Hash index is only useful for exact matches, a full table scan would be required for any other kinds of queries. That is why most database engines have settled

Hash index is only useful for exact matches, a full table scan would be required for any other kinds of queries. That is why most database engines have settled with B+ tree. Regardless the type of index, field length still matters if you care about performance. Smaller fields and smaller indices means you can fits more things (the working set) in RAM and less I/O when you have to hit disks.