Playing devil's advocate here, especially for Postgres this isn't a clear cut case, as because of Postgres' MVCC implementation, depending on whether the rows w

Playing devil's advocate here, especially for Postgres this isn't a clear cut case, as because of Postgres' MVCC implementation, depending on whether the rows were already frozen or not (caused by lots of mutations ongoing and not enough VACUUMs), it might have either used an index-only scan (only from 9.2 on and only if most pages are visible-for-all, so the query scanner allows the optimization). In the other cases, it would have had to fetch all rows from disk anyway, in which case "not enough information" would have been true...