A composite index in the wrong column order

An index on (created_at, status) serving a query that filters on status and ranges on date, which uses the first column and stops.

-- the query
WHERE status = 'pending' AND created_at > '2023-11-01'

-- idx (created_at, status): a range on the first column,
-- so the second cannot be used for filtering.
--   rows examined: 412,000

-- idx (status, created_at): equality first, then the range
--   rows examined: 1,204

The rule is equality columns before range columns, and it follows from how a composite index is ordered — once a range is scanned, the remaining columns are not sorted within it. This is the most common index mistake and it does not show up as a missing index in any tooling, because an index is being used. The row counts in EXPLAIN are what expose it.