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GIN doesn't support full index scans. The reason for
this is that extractValue is allowed to return zero keys,
as for example might happen with an empty string or empty array. In such
a case the indexed value will be unrepresented in the index. It is
therefore impossible for GIN to guarantee that a
scan of the index can find every row in the table.
Because of this limitation, when extractQuery returns
nkeys = 0 to indicate that all values match the query,
GIN will emit an error. (If there are multiple ANDed
indexable operators in the query, this happens only if they all return zero
for nkeys.)
It is possible for an operator class to circumvent the restriction against
full index scan. To do that, extractValue must return at least
one (possibly dummy) key for every indexed value, and
extractQuery must convert an unrestricted search into
a partial-match query that will scan the whole index. This is inefficient
but might be necessary to avoid corner-case failures with operators such
as LIKE or subset inclusion.
GIN assumes that indexable operators are strict.
This means that extractValue will not be called at all on
a NULL value (so the value will go unindexed), and
extractQuery will not be called on a NULL comparison
value either (instead, the query is presumed to be unmatchable).
A possibly more serious limitation is that GIN cannot handle NULL keys — for example, an array containing a NULL cannot be handled except by ignoring the NULL.
