What is the expected behaviour of sequence numbers under pg concurrence model? E.g. Uncommitted transaction 1 in progress has inserted 5 tuples into table foo. Meanwhile uncommitted transaction 2 in progress inserts 100 tuples into table foo. If the sequence number for the pk started at 1000 before these transactions, what are the sequence numbers for transaction 1, 2, and the final sequence number if they both commit? One rollback's? Both rollback? One hangs open while the other commits? One hang and one rollback's?