Excellent thread!
I have a question: do really this kind of indexes make some improvements? I never thought about something like this, because my business rules never lead to this kind of (specific) situation. But if I'll ever be in this situation ... hm, how big should a column be to make such a index usefull, and how many rows in the table. I think is a dificult situation, to need indexes on a 4XXX chars length column.
Of course, if the dynamic is low, it is very usefull, but if the rows are often updated, I do not know.
Of course, I could be very wrong, but this is why I try to find out...
Any oppinions here?