postgres=# \timing 
Timing is on.
postgres=# CREATE TABLE sensor_log (
postgres(#   id            SERIAL NOT NULL PRIMARY KEY,
postgres(#   location      VARCHAR NOT NULL,
postgres(#   reading       BIGINT NOT NULL,
postgres(#   reading_date  TIMESTAMP NOT NULL
postgres(# );
CREATE TABLE
Time: 100.977 ms
postgres=# INSERT INTO sensor_log (id, location, reading, reading_date)
postgres-# SELECT s.id, s.id % 1000, s.id % 100,
postgres-#        CURRENT_DATE - ((s.id * 10) || 's')::INTERVAL
postgres-#   FROM generate_series(1, 1000000) s(id);
INSERT 0 1000000
Time: 10052.855 ms (00:10.053)
postgres=# select count(*) from sensor_log;
  count  
---------
 1000000
(1 row)

Time: 46.298 ms

postgres=# select count(*) from sensor_log where location::int % 20 = 0;
 count 
-------
 50000
(1 row)

Time: 85.917 ms

postgres=# delete from sensor_log where location::int % 23 = 0;
DELETE 44000
Time: 1137.541 ms (00:01.138)
postgres=# 
postgres=# select count(*) from sensor_log;
 count  
--------
 956000
(1 row)

Time: 55.527 ms