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Oracle DB 序列

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Jun 07, 2016 pm 05:32 PM

oracle sequencedatabase

可以将序列高速缓存在内存中，以便于更快地对这些值进行访问。当首次引用序列时，序列值会被填充到高速缓存中。因此会从高速缓存

CYCLE | NOCYCLE ：指定在达到最大值或最小值之后，序列是否继续生成值（NOCYCLE是默认选项）

CACHE n | NOCACHE ：指定Oracle Server 预先分配并保留在内存中的值的数量（默认情况下，Oracle Server 会高速缓存20 个值）

示例中创建了一个名为DEPT_DEPTID_SEQ的序列，用作DEPARTMENTS 表的DEPARTMENT_ID 列。该序列从120 开始，，不允许高速缓存，也不进行循环。

请勿在序列用于生成主键值时使用CYCLE选项，除非有一个可靠的机制与序列循环相比可以更快地清除旧行。

注：序列与表没有关系。通常，应按序列的预期用途来命名序列。但是，序列可以在任何地方使用，而与其名称无关。

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