For some reason, we ended up implementing in the database layer a function
DatabaseStatementInterface::fetchField() that is nothing more then an alias of
Probably as a consequence of that, we forgot to implement
DatabaseStatementPrefetch::fetchColumn() in our user-space implementation of
PDOStatement. Bummer, because we recently started using it in core (in
DateTimeFunctionalTest::testDateFormatStorage()). Let's fix this in
DatabaseStatementPrefetch before considering removing this function altogether in Drupal 8.