GIN doesn't support full index scans: because there are often many keys per value, each heap pointer would be returned many times, and there is no easy way to prevent this.
extractQuery returns zero
keys, GIN will emit an error.
Depending on the operator, a void query might match all, some, or
none of the indexed values (for example, every array contains the
empty array, but does not overlap the empty array), and
GIN can't determine the
correct answer, nor produce a full-index-scan result if it could
determine that that was correct.
It is not an error for
extractValue to return zero keys, but in this
case the indexed value will be unrepresented in the index. This
is another reason why full index scan is not useful — it would
miss such rows.
GIN searches keys only by equality matching. This may be improved in future.