Speeding up group by and max query

Question

In my iot use case, I have a table which stores sensor readings. I am using the timescaledb extension.

I have the following table definition:

CREATE TABLE readings (
  id          BIGSERIAL      PRIMARY KEY,
  time        TIMESTAMPTZ       NOT NULL,
  value       DOUBLE PRECISION  NOT NULL,
  device      VARCHAR(255)      NOT NULL,
  sensor      VARCHAR(255)      NOT NULL,
  resolution  VARCHAR(255)      NOT NULL,
  UNIQUE(time, device, sensor, resolution)
);

I want last reported time for every (device,sensor) pair. Here is the query I am running:

select MAX(time), device, sensor from readings group by device, sensor

This query takes a significant time to complete. Is there any way to improve the performance? Or any better query to achieve the same result?

I tried doing this and it did not help:

create index if not exists readings_idx on readings (time, device, sensor, resolution);

Answer 1

(Timescale cofounder here)

It sounds like you are trying to perform a "last point" query, e.g., find the latest record submitted by each (device, sensor). The challenge with the query as above is that the database doesn't know when it has found the last device/sensor, so needs to scan back to the start of time to conclude there are no new devices around.

Much better is the following approach, which performs a LATERAL SCAN from a list of devices against the time-series records:

https://docs.timescale.com/latest/using-timescaledb/reading-data#last-point

Good luck, and you might also find our Slack group at https://slack.timescale.com useful for these types of questions.