If you replace "especially when sorting by items in two directions" with "only when sorting by more than one item in different directions" above when talking ab

If you replace "especially when sorting by items in two directions" with "only when sorting by more than one item in different directions" above when talking about composite keys, then yes that's a factual statement of MySQL and something I believe Postgres supports. The same composite index in MySQL can be used to do ASC-ASC and DESC-DESC sorts but cannot be used to perform an ASC-DESC sort.