Yes, NULL values are not stored in the index, but you may create functional index on
Are you sure NULL values are not stored? btree, gist and bitmap index and search for NULL values.
select amname, amindexnulls, amsearchnulls from pg_am;
amname | amindexnulls | amsearchnulls
--------+--------------+---------------
btree | t | t
hash | f | f
gist | t | t
gin | f | f
bitmap | t | t
(5 rows)
Sincerely yours,
Vladimir Sitnikov