As I understand it, if you'd ask for "latest test with item == X", it
would just go through the database sorted by the time and returned on
first item with "item == X" it would find. But, if you'd ask for
"latest tests with item == X for each unique value 'scenario'", it
would not stop on the first item with "item == X", it would have to go
through the whole DB, because what if there is some other item with
"item == X" but different scenario value? You would have to create a
new table with all possible "scenario" values, but that wouldn't be a
generic solution.