14.13.1 Online DDL Operations

Online support details, syntax examples, and usage notes for DDL operations are provided under the following topics in this section.

• Index Operations

• Primary Key Operations

• Column Operations

• Foreign Key Operations

• Table Operations

• Partitioning Operations

Index Operations

The following table provides an overview of online DDL support for index operations. An asterisk indicates additional information, an exception, or a dependency. For details, see Syntax and Usage Notes.

Table 14.9 Online DDL Support for Index Operations

Operation	In Place	Rebuilds Table	Permits Concurrent DML	Only Modifies Metadata
Creating or adding a secondary index	Yes	No	Yes	No
Dropping an index	Yes	No	Yes	Yes
Adding a FULLTEXT index	Yes*	No*	No	No
Changing the index type	Yes	No	Yes	Yes

Syntax and Usage Notes

• Creating or adding a secondary index

  CREATE INDEX name ON table (col_list);

  ALTER TABLE tbl_name ADD INDEX name (col_list);

  The table remains available for read and write operations while the index is being created. The CREATE 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.

  Online DDL support for adding secondary indexes means that you can generally speed the overall process of creating and loading a table and associated indexes by creating the table without secondary indexes, then adding secondary indexes after the data is loaded.

  A newly created secondary index contains only the committed data in the table at the time the CREATE INDEX or ALTER TABLE statement finishes executing. It does not contain any uncommitted values, old versions of values, or values marked for deletion but not yet removed from the old index.

  If the server exits while creating a secondary index, upon recovery, MySQL drops any partially created indexes. You must re-run the ALTER TABLE or CREATE INDEX statement.

  Some factors affect the performance, space usage, and semantics of this operation. For details, see Section 14.13.6, “Online DDL Limitations”.

• Dropping an index

  DROP INDEX name ON table;

  ALTER TABLE tbl_name DROP INDEX name;

  The table remains available for read and write operations while the index is being dropped. The 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.

• Adding a FULLTEXT index

  CREATE FULLTEXT INDEX name ON table(column);

  Adding the first FULLTEXT index rebuilds the table if there is no user-defined FTS_DOC_ID column. Additional FULLTEXT indexes may be added without rebuilding the table.

• Changing the index type (USING {BTREE | HASH})

  ALTER TABLE tbl_name DROP INDEX i1, ADD INDEX i1(key_part,...) USING BTREE, ALGORITHM=INPLACE;

Primary Key Operations

The following table provides an overview of online DDL support for primary key operations. An asterisk indicates additional information, an exception, or a dependency. See Syntax and Usage Notes.

Table 14.10 Online DDL Support for Primary Key Operations

Operation	In Place	Rebuilds Table	Permits Concurrent DML	Only Modifies Metadata
Adding a primary key	Yes*	Yes*	Yes	No
Dropping a primary key	No	Yes	No	No
Dropping a primary key and adding another	Yes	Yes	Yes	No

Syntax and Usage Notes

• Adding a primary key

  ALTER TABLE tbl_name ADD PRIMARY KEY (column), ALGORITHM=INPLACE, LOCK=NONE;

  Rebuilds the table in place. Data is reorganized substantially, making it an expensive operation. ALGORITHM=INPLACE is not permitted under certain conditions if columns have to be converted to NOT NULL.

  Restructuring the clustered index always requires copying table data. Thus, it is best to define the primary key when you create a table, rather than issuing ALTER TABLE ... ADD PRIMARY KEY later.

  When you create a UNIQUE or PRIMARY KEY index, MySQL must do some extra work. For UNIQUE indexes, MySQL checks that the table contains no duplicate values for the key. For a PRIMARY KEY index, MySQL also checks that none of the PRIMARY KEY columns contains a NULL.

  When you add a primary key using the ALGORITHM=COPY clause, MySQL converts NULL values in the associated columns to default values: 0 for numbers, an empty string for character-based columns and BLOBs, and 0000-00-00 00:00:00 for DATETIME. This is a non-standard behavior that Oracle recommends you not rely on. Adding a primary key using ALGORITHM=INPLACE is only permitted when the SQL_MODE setting includes the strict_trans_tables or strict_all_tables flags; when the SQL_MODE setting is strict, ALGORITHM=INPLACE is permitted, but the statement can still fail if the requested primary key columns contain NULL values. The ALGORITHM=INPLACE behavior is more standard-compliant.

  If you create a table without a primary key, InnoDB chooses one for you, which can be the first UNIQUE key defined on NOT NULL columns, or a system-generated key. To avoid uncertainty and the potential space requirement for an extra hidden column, specify the PRIMARY KEY clause as part of the CREATE TABLE statement.

