問題： Db2 sequence如何確保先產生的序列號小，後產生的序列號大？ 這裏的先後是指時間先後。
答：需要在創建Sequence的時候指定order選項。 當然，只指定order選項只有保證序列號是有順序的，不能保證是追次增加的。
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.