Why Timestamp isn't allowed as a primary key?

Hi,
In JPA, whay Date objects are allowed to be used as primary keys for entities while Timestamp objects aren't?
Thanks.

No idea, but I was going to say that with Timestamps, it is possible that two records are added at the exact same moment, therefore not unique. But dates doesn't make sense unless they have a higher precision that can guarantee uniqueness.

Mark

Perhaps I missed something in the spec, but I see nothing in JPA disallowing having a timestamp as a primary key or part of one. I'm pretty sure it is supported in the JPA reference impl.

In general you should be careful in doing so, as different databases support different precision of timestamp values (seconds, milliseconds, microsecond). Normally a generated sequential id is a much better way to go.

I have been reading "EJB 3 In Action", it indicates the at Timestamp couldn't be used as PK.
I think Date PK should also have this restriction.

Using PK as date/timestamp would cause issues for distributed applications and probably thats why they are not allowed.

Googling result: http://www.mail-archive.com/pgsql-general@postgresql.org/msg86346.html

Thanks
Ravindra