Why doesn't any wiki software have a patch function?

I'm trying to make a versioning system for database entries. I thought of fully storing only the last version of an entry and using reverse-deltas for storing the changes.

W开发者_运维技巧hile there is a lot of wiki software that has a diff function, i found none that has a patch function. How do they revert to an older version of an entry?

I'm asking this because I looked at wiki software to get an ideea how to do this, and it puzzled me even more.

How do they revert to an older version of an entry?

They (the ones I've seen) don't store diffs, they just store the entire copy of the entry every time. When you view a diff between two versions, it pulls both entire copies out of the database, does the diff on the fly, and shows you that