  MySQL creates a new clustered index by copying the existing data from the original table to a temporary table that has the desired index structure. Once the data is completely copied to the temporary table, the original table is renamed with a different temporary table name. The temporary table comprising the new clustered index is renamed with the name of the original table, and the original table is dropped from the database.

  The online performance enhancements that apply to operations on secondary indexes do not apply to the primary key index. The rows of an InnoDB table are stored in a clustered index organized based on the primary key, forming what some database systems call an “index-organized table”. Because the table structure is closely tied to the primary key, redefining the primary key still requires copying the data.

  When an operation on the primary key uses ALGORITHM=INPLACE, even though the data is still copied, it is more efficient than using ALGORITHM=COPY because:

  • No undo logging or associated redo logging is required for ALGORITHM=INPLACE. These operations add overhead to DDL statements that use ALGORITHM=COPY.

  • The secondary index entries are pre-sorted, and so can be loaded in order.

  • The change buffer is not used, because there are no random-access inserts into the secondary indexes.

  If the server exits while creating a new clustered index, no data is lost, but you must complete the recovery process using the temporary tables that exist during the process. Since it is rare to re-create a clustered index or re-define primary keys on large tables, or to encounter a system crash during this operation, this manual does not provide information on recovering from this scenario.

• Dropping a primary key

  ALTER TABLE tbl_name DROP PRIMARY KEY, ALGORITHM=COPY;

  Only ALGORITHM=COPY supports dropping a primary key without adding a new one in the same ALTER TABLE statement.

• Dropping a primary key and adding another

  ALTER TABLE tbl_name DROP PRIMARY KEY, ADD PRIMARY KEY (column), ALGORITHM=INPLACE, LOCK=NONE;

  Data is reorganized substantially, making it an expensive operation.

Column Operations

The following table provides an overview of online DDL support for column operations. An asterisk indicates additional information, an exception, or a dependency. For details, see Syntax and Usage Notes.

Table 14.11 Online DDL Support for Column Operations

Operation	In Place	Rebuilds Table	Permits Concurrent DML	Only Modifies Metadata
Adding a column	Yes	Yes	Yes*	No
Dropping a column	Yes	Yes	Yes	No
Renaming a column	Yes	No	Yes*	Yes
Reordering columns	Yes	Yes	Yes	No
Setting a column default value	Yes	No	Yes	Yes
Changing the column data type	No	Yes	No	No
Dropping the column default value	Yes	No	Yes	Yes
Changing the auto-increment value	Yes	No	Yes	No*
Making a column NULL	Yes	Yes*	Yes	No
Making a column NOT NULL	Yes*	Yes*	Yes	No
Modifying the definition of an ENUM or SET column	Yes	No	Yes	Yes

Syntax and Usage Notes

• Adding a column

  ALTER TABLE tbl_name ADD COLUMN column_name column_definition, ALGORITHM=INPLACE, LOCK=NONE;

  Concurrent DML is not permitted when adding an auto-increment column. Data is reorganized substantially, making it an expensive operation. At a minimum, ALGORITHM=INPLACE, LOCK=SHARED is required.

• Dropping a column

  ALTER TABLE tbl_name DROP COLUMN column_name, ALGORITHM=INPLACE, LOCK=NONE;

  Data is reorganized substantially, making it an expensive operation.

• Renaming a column

  ALTER TABLE tbl CHANGE old_col_name new_col_name data_type, ALGORITHM=INPLACE, LOCK=NONE;

  To permit concurrent DML, keep the same data type and only change the column name.

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

  You can also rename a column that is part of a foreign key constraint. The foreign key definition is automatically updated to use the new column name. Renaming a column participating in a foreign key only works with ALGORITHM=INPLACE. 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 fails.

• Reordering columns

  To reorder columns, use FIRST or AFTER in CHANGE or MODIFY operations.

  ALTER TABLE tbl_name MODIFY COLUMN col_name column_definition FIRST, ALGORITHM=INPLACE, LOCK=NONE;

  Data is reorganized substantially, making it an expensive operation.

• Changing the column data type

  ALTER TABLE tbl_name CHANGE c1 c1 BIGINT, ALGORITHM=COPY;

  Changing the column data type is only supported with ALGORITHM=COPY.

• Setting a column default value

  ALTER TABLE tbl_name ALTER COLUMN col SET DEFAULT literal, ALGORITHM=INPLACE, LOCK=NONE;

  Only modifies table metadata. Default column values are stored in the .frm file for the table, not the InnoDB data dictionary.

