Query performance

I have a table with some seven million rows.

The query:

select count(rdate),rdate from reading where sensor_id in
(1137,1138,1139) group by rdate order by rdate desc limit 1;

takes a few seconds but:

select count(rdate),rdate from reading where sensor_id in
(1137,1138,1139,1140) group by rdate order by rdate desc limit 1;

(anything with four or more values in the "in" list) takes several
minutes.

Is there any way to make the "larger" queries more efficient?

Both rdate and sensor_id are indexed and the database is vacuumed every
night.

Explain outputs are:

benparts=# explain select count(rdate),rdate from reading where
sensor_id in (1137,1138,1139,1140) group by rdate order by rdate desc
limit 1;
QUERY PLAN
---------------------------------------------------------------------------------------------------
Limit (cost=0.00..39890.96 rows=1 width=8)
-> GroupAggregate (cost=0.00..7938300.21 rows=199 width=8)
-> Index Scan Backward using date on reading
(cost=0.00..7937884.59 rows=82625 width=8)
Filter: (sensor_id = ANY
('{1137,1138,1139,1140}'::integer[]))
(4 rows)

benparts=# explain select count(rdate),rdate from reading where
sensor_id in (1137,1138,1139) group by rdate order by rdate desc limit
1;
QUERY PLAN
-----------------------------------------------------------------------------------------------------
Limit (cost=48364.32..48364.32 rows=1 width=8)
-> Sort (cost=48364.32..48364.49 rows=69 width=8)
Sort Key: rdate
-> HashAggregate (cost=48361.35..48362.21 rows=69 width=8)
-> Bitmap Heap Scan on reading (cost=535.53..48218.10
rows=28650 width=8)
Recheck Cond: (sensor_id = ANY
('{1137,1138,1139}'::integer[]))
-> Bitmap Index Scan on reading_sensor
(cost=0.00..528.37 rows=28650 width=0)
Index Cond: (sensor_id = ANY
('{1137,1138,1139}'::integer[]))
(8 rows)

TIA,
Stephen Davies
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