The index that answered the whole query
sql
SELECT student_id, score FROM submissions WHERE problem_id = 42;
An index on (problem_id) makes this fast. An index on
(problem_id, student_id, score) makes it faster still. Why?
sql
SELECT student_id, score FROM submissions WHERE problem_id = 42;
An index on (problem_id) makes this fast. An index on
(problem_id, student_id, score) makes it faster still. Why?