As I understand, pg_dump first take snapshot and then lock all tables it intended to dump. What happens if not MVCC-safe command committed after snapshot but before lock? From comment to pg_dump.c I understand that it may fail with 'cache lookup failed' error. But, can it happen, that pg_dump not fail, but instead capture inconsistent dump? For example TRUNCATE committed after snapshot and pg_dump will see result of TRUNCATE but not result of other commands in TRUNCATE transaction?
You can't truncate an already existing table while pg_dump is running. TRUNCATE needs an exclusive lock, and pg_dump already has a lock on all tables of the database it's dumping. So TRUNCATE will be blocked until pg_dump finishes all its work.
(The same will happen for VACUUM FULL, CLUSTER and some (all?) ALTER TABLE commands.)
