Historically, many DDL operations
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
MySQL 5.5, and MySQL 5.1 with the InnoDB Plugin, optimized
CREATE INDEX and
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
(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
statement and shows how the online DDL feature applies to each
The “In-Place?” column shows which operations
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|
|Yes*||No*||Yes||Yes||Some restrictions for |
|Yes||No*||No||Yes||Creating the first |
|Set default value for a column||Yes||No||Yes||Yes||Modifies |
|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
|Drop a foreign key constraint||Yes||No||Yes||Yes||The |
|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
column. Although |
|Drop a column||Yes||Yes||Yes||Yes||Although |
|Reorder columns||Yes||Yes||Yes||Yes||Although |
|Change ||Yes||Yes||Yes||Yes||Although |
|Change ||Yes||Yes||Yes||Yes||Although |
|Make column ||Yes||Yes||Yes||Yes||Although |
|Make column ||Yes*||Yes||Yes||Yes||When |
|Change data type of column||No||Yes||No||Yes|
|Add primary key||Yes*||Yes||Yes||Yes||Although |
|Drop primary key and add another||Yes||Yes||Yes||Yes|
|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 |
|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 ||No||Yes||No||Yes||Acts like the |
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:
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
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
DELETE statement on the table.
Set a default value for a column:
tbl ALTER COLUMN
col SET DEFAULT
tbl ALTER COLUMN
col DROP DEFAULT
value for a column:
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:
When you keep the same data type and
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.
Adding or dropping a foreign key constraint:
referential_actions; ALTER TABLE
tblDROP FOREIGN KEY
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
for each foreign key:
show create table
Or, query the
table and use the
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:
tableDROP FOREIGN KEY
constraint, DROP 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.)
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
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
DELETE statement caused an
ON UPDATE or
action in the child table.
ALTER TABLE operation run with
ALGORITHM=COPY clause prevents concurrent
DML operations. Concurrent queries are still allowed. That is, a
table-copying operation always includes at least the concurrency
LOCK=SHARED (allow queries but
not DML). You can further restrict concurrency for such operations
LOCK=EXCLUSIVE (prevent DML and
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
Adding, dropping, or reordering columns.
Adding or dropping a primary key.
KEY_BLOCK_SIZE properties for a table.
Changing the nullable status for a column.
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
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
\G required for tidy formatting). This output
shows clauses such as numeric precision,
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.