SQL/PSQL function to get latest update from inner joined table

Question

I have 2 tables: drivers and drivers_locations. I want to create a computed field in the drivers table that returns the last inserted location of a driver.

The drivers_locations table has these columns:

- id: uuid
- driver_id: uuid
- location: geography
- timestamp: bigint

The timestamp is in milliseconds since the unix epoch. (e.g. 1673129623999).

Here is my noobish failed attempt at achieving this:

CREATE OR REPLACE FUNCTION public.fn_driver_last_location(driver_row drivers)
    RETURNS geometry
 LANGUAGE sql
 IMMUTABLE
AS $function$
    SELECT loc.location 
    FROM driver_locations loc
    inner join drivers drv
    on loc.driver_id  = drv.id
    group by loc.location_recorded_time_unix, loc.location, drv.id
    HAVING loc.location_recorded_time_unix = MAX(loc.location_recorded_time_unix) and drv.id = driver_row.id;
$function$
;;

Any ideas?

Answer 1

You can sort the locations by the time and limit the result to one row. You also don't need the join.

CREATE OR REPLACE FUNCTION public.fn_driver_last_location(driver_row drivers)
 RETURNS geometry
 LANGUAGE sql
 STABLE
AS $function$
    SELECT loc.location
    FROM driver_locations loc
    where loc.driver_id = driver_row.id
    order by loc.location_recorded_time_unix desc 
    limit 1;
$function$
;

A function that selects from a database tables should never be declared as immutable as it can return different results even when called with the same parameter.