Question

我正在使用MySQL 5.6，并且想修改一个表的默认编码（从latin1到utf8），而无需修改现有的列和行。基于documentation，我尝试了以下命令：

ALTER TABLE mytable DEFAULT CHARACTER SET utf8;

它修改了我的表的默认字符集编码，并且没有修改列的排序规则，但是，我真的很惊讶地看到：

Query OK, 32141 rows affected (6.31 sec)
Records: 32141 Duplicates: 0  Warnings: 0

除了“受影响的32141行”之外，结果与预期的一样，如下所示：

MySQL> select count(*) from mytable;
+----------+
| count(*) |
+----------+
|    32141 |
+----------+
1 row in set (0.01 sec)

MySQL> show table status like 'mytable';
+-----------------------+--------+---------+------------+-------+----------------+-------------+-----------------+--------------+-----------+----------------+-------------+-------------+------------+-----------------+----------+----------------+---------+
| Name                  | Engine | Version | Row_format | Rows  | Avg_row_length | Data_length | Max_data_length | Index_length | Data_free | Auto_increment | Create_time | Update_time | Check_time | Collation       | Checksum | Create_options | Comment |
+-----------------------+--------+---------+------------+-------+----------------+-------------+-----------------+--------------+-----------+----------------+-------------+-------------+------------+-----------------+----------+----------------+---------+
| mytable               | InnoDB |      10 | Compact    | 16723 |          20798 |   347815936 |               0 |     21561344 |  15728640 |           NULL | NULL        | NULL        | NULL       | utf8_general_ci |     NULL | partitioned    |         |
+-----------------------+--------+---------+------------+-------+----------------+-------------+-----------------+--------------+-----------+----------------+-------------+-------------+------------+-----------------+----------+----------------+---------+

MySQL> show create table mytable;
CREATE TABLE `mytable` (
  `ID` varchar(255) NOT NULL,
  `COL1` double DEFAULT NULL,
  `COL2` longtext CHARACTER SET latin1,
  `COL3` datetime DEFAULT NULL,
  `COL4` varchar(255) CHARACTER SET latin1 DEFAULT NULL,
  `COL5` int(11) DEFAULT NULL,
  `COL6` datetime DEFAULT NULL,
  `COL7` varchar(255) CHARACTER SET latin1 DEFAULT NULL,
  `COL8` datetime(3) NOT NULL,
  `COL9` int(11) NOT NULL DEFAULT '-1',
  `COL10` int(11) DEFAULT '0',
  `COL11` double DEFAULT '0',
  PRIMARY KEY (`ID`,`COL9`),
  KEY `idx1` (`COL7`,`COL3`,`COL6`),
  KEY `idx2` (`COL1`,`COL4`,`COL3`,`COL6`),
  KEY `idx3` (`ID`,`COL3`,`COL6`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
/*!50100 PARTITION BY RANGE (`COL9`)
(PARTITION p0 VALUES LESS THAN (1) ENGINE = InnoDB,
 PARTITION p1 VALUES LESS THAN (2) ENGINE = InnoDB,
 PARTITION p2 VALUES LESS THAN (3) ENGINE = InnoDB,
 PARTITION p3 VALUES LESS THAN (4) ENGINE = InnoDB,
 PARTITION p4 VALUES LESS THAN (5) ENGINE = InnoDB,
 PARTITION p5 VALUES LESS THAN (6) ENGINE = InnoDB,
 PARTITION p6 VALUES LESS THAN (7) ENGINE = InnoDB,
 PARTITION p7 VALUES LESS THAN (8) ENGINE = InnoDB,
 PARTITION p8 VALUES LESS THAN (9) ENGINE = InnoDB,
 PARTITION p9 VALUES LESS THAN (10) ENGINE = InnoDB,
 PARTITION p10 VALUES LESS THAN (11) ENGINE = InnoDB,
 PARTITION p11 VALUES LESS THAN (100) ENGINE = InnoDB,
 PARTITION p12 VALUES LESS THAN (101) ENGINE = InnoDB,
 PARTITION p13 VALUES LESS THAN (102) ENGINE = InnoDB,
 PARTITION p14 VALUES LESS THAN (103) ENGINE = InnoDB,
 PARTITION p15 VALUES LESS THAN (104) ENGINE = InnoDB,
 PARTITION p16 VALUES LESS THAN (105) ENGINE = InnoDB,
 PARTITION p17 VALUES LESS THAN (106) ENGINE = InnoDB,
 PARTITION p18 VALUES LESS THAN (107) ENGINE = InnoDB,
 PARTITION p19 VALUES LESS THAN (108) ENGINE = InnoDB,
 PARTITION p20 VALUES LESS THAN (109) ENGINE = InnoDB,
 PARTITION p21 VALUES LESS THAN (110) ENGINE = InnoDB,
 PARTITION p22 VALUES LESS THAN (111) ENGINE = InnoDB,
 PARTITION p23 VALUES LESS THAN (200) ENGINE = InnoDB,
 PARTITION p24 VALUES LESS THAN (201) ENGINE = InnoDB,
 PARTITION p25 VALUES LESS THAN (202) ENGINE = InnoDB,
 PARTITION p26 VALUES LESS THAN (203) ENGINE = InnoDB,
 PARTITION p27 VALUES LESS THAN (204) ENGINE = InnoDB,
 PARTITION p28 VALUES LESS THAN (205) ENGINE = InnoDB,
 PARTITION p29 VALUES LESS THAN (206) ENGINE = InnoDB,
 PARTITION p30 VALUES LESS THAN (207) ENGINE = InnoDB,
 PARTITION p31 VALUES LESS THAN (208) ENGINE = InnoDB,
 PARTITION p32 VALUES LESS THAN (209) ENGINE = InnoDB,
 PARTITION p33 VALUES LESS THAN (210) ENGINE = InnoDB,
 PARTITION p34 VALUES LESS THAN (211) ENGINE = InnoDB,
 PARTITION p35 VALUES LESS THAN MAXVALUE ENGINE = InnoDB) */

