Опитайте тази заявка -
SELECT t1.* FROM foo t1
JOIN (SELECT uid, MAX(id) id FROM foo GROUP BY uid) t2
ON t1.id = t2.id AND t1.uid = t2.uid;
След това използвайте EXPLAIN за анализиране на заявки.
SELECT t1.* FROM foo t1
LEFT JOIN foo t2
ON t1.id < t2.id AND t1.uid = t2.uid
WHERE t2.id is NULL;