• Dropping a column default value

  ALTER TABLE tbl ALTER COLUMN col DROP DEFAULT, ALGORITHM=INPLACE, LOCK=NONE;

• Changing the auto-increment value

  ALTER TABLE table AUTO_INCREMENT=next_value, ALGORITHM=INPLACE, LOCK=NONE;

  Modifies a value stored in memory, not the data file.

  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 restart the auto-increment sequence from 1.

• Making a column NULL

  ALTER TABLE tbl_name MODIFY COLUMN column_name data_type NULL, ALGORITHM=INPLACE, LOCK=NONE;

  Rebuilds the table in place. Data is reorganized substantially, making it an expensive operation.

• Making a column NOT NULL

  ALTER TABLE tbl_name MODIFY COLUMN column_name data_type NOT NULL, ALGORITHM=INPLACE, LOCK=NONE;

  Rebuilds the table in place. STRICT_ALL_TABLES or STRICT_TRANS_TABLES SQL_MODE is required for the operation to succeed. The operation fails if the column contains NULL values. The server prohibits changes to foreign key columns that have the potential to cause loss of referential integrity. See Section 13.1.7, “ALTER TABLE Syntax”. Data is reorganized substantially, making it an expensive operation.

• Modifying the definition of an ENUM or SET column

  CREATE TABLE t1 (c1 ENUM('a', 'b', 'c'));
  ALTER TABLE t1 MODIFY COLUMN c1 ENUM('a', 'b', 'c', 'd'), ALGORITHM=INPLACE, LOCK=NONE;

  Modifying the definition of an ENUM or SET column by adding new enumeration or set members to the end of the list of valid member values may be performed in place, as long as the storage size of the data type does not change. For example, adding a member to a SET column that has 8 members changes the required storage per value from 1 byte to 2 bytes; this requires a table copy. Adding members in the middle of the list causes renumbering of existing members, which requires a table copy.

Foreign Key Operations

The following table provides an overview of online DDL support for foreign key operations. An asterisk indicates additional information, an exception, or a dependency. For details, see Syntax and Usage Notes.

Table 14.12 Online DDL Support for Foreign Key Operations

Operation	In Place	Rebuilds Table	Permits Concurrent DML	Only Modifies Metadata
Adding a foreign key constraint	Yes*	No	Yes	Yes
Dropping a foreign key constraint	Yes	No	Yes	Yes

Syntax and Usage Notes

• Adding a foreign key constraint

  The INPLACE algorithm is supported when foreign_key_checks is disabled. Otherwise, only the COPY algorithm is supported.

  ALTER TABLE tbl1 ADD CONSTRAINT fk_name FOREIGN KEY index (col1) 
    REFERENCES tbl2(col2) referential_actions;

• Dropping a foreign key constraint

  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.

  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.

  You can also drop a foreign key and its associated index in a single statement:

  ALTER TABLE table DROP FOREIGN KEY constraint, DROP INDEX index;

Note

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

• An ALTER TABLE on the child table could wait for another transaction to commit, if a change to the parent table causes 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 causes an ON UPDATE or ON DELETE action in the child table.

Table Operations

The following table provides an overview of online DDL support for table operations. An asterisk indicates additional information, an exception, or a dependency. For details, see Syntax and Usage Notes.

Table 14.13 Online DDL Support for Table Operations

Operation	In Place	Rebuilds Table	Permits Concurrent DML	Only Modifies Metadata
Changing the ROW_FORMAT	Yes	Yes	Yes	No
Changing the KEY_BLOCK_SIZE	Yes	Yes	Yes	No
Setting persistent table statistics	Yes	No	Yes	Yes
Specifying a character set	Yes	Yes*	No	No
Converting a character set	No	Yes	No	No
Optimizing a table	Yes*	Yes	Yes	No
Rebuilding with the FORCE option	Yes*	Yes	Yes	No
Performing a null rebuild	Yes*	Yes	Yes	No
Renaming a table	Yes	No	Yes	Yes

Syntax and Usage Notes

• Changing the ROW_FORMAT

  ALTER TABLE tbl_name ROW_FORMAT = row_format, ALGORITHM=INPLACE, LOCK=NONE;

  Data is reorganized substantially, making it an expensive operation.

  For additional information about the ROW_FORMAT option, see Table Options.

• Changing the KEY_BLOCK_SIZE

  ALTER TABLE tbl_name KEY_BLOCK_SIZE = value, ALGORITHM=INPLACE, LOCK=NONE;

  Data is reorganized substantially, making it an expensive operation.

