Why is the candidate composite index not used?
You have to FORCE the index, the optimizer will only then  use your wanted index, else you give it only a hint
SELECT 
    BIN_TO_UUID(album_id) AS e,
    firstdateofinterval AS p,
    BIN_TO_UUID(territory_id) AS t,
    SUM(playbacks) AS c1,
    SUM(userplaybacks) AS c2,
    SUM(userdownloads) AS c3,
    SUM(usersingledownloads) AS c4
FROM
    entityusage_trackbyyear
    FORCE INDEX (territory_id)
WHERE
    firstdateofinterval &gt;= '2018-01-01'
        AND firstdateofinterval &lt;= '2018-09-30'
        AND library_id = UUID_TO_BIN('c72cbb1e-5848-39ee-a201-664f26ee9204')
GROUP BY p , e