GIN doesn't support full index scans due to their extreme inefficiency: because there are often many keys per value, each heap pointer will returned several times.
When extractQuery
returns zero keys,
GIN will emit an error: for different opclasses and
strategies the semantic meaning of a void query may be different (for
example, any array contains the void array, but they don't overlap the
void array), and GIN can't suggest a reasonable answer.
GIN searches keys only by equality matching. This may be improved in future.