Oracle中Sequence定义和使用
Oracle中有Sequence序列生成器用于生成表的主键值，官方定义：Sequences are database objects from which multiple users can generate unique integers. The sequence generator generates sequential numbers, which can help to generate unique primary keys automatically, and to coordinate keys across multiple rows or tables.
sequence cache：Sequence numbers can be kept in the sequence cache in the System Global Area (SGA). Sequence numbers can be accessed more quickly in the sequence cache than they can be read from disk.When a sequence is read into the sequence cache, sequence values are generated and stored in a cache entry. The number of sequence values stored in the cache is determined by the CACHE parameter in the CREATE SEQUENCE statement. The default value for this parameter is 20.
CREATE SEQUENCE emp_sequence
INCREMENT BY 1
START WITH 1
NOMAXVALUE
NOCYCLE
CACHE 10;
MySQL中定义和使用Sequence
一、建立sequence table
CREATE TABLE `sequence` (
`seq_name` varchar(50) NOT NULL,
`current_value` int(11) NOT NULL,
`increment_value` int(11) NOT NULL DEFAULT '1',
PRIMARY KEY (`seq_name`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
二、创建三个函数
1、func_currval：获取当前序列的值并返回
CREATE FUNCTION `func_currval`(seq_name varchar(50))
RETURNS int(11)
begin
declare value integer;
set value = 0;
select current_value into value
from sequence
where seq_name = seq_name limit 1;
return value;
end;
2、func_setval：设置当前序列的值并返回
CREATE FUNCTION `func_setval`(seq_name varchar(50))
RETURNS int(11)
begin
update sequence
set current_value = current_value + increment_value
where seq_name = seq_name;
return func_currval(seq_name);
end;
3、func_nextval：获取序列中的一个可用的值
CREATE FUNCTION `func_nextval`(seq_name varchar(50),value integer)
RETURNS int(11)
begin
update sequence
set current_value = value
where seq_name = seq_name;
return func_currval(seq_name);
end;
三、调用函数生成序列
INSERT INTO sequence VALUES ('seq_xxx, '0', '1');
SELECT func_nextval(''seq_xxx');
SELECT func_currval(''seq_xxx');
使用Auto-Increments(MySQL官方出品)
1、创建一个序列表：
CREATE TABLE `sequence` (
`id` bigint(20) unsigned NOT NULL auto_increment,
`stub` char(1) NOT NULL default '',
PRIMARY KEY (`id`),
UNIQUE KEY `stub` (`stub`)
) ENGINE=MyISAM;
SELECT * from sequence ;
+-------------------+------+
| id | stub |
+-------------------+------+
| 72157623227190423 | a |
+-------------------+------+
2、如果需要一个新的值，执行以下的SQL获取：
REPLACE INTO sequence(stub) VALUES ('a');
SELECT LAST_INSERT_ID();
使用UUID或UUID_SHORT(MySQL官方出品)
UUID() |
Return a Universal Unique Identifier (UUID) |
UUID_SHORT() |
Return an integer-valued universal identifier |
UUID_SHORT()：
Returns a “short” universal identifier as a 64-bit unsigned integer. Values returned by UUID_SHORT() differ from the string-format 128-bit identifiers returned by the UUID() function and have different uniqueness properties. The value of UUID_SHORT() is guaranteed to be unique if the following conditions hold:
-
The server_id value of the current server is between 0 and 255 and is unique among your set of master and slave servers
-
You do not set back the system time for your server host between mysqld restarts
-
You invoke UUID_SHORT() on average fewer than 16 million times per second between mysqld restarts
The UUID_SHORT() return value is constructed this way: (server_id & 255) << 56 + (server_startup_time_in_seconds << 24) + incremented_variable++;
mysql> SELECT UUID_SHORT(); -> 92395783831158784
总结
以上三个方案均可以实现序列功能，但高并发场景肯定会存在性能问题（Oracle有序列缓存），要结合具体业务场景评估使用，建议以下等价方案：
- 使用自增INT类型作为物理主键，MySQL底层存储为B+ Tree；
- 借助第三方基础服务实现，如：分布式ID生成器SowFlake；
- 使用UUID或者UUID_SHORT函数生成唯一值；
美团的技术博客介绍比较详细：Leaf——美团点评分布式ID生成系统
参考
sequence cache
MySQL实现类似Oracle的序列 - sequence
Leaf——美团点评分布式ID生成系统
Ticket Servers: Distributed Unique Primary Keys on the Cheap
