本文介绍了Sequence的隐式用法。

创建Sequence

在为拆分表或广播表的主键定义AUTO_INCREMENT后,Sequence可以用于自动填充主键,由DRDS自动维护。

扩展标准建表语法,增加了自增列的Sequence类型,如果未指定类型关键字,则默认类型为GROUP。自动创建的、跟表相关联的Sequence名称,都是以AUTO_SEQ_为前缀,后面加上表名。

创建Group Sequence、Time-based Sequence或Simple Sequence
语法
CREATE TABLE <name> (
   <column> ... AUTO_INCREMENT [ BY GROUP | SIMPLE | TIME ],
   <column definition>,
   ...
) ... AUTO_INCREMENT=<start value>
说明 如果指定了BY TIME,即Time-based Sequence,则该列类型必须为BIGINT。
创建单元化Group Sequence

语法

CREATE TABLE <name> (
   <column> ... AUTO_INCREMENT [ BY GROUP ] [ UNIT COUNT <numeric value> INDEX <numeric value> ],
   <column definition>,
   ...
) ... AUTO_INCREMENT=<start value>
示例
  • 示例一:默认创建一张使用Group Sequence作为自增列的表。
    mysql> CREATE TABLE tab1 (
    col1 BIGINT NOT NULL AUTO_INCREMENT,
    col2 VARCHAR(16),
    PRIMARY KEY(col1)
    ) DBPARTITION BY HASH(col1);
  • 示例二:创建3张同名的、使用相同单元数量和不同单元索引的单元化Group Sequence作为自增列的表,分别用于3个不同的实例或库。
    1. 实例1/库1

      请在命令行输入如下代码:

      mysql> CREATE TABLE tab2 (
      col1 BIGINT NOT NULL AUTO_INCREMENT UNIT COUNT 3 INDEX 0,
      col2 VARCHAR(16),
      PRIMARY KEY(col1)
      ) DBPARTITION BY HASH(col1);
    2. 实例2/库2

      请在命令行输入如下代码:

      mysql> CREATE TABLE tab2 (
      col1 BIGINT NOT NULL AUTO_INCREMENT UNIT COUNT 3 INDEX 1,
      col2 VARCHAR(16),
      PRIMARY KEY(col1)
      ) DBPARTITION BY HASH(col1);
    3. 实例3/库3

      请在命令行输入如下代码:

      mysql> CREATE TABLE tab2 (
      col1 BIGINT NOT NULL AUTO_INCREMENT UNIT COUNT 3 INDEX 2,
      col2 VARCHAR(16),
      PRIMARY KEY(col1)
      ) DBPARTITION BY HASH(col1);
  • 示例三:创建一张使用Time-based Sequence作为自增列的表。
    mysql> CREATE TABLE tab3 (
    col1 BIGINT NOT NULL AUTO_INCREMENT BY TIME, 
    col2 VARCHAR(16), 
    PRIMARY KEY(col1)
    ) DBPARTITION BY HASH(col1);
  • 示例四:创建一张使用Simple Sequence作为自增列的表。
    mysql> CREATE TABLE tab4 ( 
    col1 BIGINT NOT NULL AUTO_INCREMENT BY SIMPLE, 
    col2 VARCHAR(16), 
    PRIMARY KEY(col1)
    ) DBPARTITION BY HASH(col1);

修改Sequence

暂不支持通过ALTER TABLE来修改对应Sequence的类型,但您可以参见如下语法通过ALTER TABLE修改起始值:

ALTER TABLE <name> ... AUTO_INCREMENT=<start value>
说明
  • 如果想要修改表相关的Sequence类型,需要通过SHOW SEQUENCES指令查找出Sequence的具体名称和类型,然后再用ALTER SEQUENCE指令去修改,具体操作请参见修改Sequence
  • 使用Sequence后,请谨慎修改AUTO_INCREMENT的起始值(仔细评估已经产生的Sequence值,以及生成新Sequence值的速度,防止产生冲突)。

查看表信息及相关Sequence类型

SHOW CREATE TABLE

当表为拆分表或者广播表时,显示自增列Sequence的类型。

查看已创建的表语法如下:

SHOW CREATE TABLE <name>
说明
  • SHOW CREATE TABLE仅显示相关Sequence的类型,并不显示Sequence详细信息,如需查看,请使用SHOW SEQUENCES命令。
  • 关联了单元化Group Sequence的表并不显示单元数量和单元索引,因此不能将SHOW CREATE TABLE显示的DDL直接用于创建具备同样单元化Group Sequence能力的表。
  • 如果需要创建具备同样单元化能力的表,必须使用SHOW SEQUENCES查看单元数量和单元索引,然后参照CREATE TABLE的语法修改通过SHOW CREATE TABLE获取的建表DDL。
示例
  • 示例一:建表时指定AUTO_INCREMENT,但没有指定Sequence类型关键字,则默认使用Group Sequence
    mysql> SHOW CREATE TABLE tab1;

