The accepted answer misses an important detail, as an index also works backwards. So if you have ASC index, a DESC query uses that index without having to scan the table.

So to extend the answer:

You might wonder why bother providing all four options, when two options together with the possibility of backward scan would cover all the variants of ORDER BY . In single-column indexes the options are indeed redundant, but in multicolumn indexes they can be useful. Consider a two-column index on (x, y) : this can satisfy ORDER BY x, y if we scan forward, or ORDER BY x DESC, y DESC if we scan backward. But it might be that the application frequently needs to use ORDER BY x ASC, y DESC . There is no way to get that ordering from a plain index, but it is possible if the index is defined as (x ASC, y DESC) or (x DESC, y ASC) .