MySQL> show full columns from mytable;
+--------------------------+--------------+-------------------+------+-----+---------+-------+---------------------------------+---------+
| Field                    | Type         | Collation         | Null | Key | Default | Extra | Privileges                      | Comment |
+--------------------------+--------------+-------------------+------+-----+---------+-------+---------------------------------+---------+
| ID                       | varchar(255) | latin1_swedish_ci | NO   | PRI | NULL    |       | select,insert,update,references |         |
| COL1                     | double       | NULL              | YES  | MUL | NULL    |       | select,insert,update,references |         |
| COL2                     | longtext     | latin1_swedish_ci | YES  |     | NULL    |       | select,insert,update,references |         |
| COL3                     | datetime     | NULL              | YES  |     | NULL    |       | select,insert,update,references |         |
| COL4                     | varchar(255) | latin1_swedish_ci | YES  |     | NULL    |       | select,insert,update,references |         |
| COL5                     | int(11)      | NULL              | YES  |     | NULL    |       | select,insert,update,references |         |
| COL6                     | datetime     | NULL              | YES  |     | NULL    |       | select,insert,update,references |         |
| COL7                     | varchar(255) | latin1_swedish_ci | YES  | MUL | NULL    |       | select,insert,update,references |         |
| COL8                     | datetime(3)  | NULL              | NO   |     | NULL    |       | select,insert,update,references |         |
| COL9                     | int(11)      | NULL              | NO   | PRI | -1      |       | select,insert,update,references |         |
| COL10                    | int(11)      | NULL              | YES  |     | 0       |       | select,insert,update,references |         |
| COL11                    | double       | NULL              | YES  |     | 0       |       | select,insert,update,references |         |
+--------------------------+--------------+-------------------+------+-----+---------+-------+---------------------------------+---------+

我的连接参数如下：

MySQL> show variables where variable_name like '%char%' or variable_name like '%collation%';
+--------------------------+--------------------------------------------------+
| Variable_name            | Value                                            |
+--------------------------+--------------------------------------------------+
| character_set_client     | utf8mb4                                          |
| character_set_connection | utf8mb4                                          |
| character_set_database   | utf8mb4                                          |
| character_set_filesystem | binary                                           |
| character_set_results    | utf8mb4                                          |
| character_set_server     | utf8mb4                                          |
| character_set_system     | utf8                                             |
| collation_connection     | utf8mb4_general_ci                               |
| collation_database       | utf8mb4_general_ci                               |
| collation_server         | utf8mb4_general_ci                               |
+--------------------------+--------------------------------------------------+

