"xmin" system column

["Eric B. Ridge" <ebr@xxxxxxxx>]

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?  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.

I've tried both "VACUUM" and "VACUUM FULL" on specific tables and  
neither seem to have an impact, but I haven't done extensive testing  
against very large tables that have experienced lots of churn.

Any input will be greatly appreciated!

eric