高并发（压力测试）下，
发现postgresql数据库中出现自增长的ID断层的情况，如下所示：
ID
1
2
3
4
7
8
9
少了ID为5和6的数据，经过确认，是postgresql对高并发处理机制导致的，
在pg中，每次新增数据，都会将一个ID锁定到这条数据上，如果这条数据因为某些原因插入失败（主键重复等），那么这个ID也不会再赋予新的数据，因此出现ID跳过的现象，官方解释：
If transactions rollback, the serial value assigned during the rolled
back transaction is skipped. This has been discussed many times, it's
a tradeoff between losing some #s now and again and taking a huge
performance and code complexity hit to avoid it.
If you absolutely need consecutive #s, then serial is not for you and
you should implement your own method of acquiring sequential numbers.
