sql - How do I make my ORDER BY DESC queries efficient in MySQL if descending indexes are ignored? -

sql - How do I make my ORDER BY DESC queries efficient in MySQL if descending indexes are ignored? -

select * table from_user=? , to_user=? order created_at desc

created_at datetime column.

according previous question (here), learned don't need create descending index.

is index ok if have billions of records? (notice i'm not specifying desc index created_at)

>> from_user_id, to_user_id, created_at

imo having index on from_user_id, to_user_id, created_at not help much. don't think have many records same from_user_id, to_user_id, created_at values. (date time millisecond!) if assumption correct, in index such low cardinality effectiveness reduced linear search plus wasting storage. suggest having index on from_user_id, to_user_id, matching criteria, index applied before ordering. in case can see query execution plan see query cost, when having these indexes. (indexing mechanism overview interesting)

mysql sql database indexing