mysql 的 NO_ENGINE_SUBSTITUTION

今天lys问了一个问题， NO_ENGINE_SUBSTITUTION 是干啥的，这个还真没注意过以前。

研究了一下，原来这个是在创建表指定engine子句的时候，让mysql对此DDL做判断用的。看一个例子：

mysql> show variables like '%sql_mode%'; 
+---------------+------------------------+
| Variable_name | Value                  |
+---------------+------------------------+
| sql_mode      | NO_ENGINE_SUBSTITUTION |
+---------------+------------------------+
1 row in set (0.01 sec)

mysql> show engines;
+--------------------+---------+----------------------------------------------------------------------------+--------------+------+------------+
| Engine             | Support | Comment                                                                    | Transactions | XA   | Savepoints |
+--------------------+---------+----------------------------------------------------------------------------+--------------+------+------------+
| CSV                | YES     | CSV storage engine                                                         | NO           | NO   | NO         |
| MRG_MYISAM         | YES     | Collection of identical MyISAM tables                                      | NO           | NO   | NO         |
| MEMORY             | YES     | Hash based, stored in memory, useful for temporary tables                  | NO           | NO   | NO         |
| BLACKHOLE          | YES     | /dev/null storage engine (anything you write to it disappears)             | NO           | NO   | NO         |
| MyISAM             | YES     | MyISAM storage engine                                                      | NO           | NO   | NO         |
| PERFORMANCE_SCHEMA | YES     | Performance Schema                                                         | NO           | NO   | NO         |
| ARCHIVE            | YES     | Archive storage engine                                                     | NO           | NO   | NO         |
| FEDERATED          | NO      | Federated MySQL storage engine                                             | NULL         | NULL | NULL       |
| InnoDB             | DEFAULT | Percona-XtraDB, Supports transactions, row-level locking, and foreign keys | YES          | YES  | YES        |
+--------------------+---------+----------------------------------------------------------------------------+--------------+------+------------+
9 rows in set (0.00 sec)

mysql> use test ;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> create table t_e (it int ) engine=FEDERATE1;
ERROR 1286 (42000): Unknown storage engine 'FEDERATE1'  #FEDERATE1引擎不存在，直接报错。

当前 sql_mode=”NO_ENGINE_SUBSTITUTION”，表示不进行引擎替换。就是说，如果DDL的engine子句指定的引擎不存在，直接报错。

再看下面的例子。

mysql> set sql_mode='';
Query OK, 0 rows affected (0.00 sec)

mysql> create table t_e (it int ) engine=FEDERATE1;
Query OK, 0 rows affected, 2 warnings (0.04 sec)


1 row in set (0.00 sec)
mysql> show warnings ;
+---------+------+---------------------------------------------+
| Level   | Code | Message                                     |
+---------+------+---------------------------------------------+
| Warning | 1286 | Unknown storage engine 'FEDERATE1'          |
| Warning | 1266 | Using storage engine InnoDB for table 't_e' | 
+---------+------+---------------------------------------------+
2 rows in set (0.00 sec)

mysql> show create table t_e;
+-------+-----------------------------------------------------------------------------------------+
| Table | Create Table                                                                            |
+-------+-----------------------------------------------------------------------------------------+
| t_e   | CREATE TABLE `t_e` (
  `it` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+-------+-----------------------------------------------------------------------------------------+
#引擎已被替换成InnoDB

去掉 NO_ENGINE_SUBSTITUTION ，此时，依然指定一个不存在的引擎，mysql自动将引擎替换成默认的innodb引擎。

去掉 NO_ENGINE_SUBSTITUTION ，对于alter操作的行为是：alter操作并不成功。

mysql> show create table t_e;
+-------+-----------------------------------------------------------------------------------------+
| Table | Create Table                                                                            |
+-------+-----------------------------------------------------------------------------------------+
| t_e   | CREATE TABLE `t_e` (
  `it` int(11) DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1 |
+-------+-----------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
#以前是MyISAM引擎。

mysql> alter table t_e engine=FEDERATE1;  #修改成不存在的FEDERATE1引擎
Query OK, 0 rows affected, 1 warning (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 1

mysql> show warnings ;                  
+---------+------+------------------------------------+
| Level   | Code | Message                            |
+---------+------+------------------------------------+
| Warning | 1286 | Unknown storage engine 'FEDERATE1' |
+---------+------+------------------------------------+
1 row in set (0.00 sec)

mysql> show create table t_e;           
+-------+-----------------------------------------------------------------------------------------+
| Table | Create Table                                                                            |
+-------+-----------------------------------------------------------------------------------------+
| t_e   | CREATE TABLE `t_e` (
  `it` int(11) DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1 | #引擎并没修改。
+-------+-----------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

如下是官网解释：

NO_ENGINE_SUBSTITUTION

Control automatic substitution of the default storage engine when a statement such as CREATE TABLE or ALTER TABLE specifies a storage engine that is disabled or not compiled in.

The default SQL mode includes NO_ENGINE_SUBSTITUTION.

Because storage engines can be pluggable at runtime, unavailable engines are treated the same way:

With NO_ENGINE_SUBSTITUTION disabled, for CREATE TABLE the default engine is used and a warning occurs if the desired engine is unavailable. For ALTER TABLE, a warning occurs and the table is not altered.

With NO_ENGINE_SUBSTITUTION enabled, an error occurs and the table is not created or altered if the desired engine is unavailable.