請大家耐心看下去，受益非淺！
Identity columns (sometimes referred to as "auto-increment" columns) remain a somewhat controversial topic in ASE: while the functionality is very useful, the implementation in ASE leaves quite a few things to be desired. In practice, this means that there is a risk of running into "identity gaps" : these are occasions where the identity value suddenly make large jumps, like from 10031 to 5000002 instead of to 10032, usually as a result of a shutdown with nowait.
Because shutdown with nowait cannot always be avoided, identity gaps may occur at some point, and they can cause real problems for applications that are not expecting such large numbers.
It is unfortunate that ASE does not have any features to make repairing an identity gap easy: the repair procedure documented by Sybase is a bit clumsy and can cause significant application downtime (it requires a full BCP-out and BCP-in of the table). For this reason, some developers or DBAs have decided not to use identity columns at all.
The problems caused by identity gaps can actually be limited; the precise method depends on your ASE version:
1)When running ASE 12.0 or later, you should use the identity_gap feature for each table with an identity column to limit the size of possible identity gaps. Details are described here . 見文1
2)For all ASE versions, you can design your data model in such a way that identity gaps will never occur.見文2
---------------------------------------------------------------------------
[color=#00008B]A better method for dealing with "identity gaps" [/color]
Introduction
A well-known feature of Sybase Adaptive Server Enterprise is the "identity column". Identity columns are useful because they automatically generate consecutive numeric values upon insertion of new rows, while having minimal impact on performance.
A disadvantage of using identity columns is that so-called "identity gaps" may occasionally occur. Identity gaps are large, sudden jumps in the value of an identity column, which often cause problems for applications.
Unfortunately, there is currently no quick or easy way of repairing an identity gap.
