<p>I'll point to rather different reasoning...<p>Libpq is not returning tables, or relations, for that matter, but
ratherthe results of queries.<p>It is reasonable to expect to know which attributes of a table are or are not nullable,
andthat is commonly available as an attribute of pg_attribute, however...<p>General purpose queries are nowhere near so
predetermined. Indeed, whether a column is nullable may not be at all visible, as the value of a column may be computed
bya function and thereby be quite opaque to static analysis.<p>That makes me think that guessing which attributes of a
querymay be null seems like a pretty futile exercise. At first blush, we could simplify to PQnullable() always
returningtrue, but that's not terribly revealing. However, often, there mayn't be a much better solution that isn't
reallytough to implement.<p>I'd not be keen on people putting much effort into futile exercises ; better to work on
thingsthat are "less futile."