  For additional information about the KEY_BLOCK_SIZE option, see Table Options.

• Setting persistent table statistics options

  ALTER TABLE tbl_name STATS_PERSISTENT=0, STATS_SAMPLE_PAGES=20, STATS_AUTO_RECALC=1, ALGORITHM=INPLACE, LOCK=NONE;

  Only modifies table metadata.

  Persistent statistics include STATS_PERSISTENT, STATS_AUTO_RECALC, and STATS_SAMPLE_PAGES. For more information, see Section 14.8.11.1, “Configuring Persistent Optimizer Statistics Parameters”.

• Specifying a character set

  ALTER TABLE tbl_name CHARACTER SET = charset_name, ALGORITHM=INPLACE, LOCK=NONE;

  Rebuilds the table if the new character encoding is different.

• Converting a character set

  ALTER TABLE tbl_name CONVERT TO CHARACTER SET charset_name, ALGORITHM=COPY;

  Rebuilds the table if the new character encoding is different.

• Optimizing a table

  OPTIMIZE TABLE tbl_name;

  Performed in-place as of MySQL 5.6.17. In-place operation is not supported for tables with FULLTEXT indexes. The operation uses the INPLACE algorithm, but ALGORITHM and LOCK syntax is not permitted.

• Rebuilding a table with the FORCE option

  ALTER TABLE tbl_name FORCE, ALGORITHM=INPLACE, LOCK=NONE;

  Uses ALGORITHM=INPLACE as of MySQL 5.6.17. ALGORITHM=INPLACE is not supported for tables with FULLTEXT indexes.

• Performing a "null" rebuild

  ALTER TABLE tbl_name ENGINE=InnoDB, ALGORITHM=INPLACE, LOCK=NONE;

  Uses ALGORITHM=INPLACE as of MySQL 5.6.17. ALGORITHM=INPLACE is not supported for tables with FULLTEXT indexes.

• Renaming a table

  ALTER TABLE old_tbl_name RENAME TO new_tbl_name, ALGORITHM=INPLACE, LOCK=NONE;

  MySQL renames files that correspond to the table tbl_name without making a copy. (You can also use the RENAME TABLE statement to rename tables. See Section 13.1.32, “RENAME TABLE Syntax”.) Privileges granted specifically for the renamed table are not migrated to the new name. They must be changed manually.

Partitioning Operations

With the exception of ALTER TABLE partitioning clauses, online DDL operations for partitioned InnoDB tables follow the same rules that apply to regular InnoDB tables.

ALTER TABLE partitioning clauses do not go through the same internal online DDL API as regular non-partitioned InnoDB tables, and most do not support ALGORITHM and LOCK clauses.

If you use a partitioning clause in an ALTER TABLE statement, the partitioned table is repartitioned using the ALTER TABLE COPY algorithm. In other words, a new partitioned table is created with the new partitioning scheme. The newly created table includes any changes applied by the ALTER TABLE statement and the table data is copied into the new table structure.

If you do not modify table partitioning using ALTER TABLE partitioning clauses that use the COPY algorithm or perform any other partition management in your ALTER TABLE statement, ALTER TABLE performs supported INPLACE operations on each table partition. Be aware, however, that when INPLACE ALTER TABLE operations are performed on each partition, there is increased demand on system resources due to operations being performed on multiple partitions.

Even though partitioning clauses of the ALTER TABLE statement do not go through the same internal online DDL API as regular non-partitioned InnoDB tables, MySQL still attempts to minimize data copying and locking where possible:

• ADD PARTITION and DROP PARTITION for tables partitioned by RANGE or LIST do not copy existing data.

• TRUNCATE PARTITION does not copy existing data.

• Concurrent queries are allowed during ADD PARTITION and COALESCE PARTITION for tables partitioned by HASH or LIST. MySQL copies the data while holding a shared lock.

• For REORGANIZE PARTITION, REBUILD PARTITION, or ADD PARTITION or COALESCE PARTITION for a table partitioned by LINEAR HASH or LIST, concurrent queries are allowed. Data from the affected partitions is copied while holding a shared metadata (read) lock on the table.

• PARTITION BY and REMOVE PARTITIONING support the ALGORITHM clause with the DEFAULT or COPY option. Both operations permit concurrent queries.

For additional information about ALTER TABLE partitioning clauses, see Partitioning Options, and Section 13.1.7.1, “ALTER TABLE Partition Operations”. For information about partitioning in general, see Chapter 19, Partitioning.