请注意：

数据是通过Java应用程序创建的
在创建数据时，将连接参数设置为utf8
此表没有链接的FK

当我尝试使用一些新创建的表进行复制时，似乎未修改行。参见下面的“受影响的0行”：

MySQL> select count(*) from mytesttable;
+----------+
| count(*) |
+----------+
|        3 |
+----------+
3 row in set (0.10 sec)

MySQL> alter table mytesttable character set utf8;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

在数据创建过程中，我尝试将连接参数改回latin1，但并没有改变结果：仍然“影响0行”。

所以我的问题：

我对命令的理解正确吗？（不应修改行）
什么能解释在第一种情况下行受到影响？

编辑

我刚刚发现，如果删除分区，则不会发生此问题。

通过分区，我得到“ XXX个受影响的行”
没有分区，我得到“ 0条受影响的行”

是预期的吗？

带有摘要的EDIT 2

最初：

表使用latin1作为默认编码（与列相同）
该连接被声明为utf8

有效方法：

在执行任何ALTER TABLE命令之前，像“é”这样的字符似乎已被latin1编码（E9）
运行命令ALTER TABLE mytable CHARACTER SET utf8mb4;不会修改数据（十六进制命令仍显示E9）列仍声明为latin1。
运行命令ALTER TABLE mytable MODIFY COL2 LONGTEXT CHARACTER SET utf8mb4将列更改为utf8mb4（C3A9）

到目前为止很好。

剩余问题：

如何确保表中存在的所有数据均为latin1？我尝试过SELECT COL2 FROM mytable WHERE LENGTH(COL2) != CHAR_LENGTH(COL2) LIMIT 1，但得到0个结果。够了吗？
为什么命令ALTER TABLE mytable CHARACTER SET utf8mb4;显示似乎未修改数据时“ 32141行受影响”？（当表在同一列上具有分区和索引时会发生这种情况）
根据上一点，更改表的默认编码是否安全（需要？）？还是我应该坚持修改列？

非常感谢您的帮助

Answer 1

您一团糟，ALTER使一团糟。

首先，将表列声明为latin1 ，并且连接声明客户端正在使用latin1（通过SET NAMES latin1）。如果é实际上是客户端中的E9十六进制，那会很好。但是客户端中的数据是UTF-8。因此é是两个字节C3A9作为2个latin1字符发送到数据库的。损坏不明显，因为当您SELECTed时，损坏已经恢复。

随后的步骤将这些字节的每个视为latin1并将其转换为utf8，从而进行了“双重”编码。

请参见Trouble with UTF-8 characters; what I see is not what I stored中的“ Mojibake”和“双重编码”。如果要尝试恢复数据，请参见http://mysql.rjweb.org/doc.php/charcoll#fixes_for_various_cases

中的相应情况

好吧，显然ALTER TABLE mytable DEFAULT CHARACTER SET utf8;不仅在更改默认值，而且还在复制表，并在此过程中引入了双重编码。

十多年来，我一直在追求MySQL字符集问题。这是我尚未观察到的新皱纹。

我很确定character_set_system与您的问题无关。（但我可能错了！）

错误的设置名称

测试用例：

CREATE TABLE mytest ( MYDATA longtext ) ENGINE=InnoDB DEFAULT CHARSET=latin1;
SET NAMES latin1;
INSERT INTO mytest VALUES ( "é" );
SELECT MYDATA, HEX(MYDATA) FROM mytest;

运行该测试用例：

mysql> SET NAMES latin1;

mysql> SHOW CREATE TABLE mytest\G
*************************** 1. row ***************************
       Table: mytest
Create Table: CREATE TABLE `mytest` (
  `MYDATA` longtext
) ENGINE=InnoDB DEFAULT CHARSET=latin1

mysql> INSERT INTO mytest VALUES ( "é" );

mysql> SELECT MYDATA, HEX(MYDATA), LENGTH(MYDATA),
              CHAR_LENGTH(MYDATA) FROM mytest;
+--------+-------------+----------------+---------------------+
| MYDATA | HEX(MYDATA) | LENGTH(MYDATA) | CHAR_LENGTH(MYDATA) |
+--------+-------------+----------------+---------------------+
| é      | C3A9        |              2 |                   2 |
+--------+-------------+----------------+---------------------+

