CREATE TABLE metrics(
machine_id int NOT NULL,
ts timestamptz NOT NULL,
"V1_2" int NOT NULL,
"V2_3" int NOT NULL,
"V3_1" int NOT NULL
);

INSERT INTO  metrics(machine_id, ts, "V1_2", "V2_3", "V3_1")
SELECT n % 2 + 1, '20180711'::timestamptz + (n * interval '5 second'), n % 123, n % 438, n % 841
  FROM generate_series(1, 100000) AS g(n);

CREATE INDEX ON metrics(machine_id, (date_trunc('day', ts AT TIME ZONE 'CEST')), ("V1_2" + "V2_3" + "V3_1"));

-- This could be used in LATERAL join to generated dates (generate_series):
SELECT MAX ("V1_2" + "V2_3" + "V3_1") AS vmax,
       MIN ("V1_2" + "V2_3" + "V3_1") AS vmin
  FROM metrics AS m
 WHERE m.machine_id = 1
   AND date_trunc('day', ts AT TIME ZONE 'CEST') = '20180713';