On 19t of January, Heikki Linnakangas committed patch:
Make pg_relation_size() and friends return NULL if the object doesn't exist. That avoids errors when the functions are used in queries like "SELECT pg_relation_size(oid) FROM pg_class", and a table is dropped concurrently. Phil Sorber
Continue reading Waiting for 9.2 – NULLS from pg_*_size() functions