问题： Db2 sequence如何确保先产生的序列号小，后产生的序列号大？ 这里的先后是指时间先后。
答：需要在创建Sequence的时候指定order选项。 当然，只指定order选项只有保证序列号是有顺序的，不能保证是追次增加的。
ORDER or NO ORDER
Specifies whether the sequence numbers must be generated in order of request. The default is NO ORDER.
ORDER
Specifies that the sequence numbers are generated in order of request. Specifying ORDER might disable the caching of values. There is no guarantee that values are assigned in order across the entire server unless NO CACHE is also specified. ORDER applies only to a single-application process.
NO ORDER
Specifies that the sequence numbers do not need to be generated in order of request.
In a data sharing environment, if the CACHE and NO ORDER options are in effect, multiple caches can be active simultaneously, and the requests for next value assignments from different DB2 members might not result in the assignment of values in strict numeric order. For example, if members DB2A and DB2B are using the same sequence, and DB2A gets the cache values 1 to 20 and DB2B gets the cache values 21 to 40, the actual order of values assigned would be 1,21,2 if DB2A requested for next value first, then DB2B requested, and then DB2A again requested. Therefore, to guarantee that sequence numbers are generated in strict numeric order among multiple DB2 members using the same sequence concurrently, specify the ORDER option.