Excuse my ignorance on FDWs but how does this work with respect to DB constraints? For example, if a postgres column is supposed to be a date, but it's not a pr

Excuse my ignorance on FDWs but how does this work with respect to DB constraints? For example, if a postgres column is supposed to be a date, but it's not a properly formatted date, does it throw an error at query time? Or is everything treated as a varchar/text?