角色看起来不错。但是十六进制看起来像UTF-8，而不是latin1。而CHAR_LENGTH是“错误的”。

情况是：CHARACTER SET latin1，但是其中有utf8字节。在修复字符集时，不保留字节：

然后在不更改字节的情况下转换列：

ALTER TABLE tbl MODIFY COLUMN MYDATA LONGBLOB;
ALTER TABLE tbl MODIFY COLUMN MYDATA LONGTEXT CHARACTER SET utf8mb4;

（请确保拥有您最初拥有的所有属性，例如NOT NULL。）

这是http://mysql.rjweb.org/doc.php/charcoll中讨论的“两步ALTER”。）（请确保其他规范保持不变-VARCHAR，NOT NULL等）

分区测试用例：

DROP TABLE IF EXISTS ptest;
CREATE TABLE ptest (
        nn INT NOT NULL,
        ee LONGTEXT
    ) ENGINE=InnoDB DEFAULT CHARSET=latin1
    PARTITION BY RANGE (nn)
        (PARTITION p0 VALUES LESS THAN (1),
         PARTITION p1 VALUES LESS THAN MAXVALUE);
SET NAMES latin1;
INSERT INTO ptest (nn, ee)  VALUES ( 0, "é" ), ( 1, "ü" );
SELECT nn, ee, HEX(ee), LENGTH(ee), CHAR_LENGTH(ee) FROM ptest;
ALTER TABLE ptest
    DEFAULT CHARSET utf8;
SELECT nn, ee, HEX(ee), LENGTH(ee), CHAR_LENGTH(ee) FROM ptest;
SELECT @@version;
SHOW CREATE TABLE ptest\G

分区结果：

mysql>     DROP TABLE IF EXISTS ptest;
Query OK, 0 rows affected (0.02 sec)

mysql>     CREATE TABLE ptest (
    ->             nn INT NOT NULL,
    ->             ee LONGTEXT
    ->         ) ENGINE=InnoDB DEFAULT CHARSET=latin1
    ->         PARTITION BY RANGE (nn)
    ->             (PARTITION p0 VALUES LESS THAN (1),
    ->              PARTITION p1 VALUES LESS THAN MAXVALUE);
Query OK, 0 rows affected (0.03 sec)

mysql>     SET NAMES latin1;
Query OK, 0 rows affected (0.00 sec)

mysql>     INSERT INTO ptest (nn, ee)  VALUES ( 0, "é" ), ( 1, "ü" );
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql>     SELECT nn, ee, HEX(ee), LENGTH(ee), CHAR_LENGTH(ee) FROM ptest;
+----+------+---------+------------+-----------------+
| nn | ee   | HEX(ee) | LENGTH(ee) | CHAR_LENGTH(ee) |
+----+------+---------+------------+-----------------+
|  0 | é    | C3A9    |          2 |               2 |
|  1 | ü    | C3BC    |          2 |               2 |
+----+------+---------+------------+-----------------+
2 rows in set (0.00 sec)

mysql>     ALTER TABLE ptest
    ->         DEFAULT CHARSET utf8;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql>     SELECT nn, ee, HEX(ee), LENGTH(ee), CHAR_LENGTH(ee) FROM ptest;
+----+------+---------+------------+-----------------+
| nn | ee   | HEX(ee) | LENGTH(ee) | CHAR_LENGTH(ee) |
+----+------+---------+------------+-----------------+
|  0 | é    | C3A9    |          2 |               2 |
|  1 | ü    | C3BC    |          2 |               2 |
+----+------+---------+------------+-----------------+
2 rows in set (0.00 sec)

mysql>     SELECT @@version;
+-----------------+
| @@version       |
+-----------------+
| 5.6.22-71.0-log |
+-----------------+
1 row in set (0.00 sec)

mysql>     SHOW CREATE TABLE ptest\G
*************************** 1. row ***************************
       Table: ptest
Create Table: CREATE TABLE `ptest` (
  `nn` int(11) NOT NULL,
  `ee` longtext CHARACTER SET latin1
) ENGINE=InnoDB DEFAULT CHARSET=utf8
/*!50100 PARTITION BY RANGE (nn)
(PARTITION p0 VALUES LESS THAN (1) ENGINE = InnoDB,
 PARTITION p1 VALUES LESS THAN MAXVALUE ENGINE = InnoDB) */
1 row in set (0.00 sec)

嗯...我看不到ALTER问题。您使用什么版本？您看到这个测试用例有问题吗？

Alter Table默认字符集修改MySQL 5.6中的行

1 个答案: