On Thu, Jan 26, 2006 at 04:19:34PM -0500, Eric B. Ridge wrote: > 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 don't know about tuples from different transactions having the same xmin (aside from 1/BootstrapXID and 2/FrozenXID), but tuples from the same outer transaction could have different xmin values due to savepoints. test=> CREATE TABLE foo (x integer); test=> BEGIN; test=> INSERT INTO foo VALUES (1); test=> SAVEPOINT s; test=> INSERT INTO foo VALUES (2); test=> RELEASE SAVEPOINT s; test=> INSERT INTO foo VALUES (3); test=> COMMIT; test=> SELECT xmin, * FROM foo; xmin | x --------+--- 424584 | 1 424585 | 2 424584 | 3 (3 rows) Explicit savepoints aren't the only way to get this effect; you'll also see it if the savepoint is implicit, as when trapping errors in a function. -- Michael Fuhr
