btree gist indices, null and open-ended tsranges

Hi All,

Working with the exclude constraint example from
https://www.postgresql.org/docs/current/static/rangetypes.html:

CREATE EXTENSION btree_gist;
CREATE TABLE room_reservation (
room text,
during tsrange,
EXCLUDE USING GIST (room WITH =, during WITH &&)
);

So, first observation: if I make room nullable, the exclude constraint
does not apply for rows that have a room of null. I guess that's to be
expected, right?

Next question: if lots of rows have open-ended periods
(eg: [, 2010-01-01 15:00) or [2010-01-01 14:00,)), how does that affect
the performance of the btree gist index backing the exclude constraint?

cheers,

Chris