Speeding Up Query on a Time Series Table. It's Slow to get (MAX - MIN) Aggregation Value
An index on (point_id, datetime, value) will likely speed up the query, as it will only have to do an index seek, read only the relevant part of the index and also have available (from the index) all the values of the value column to calculate the MIN and MAX.

An index on (A) (point_id) or (B) (point_id, datetime) would need to also find the read the relevant tuples from the table, in order (A) to filter them for the datetime range and (A and B) to read the values of the value column.