We recently had an issue where a misbehaving application was running a long transaction that modified a bunch of rows, and this was holding up other transactions that wanted to do similar modifications. No surprising there. But what I'm unclear of is how this was showing up in pg_locks. The blocked transactions were all waiting on the transactionid of the long-running transaction, not any particular relation or tuple. Why doesn't pg_locks show the actual blockage? Isn't that what MVCC is good for?