MySQL natively doesn't support hash indexes. So, making a pseudo hash column and creating an index on a hash column needs some thinking.
And it seems to be widely understood that if the text field is long then hashes are worth the overhead they require. But how long should text be to start thinking about hash?
If I intend to use 128bit hash then how many characters threshold would be a sufficient minimum for a text column?
Edit Details listed in my another question maybe useful http://stackoverflow.com/questions/7403167/index-on-url-or-hashing-considering-ram