I've some specific use case for certain default wordrpess tables, which I need to add a few indexes to certain columns to improve performance. I'm wondering will adding these indexes break future Wordpress update when they choose to add index to the same column?
HASH index shouldn't break anything, and I imagine that is what you want. At worst you could end up with multiple indexes, or less than optimal ones, which might negatively effect performance. I assume that if you are going to do this you will be evaluating the change to see if it actually does help, rather than hurt.
I guess you could run into trouble with a
UNIQUE index, since WordPress does use a couple of those.