Re: "xmin" system column

"Eric B. Ridge" <ebr@tcdi.com> writes:
> Outside of "VACUUM FREEZE", is there any way the "xmin" column in a
> relation can change, assuming of course the tuple is never updated
> again?

If the tuple lives long enough, VACUUM will change it to FrozenTransactionId
eventually, even without the FREEZE option.

> I'm considering using this as a way to identify all tuples
> modified in the same transaction (in an effort to group them
> together), and am wondering if there's any way tuples from different
> transactions could end up with the same xmin value.

This seems OK as long as the transaction was fairly recent.  Note that
you will need a fairly restrictive definition of "same transaction"
(no subtransactions).

            regards, tom lane