MySQL 8.4 Reference Manual Including MySQL NDB Cluster 8.4
MySQL 8.4 supports atomic Data Definition Language (DDL) statements. This feature is referred to as atomic DDL. An atomic DDL statement combines the data dictionary updates, storage engine operations, and binary log writes associated with a DDL operation into a single, atomic operation. The operation is either committed, with applicable changes persisted to the data dictionary, storage engine, and binary log, or is rolled back, even if the server halts during the operation.
Atomic DDL is not transactional
DDL. DDL statements, atomic or otherwise, implicitly
end any transaction that is active in the current session, as if
you had done a COMMIT before
executing the statement. This means that DDL statements cannot
be performed within another transaction, within transaction
control statements such as
START TRANSACTION ...
COMMIT, or combined with other statements within the
same transaction.
Atomic DDL is made possible by the MySQL data dictionary, which provides centralized, transactional metadata storage.
The atomic DDL feature is described under the following topics in this section:
The atomic DDL feature supports both table and non-table DDL
statements. Table-related DDL operations require storage engine
support, whereas non-table DDL operations do not. Currently,
only the InnoDB storage engine supports
atomic DDL.
Supported table DDL statements include
CREATE, ALTER, and
DROP statements for databases,
tablespaces, tables, and indexes, and the
TRUNCATE TABLE statement.
Supported non-table DDL statements include:
The following statements are not supported by the atomic DDL feature:
Table-related DDL statements that involve a storage engine
other than InnoDB.
INSTALL PLUGIN and
UNINSTALL PLUGIN
statements.
INSTALL COMPONENT and
UNINSTALL COMPONENT
statements.
CREATE SERVER,
ALTER SERVER, and
DROP SERVER statements.
The characteristics of atomic DDL statements include the following:
Metadata updates, binary log writes, and storage engine operations, where applicable, are combined into a single atomic operation.
There are no intermediate commits at the SQL layer during the DDL operation.
Where applicable:
The state of data dictionary, routine, event, and loadable function caches is consistent with the status of the DDL operation, meaning that caches are updated to reflect whether or not the DDL operation was completed successfully or rolled back.
The storage engine methods involved in a DDL operation do not perform intermediate commits, and the storage engine registers itself as part of the DDL operation.
The storage engine supports redo and rollback of DDL operations, which is performed in the Post-DDL phase of the DDL operation.
The visible behaviour of DDL operations is atomic.
This section describes some important aspects of DDL statement
behavior when using a storage engine that support atomic DDL,
such as InnoDB.
DROP TABLE operations are
fully atomic if all named tables use a storage engine which
supports atomic DDL. The statement either drops all tables
successfully or is rolled back.
DROP TABLE fails with an
error if a named table does not exist, and no changes are
made, regardless of the storage engine.
DROP DATABASE is atomic if
all tables use a storage engine which supports atomic DDL.
The statement either drops all objects successfully or is
rolled back. You should be aware that removal of the
database directory from the file system occurs last and is
not part of the atomic operation; thus, if removal of the
database directory fails due to a file system error or
server halt, the DROP
DATABASE transaction is not rolled back.
For tables that do not use a storage engine which supports
atomic DDL, table deletion occurs outside of the atomic
DROP TABLE or
DROP DATABASE transaction.
Such table deletions are written to the binary log
individually, which limits the discrepancy between the
storage engine, data dictionary, and binary log to one table
at most in the case of an interrupted
DROP TABLE or
DROP DATABASE operation. For
operations that drop multiple tables, any tables that do not
use a storage engine which supports atomic DDL are dropped
before tables that do so.
CREATE TABLE,
ALTER TABLE,
RENAME TABLE,
TRUNCATE TABLE,
CREATE TABLESPACE, and
DROP TABLESPACE operations
for tables that use a storage engine which supports atomic
DDL are either fully committed or rolled back if the server
halts during their operation. RENAME
TABLE operations are atomic only if all named
tables use a storage engine which supports atomic DDL.
For storage engines that support atomic DDL, the
CREATE
TABLE ... SELECT statement is logged as one
transaction in the binary log when row-based replication is
in use.
On storage engines that support both atomic DDL and foreign
key constraints, creation of foreign keys is not permitted
in
CREATE
TABLE ... SELECT statements when row-based
replication is in use. Foreign key constraints can be added
later using ALTER TABLE.
When
CREATE
TABLE ... SELECT is applied as an atomic
operation, a metadata lock is held on the table while data
is inserted, which prevents concurrent access to the table
for the duration of the operation.
DROP VIEW fails if a named
view does not exist, and no changes are made.
Account management statements either succeed for all named users or roll back and have no effect if an error occurs.
Currently, only the InnoDB storage engine
supports atomic DDL. Storage engines that do not support atomic
DDL are exempted from DDL atomicity. DDL operations involving
exempted storage engines remain capable of introducing
inconsistencies that can occur when operations are interrupted
or only partially completed.
To support redo and rollback of DDL operations,
InnoDB writes DDL logs to the
mysql.innodb_ddl_log table, which is a hidden
data dictionary table that resides in the
mysql.ibd data dictionary tablespace.
To view DDL logs that are written to the
mysql.innodb_ddl_log table during a DDL
operation, enable the
innodb_print_ddl_logs
configuration option. For more information, see
Viewing DDL Logs.
