MySQL无法修改主键的问题分析及解决方案

[test]> alter table test drop primary key;
ERROR 3750 (HY000): Unable to create or change a table without a primary key, 
when the system variable 'sql_require_primary_key' is set. 
Add a primary key to the table or unset this variable to avoid this message. 
Note that tables without a primary key can cause performance problems in row-based replication, 
so please consult your DBA before changing this setting.

参数名称：sql_require_primary_key
作用范围：Global & Session
动态修改：Yes
默认值：OFF
该参数设置为ON时，SQL语句create table创建新表或者alter语句对已存在的表进行修改，将会强制检查表中是否包含主键，如果没有主键，则会报错。

bisal@mysqldb:  [test]> create table t_primary_key (id int, c1 varchar(1), c2 varchar(1), constraint pk_t_id primary key(id));
Query OK, 0 rows affected (0.07 sec)

bisal@mysqldb:  [test]> alter table t_primary_key drop primary key;
ERROR 3750 (HY000): Unable to create or change a table without a primary key, when the system variable 'sql_require_primary_key' is set. Add a primary key to the table or unset this variable to avo

bisal@mysqldb:  [(none)]> show variables like '%sql_require%';
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| sql_require_primary_key | OFF   |
+-------------------------+-------+
1 row in set (0.00 sec)

bisal@mysqldb:  [(none)]> set sql_require_primary_key = ON;
Query OK, 0 rows affected (0.02 sec)

bisal@mysqldb:  [(none)]> show variables like '%sql_require%';
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| sql_require_primary_key | ON    |
+-------------------------+-------+
1 row in set (0.00 sec)

bisal@mysqldb:  [test]> alter table t_primary_key drop primary key;
Query OK, 0 rows affected (0.10 sec)
Records: 0  Duplicates: 0  Warnings: 0

bisal@mysqldb:  [test]> alter table t_primary_key drop primary key, add constraint pk_t_01 primary key (id, c1);
Query OK, 0 rows affected (0.06 sec)
Records: 0  Duplicates: 0  Warnings: 0