A couple of obvious reasons why it's not the same: a hashed index is not useful for anything but direct equality comparison. If you need any normalization like

A couple of obvious reasons why it's not the same: a hashed index is not useful for anything but direct equality comparison. If you need any normalization like lowercasing, you need to do that yourself. Next, you can't do collation/ordering using the index (and greater/less than comparisons), unless the hash function maintains ordering properties. By having an option of an expression index customised to your data, you can use it to get the fastest query performance.