A covering index, and the table it never touches

An index containing every column a query needs answers it without reading the table, which on a wide row is the difference between one page read and two.

SELECT customer_id, total_cents FROM orders
WHERE placed_at BETWEEN ? AND ?;

-- serves the WHERE, then still reads the row for the rest
KEY idx_placed (placed_at)

-- covering: everything is in the index
KEY idx_placed_cover (placed_at, customer_id, total_cents)

-- EXPLAIN Extra: Using index
--   (not "Using index condition", which is different)

Using index in the Extra column is the confirmation, and it is easy to confuse with Using index condition, which means index condition pushdown and still reads rows. The cost is index size: adding three columns to cover a query makes every write to those columns update the index too, so covering a rarely-run report is usually a bad trade and covering the hottest query is usually a good one.