The redo logs for changes to the
mysql.innodb_ddl_log table are flushed to
disk immediately regardless of the
innodb_flush_log_at_trx_commit
setting. Flushing the redo logs immediately avoids situations
where data files are modified by DDL operations but the redo
logs for changes to the
mysql.innodb_ddl_log table resulting from
those operations are not persisted to disk. Such a situation
could cause errors during rollback or recovery.
The InnoDB storage engine executes DDL
operations in phases. DDL operations such as
ALTER TABLE may perform the
Prepare and Perform
phases multiple times prior to the Commit
phase.
Prepare: Create the required objects
and write the DDL logs to the
mysql.innodb_ddl_log table. The DDL logs
define how to roll forward and roll back the DDL operation.
Perform: Perform the DDL operation. For
example, perform a create routine for a CREATE
TABLE operation.
Commit: Update the data dictionary and commit the data dictionary transaction.
Post-DDL: Replay and remove DDL logs
from the mysql.innodb_ddl_log table. To
ensure that rollback can be performed safely without
introducing inconsistencies, file operations such as
renaming or removing data files are performed in this final
phase. This phase also removes dynamic metadata from the
mysql.innodb_dynamic_metadata data
dictionary table for DROP
TABLE, TRUNCATE
TABLE, and other DDL operations that rebuild the
table.
DDL logs are replayed and removed from the
mysql.innodb_ddl_log table during the
Post-DDL phase, regardless of whether the
DDL operation is committed or rolled back. DDL logs should only
remain in the mysql.innodb_ddl_log table if
the server is halted during a DDL operation. In this case, the
DDL logs are replayed and removed after recovery.
In a recovery situation, a DDL operation may be committed or
rolled back when the server is restarted. If the data dictionary
transaction that was performed during the
Commit phase of a DDL operation is present
in the redo log and binary log, the operation is considered
successful and is rolled forward. Otherwise, the incomplete data
dictionary transaction is rolled back when
InnoDB replays data dictionary redo logs, and
the DDL operation is rolled back.
To view DDL logs that are written to the
mysql.innodb_ddl_log data dictionary table
during atomic DDL operations that involve the
InnoDB storage engine, enable
innodb_print_ddl_logs to have
MySQL write the DDL logs to stderr. Depending
on the host operating system and MySQL configuration,
stderr may be the error log, terminal, or
console window. See
Section 7.4.2.2, “Default Error Log Destination Configuration”.
InnoDB writes DDL logs to the
mysql.innodb_ddl_log table to support redo
and rollback of DDL operations. The
mysql.innodb_ddl_log table is a hidden data
dictionary table that resides in the
mysql.ibd data dictionary tablespace. Like
other hidden data dictionary tables, the
mysql.innodb_ddl_log table cannot be accessed
directly in non-debug versions of MySQL. (See
Section 16.1, “Data Dictionary Schema”.) The structure of the
mysql.innodb_ddl_log table corresponds to
this definition:
CREATE TABLE mysql.innodb_ddl_log ( id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY, thread_id BIGINT UNSIGNED NOT NULL, type INT UNSIGNED NOT NULL, space_id INT UNSIGNED, page_no INT UNSIGNED, index_id BIGINT UNSIGNED, table_id BIGINT UNSIGNED, old_file_path VARCHAR(512) COLLATE utf8mb4_bin, new_file_path VARCHAR(512) COLLATE utf8mb4_bin, KEY(thread_id) );
id: A unique identifier for a DDL log
record.
thread_id: Each DDL log record is
assigned a thread_id, which is used to
replay and remove DDL logs that belong to a particular DDL
operation. DDL operations that involve multiple data file
operations generate multiple DDL log records.
type: The DDL operation type. Types
include FREE (drop an index tree),
DELETE (delete a file),
RENAME (rename a file), or
DROP (drop metadata from the
mysql.innodb_dynamic_metadata data
dictionary table).
space_id: The tablespace ID.
page_no: A page that contains allocation
information; an index tree root page, for example.
index_id: The index ID.
table_id: The table ID.
old_file_path: The old tablespace file
path. Used by DDL operations that create or drop tablespace
files; also used by DDL operations that rename a tablespace.
new_file_path: The new tablespace file
path. Used by DDL operations that rename tablespace files.
This example demonstrates enabling
innodb_print_ddl_logs to view
DDL logs written to strderr for a
CREATE TABLE operation.
mysql> SET GLOBAL innodb_print_ddl_logs=1; mysql> CREATE TABLE t1 (c1 INT) ENGINE = InnoDB;
[Note] [000000] InnoDB: DDL log insert : [DDL record: DELETE SPACE, id=18, thread_id=7, space_id=5, old_file_path=./test/t1.ibd] [Note] [000000] InnoDB: DDL log delete : by id 18 [Note] [000000] InnoDB: DDL log insert : [DDL record: REMOVE CACHE, id=19, thread_id=7, table_id=1058, new_file_path=test/t1] [Note] [000000] InnoDB: DDL log delete : by id 19 [Note] [000000] InnoDB: DDL log insert : [DDL record: FREE, id=20, thread_id=7, space_id=5, index_id=132, page_no=4] [Note] [000000] InnoDB: DDL log delete : by id 20 [Note] [000000] InnoDB: DDL log post ddl : begin for thread id : 7 [Note] [000000] InnoDB: DDL log post ddl : end for thread id : 7