Clients waiting to acquire a lock on an entire table is something you should aim to avoid. The following will show you when this is happening:
SELECT locked.pid AS locked_pid, locker.pid AS locker_pid, locked_act.usename AS locked_user, locker_act.usename AS locker_user, locked.virtualtransaction, locked.transactionid, relname FROM pg_locks locked LEFT OUTER JOIN pg_class ON (locked.relation = pg_class.oid), pg_locks locker, pg_stat_activity locked_act, pg_stat_activity locker_act WHERE locker.granted=true AND locked.granted=false AND locked.pid=locked_act.procpid AND locker.pid=locker_act.procpid AND locked.relation=locker.relation;
Output from this query looks similar to the previous transactionid examples: ...