Postgres not using partial timestamp index on interval queries (e.g., now() - interval '7 days' ), PostgreSQL

Postgres not using partial timestamp index on interval queries (e.g., ...

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--PostgreSQL 9.5
--'\\' is a delimiter

SET TIME ZONE 'UTC';

CREATE TABLE precip (
    gauge_id smallint,
    inches numeric(8, 2),
    reading_time timestamp with time zone
);

INSERT INTO precip(gauge_id, inches, reading_time)
SELECT CASE WHEN random() > 0.7 THEN 208 ELSE (100.0 * random())::int END, (1000000.0 * random())::int / 100.0, s
FROM   generate_series('2016-02-01 00:00:00+00', '2017-03-06 00:00:00+00', interval '1 day') s;

/*
CREATE INDEX idx_precip3_id
    ON precip USING btree
    (gauge_id);

CREATE INDEX idx_precip3_reading_time
    ON precip USING btree
    (reading_time);

CREATE INDEX idx_precip_last_five_days
    ON precip USING btree
    (reading_time)
    WHERE reading_time > '2017-02-26 00:00:00+00';
*/

CREATE INDEX idx_precip_gauge_id_last_five_days
    ON precip USING btree
    (gauge_id, reading_time)
    WHERE reading_time > '2017-02-26 00:00:00+00';

select now(), now() - interval '7 days';

explain analyze select * from precip where gauge_id = 208 and reading_time > '2017-02-27';
explain analyze select * from precip where gauge_id = 208 and reading_time > '2017-02-26 00:00:00+00';
explain analyze select * from precip where gauge_id = 208 and reading_time > now();
explain analyze select * from precip where gauge_id = 208 and reading_time > now() - interval '7 days';
explain analyze select * from precip where gauge_id = 208 and reading_time > '2017-02-26 00:00:00+00' and reading_time > now();
explain analyze select * from precip where gauge_id = 208 and reading_time > '2017-02-26 00:00:00+00' and reading_time > now() - interval '7 days';


absolute service time: 0,46 sec

	now	?column?
1	06.03.2017 15:43:21	27.02.2017 15:43:21

	QUERY PLAN
1	Index Scan using idx_precip_gauge_id_last_five_days on precip (cost=0.13..8.15 rows=1 width=24) (actual time=0.106..0.108 rows=2 loops=1)
2	Index Cond: ((gauge_id = 208) AND (reading_time > '2017-02-27 00:00:00+00'::timestamp with time zone))
3	Planning time: 0.626 ms
4	Execution time: 0.153 ms

	QUERY PLAN
1	Index Scan using idx_precip_gauge_id_last_five_days on precip (cost=0.13..8.15 rows=1 width=24) (actual time=0.011..0.012 rows=2 loops=1)
2	Index Cond: ((gauge_id = 208) AND (reading_time > '2017-02-26 00:00:00+00'::timestamp with time zone))
3	Planning time: 0.088 ms
4	Execution time: 0.038 ms

	QUERY PLAN
1	Seq Scan on precip (cost=0.00..10.00 rows=1 width=24) (actual time=0.148..0.148 rows=0 loops=1)
2	Filter: ((gauge_id = 208) AND (reading_time > now()))
3	Rows Removed by Filter: 400
4	Planning time: 0.080 ms
5	Execution time: 0.178 ms

	QUERY PLAN
1	Seq Scan on precip (cost=0.00..11.00 rows=1 width=24) (actual time=0.162..0.164 rows=2 loops=1)
2	Filter: ((gauge_id = 208) AND (reading_time > (now() - '7 days'::interval)))
3	Rows Removed by Filter: 398
4	Planning time: 0.101 ms
5	Execution time: 0.203 ms

	QUERY PLAN
1	Index Scan using idx_precip_gauge_id_last_five_days on precip (cost=0.14..8.16 rows=1 width=24) (actual time=0.013..0.013 rows=0 loops=1)
2	Index Cond: ((gauge_id = 208) AND (reading_time > '2017-02-26 00:00:00+00'::timestamp with time zone) AND (reading_time > now()))
3	Planning time: 0.105 ms
4	Execution time: 0.050 ms

	QUERY PLAN
1	Index Scan using idx_precip_gauge_id_last_five_days on precip (cost=0.14..8.16 rows=1 width=24) (actual time=0.017..0.019 rows=2 loops=1)
2	Index Cond: ((gauge_id = 208) AND (reading_time > '2017-02-26 00:00:00+00'::timestamp with time zone) AND (reading_time > (now() - '7 days'::interval)))
3	Planning time: 0.109 ms
4	Execution time: 0.055 ms