is more efficient to create stable database structures at the beginning, rather than creating part of the schema and then issuing ALTER TABLE statements afterward.

The main exception to this guideline is for secondary indexes on tables with large numbers of rows. It is typically most efficient to create the table with all details specified except the secondary indexes, load the data, then create the secondary indexes. You can use the same technique with foreign keys (load the data first, then set up the foreign keys) if you know the initial data is clean and do not need consistency checks during the loading process.

Whatever sequence of CREATE TABLE, CREATE INDEX, ALTER TABLE, and similar statements went into putting a table together, you can capture the SQL needed to reconstruct the current form of the table by issuing the statement SHOW CREATE TABLE table\G (uppercase \G required for tidy formatting). This output shows clauses such as numeric precision, NOT NULL, and CHARACTER SET that are sometimes added behind the scenes, and you might otherwise leave out when cloning the table on a new system or setting up foreign key columns with identical type.

5.5.1. Overview of Online DDL

Secondary Indexes

Column Properties

Foreign Keys

Notes on ALGORITHM=COPY

Concurrent DML but Table Copy Still Required

Notes on `ALGORITHM=COPY`