    返回结果如下:

    +-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    | Table | Create Table                                                                                                                                                                                           |
    +-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    | tab1  | CREATE TABLE `tab1` (
    `col1` bigint(20) NOT NULL AUTO_INCREMENT BY GROUP,
    `col2` varchar(16) DEFAULT NULL,
    PRIMARY KEY (`col1`)
    ) ENGINE=InnoDB DEFAULT CHARSET=utf8 dbpartition by hash(`col1`) |
    +-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    1 row in set (0.02 sec)
  • 示例二:建表时为AUTO_INCREMENT指定了单元数量和单元索引,使用单元化 Group Sequence,但SHOW CREATE TABLE时并不显示单元数量和单元索引,不能将此DDL用于创建具备同样单元化 Group Sequence能力的表。
    mysql> SHOW CREATE TABLE tab2;

    返回结果如下:

    +-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    | Table | Create Table                                                                                                                                                                                           |
    +-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    | tab2  | CREATE TABLE `tab2` (
    `col1` bigint(20) NOT NULL AUTO_INCREMENT BY GROUP,
    `col2` varchar(16) DEFAULT NULL,
    PRIMARY KEY (`col1`)
    ) ENGINE=InnoDB DEFAULT CHARSET=utf8 dbpartition by hash(`col1`) |
    +-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    1 row in set (0.01 sec)
  • 示例三:建表时为AUTO_INCREMENT指定了BY TIME,即Time-based Sequence类型。
    mysql> SHOW CREATE TABLE tab3;

    返回结果如下:

    +-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    | Table | Create Table                                                                                                                                                                                            |
    +-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    | tab3  | CREATE TABLE `tab3` (
    `col1` bigint(20) NOT NULL AUTO_INCREMENT BY TIME,
    `col2` varchar(16) DEFAULT NULL,
    PRIMARY KEY (`col1`)
    ) ENGINE=InnoDB DEFAULT CHARSET=utf8 dbpartition by hash(`col1`) |
    +-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    1 row in set (0.01 sec)
  • 示例四:建表时为AUTO_INCREMENT指定了BY SIMPLE,即Simple Sequence类型。
    mysql> SHOW CREATE TABLE tab4;

    返回结果如下:

    +-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    | Table | Create Table                                                                                                                                                                                            |
    +-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    | tab3  | CREATE TABLE `tab4` (
    `col1` bigint(20) NOT NULL AUTO_INCREMENT BY TIME,
    `col2` varchar(16) DEFAULT NULL,
    PRIMARY KEY (`col1`)
    ) ENGINE=InnoDB DEFAULT CHARSET=utf8 dbpartition by hash(`col1`) |
    +-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    1 row in set (0.01 sec)
SHOW SEQUENCES

建表后相关的Sequence名称和详细信息,可通过SHOW SEQUENCES查看。

mysql> SHOW SEQUENCES;

返回结果如下:

+---------------+--------+------------+------------+------------+--------------+------------+---------------------+-------+--------+
| NAME          | VALUE  | UNIT_COUNT | UNIT_INDEX | INNER_STEP | INCREMENT_BY | START_WITH | MAX_VALUE           | CYCLE | TYPE   |
+---------------+--------+------------+------------+------------+--------------+------------+---------------------+-------+--------+
| seq1          | 100000 | 1          | 0          | 100000     | N/A          | N/A        | N/A                 | N/A   | GROUP  |
| seq2          | 400000 | 3          | 1          | 100000     | N/A          | N/A        | N/A                 | N/A   | GROUP  |
| seq3          | N/A    | N/A        | N/A        | N/A        | N/A          | N/A        | N/A                 | N/A   | TIME   |
| seq4          | 1006   | N/A        | N/A        | N/A        | 2            | 1000       | 99999999999         | N     | SIMPLE |
| AUTO_SEQ_tab1 | 100000 | 1          | 0          | 100000     | N/A          | N/A        | N/A                 | N/A   | GROUP  |
| AUTO_SEQ_tab2 | 400000 | 3          | 1          | 100000     | N/A          | N/A        | N/A                 | N/A   | GROUP  |
| AUTO_SEQ_tab3 | N/A    | N/A        | N/A        | N/A        | N/A          | N/A        | N/A                 | N/A   | TIME   |
| AUTO_SEQ_tab4 | 2      | N/A        | N/A        | N/A        | 1            | 1          | 9223372036854775807 | N     | SIMPLE |
+---------------+--------+------------+------------+------------+--------------+------------+---------------------+-------+--------+
8 rows in set (0.01 sec)