PostgreSQL 10.0 preview 功能增强 - identify column (SQL标准:T174)
背景
identify column类似PostgreSQL的serial, (自增\减序列). identify column(s)属于SQL标准中的某个特性。
PostgreSQL 10.0将实现这一标准，虽然你已经可以使用serial来达到同样效果，不过实现这一标准，可以兼容更多的数据库。
对于从其他数据库迁移到PostgreSQL来说，无疑是福音。
An Identity column is a column (also known as a field) in a database table that is made up of values generated by the database.
This is much like an AutoNumber field in Microsoft Access or a sequence in Oracle.
Because the concept is so important in database science, many RDBMS systems implement some type of generated key, although each has its own terminology.
An identity column differs from a primary key in that its values are managed by the server and usually cannot be modified.
In many cases an identity column is used as a primary key; however, this is not always the case.
It is a common misconception that an identity column will enforce uniqueness; however, this is not the case.
If you want to enforce uniqueness on the column you must include the appropriate constraint too.
In Microsoft SQL Server you have options for both the seed (starting value) and the increment. By default the seed and increment are both 1.
这个patch的讨论，详见邮件组，本文末尾URL。
PostgreSQL社区的作风非常严谨，一个patch可能在邮件组中讨论几个月甚至几年，根据大家的意见反复的修正，patch合并到master已经非常成熟，所以PostgreSQL的稳定性也是远近闻名的。
参考
https://commitfest.postgresql.org/13/767/
https://www.postgresql.org/message-id/attachment/50108/v4-0001-Identity-columns.patch
https://www.postgresql.org/message-id/flat/6adbacbf-73bc-dd1a-2033-63409180fd18@2ndquadrant.com#6adbacbf-73bc-dd1a-2033-63409180fd18@2ndquadrant.com
https://en.wikipedia.org/wiki/Identity_column