CREATE INDEX foo_my_timestamp_idx
ON foo
USING btree
((my_timestamp::date) DESC);

CLUSTER foo USING foo_my_timestamp_idx;

CREATE INDEX foo_my_timestamp_idx ON foo (my_timestamp);

SELECT * FROM foo
WHERE my_timestamp::date = '2016-07-25';

SELECT * FROM foo
WHERE  my_timestamp >= '2016-07-25'
WHERE  my_timestamp <  '2016-07-26';