How to find the reason MySQL Heatwave is not offload

set optimizer_trace="enabled=on";

explain select distinct country from aiport_geo order by country;

select query, TRACE->'$**.Rapid_Offload_Fails' 'Offload Failed',

           TRACE->'$**.secondary_engin_not_used' "HeatWave Not Used'

from information_schema.optimizer_trace\G;