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.

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