I'm curious what does SELECT COUNT(DISTINCT old_text), COUNT(*) FROM text; show on Wikipedia's database? On mine I get COUNT(DISTINCT old_text): 2913 COUNT(*): 3560 I.e., 1/7 of the rows are redundant.
Currently undos, so frequent on wikis, just blindly create a duplicate row instead of checking if the old one could be reused, https://bugzilla.wikimedia.org/show_bug.cgi?id=18333 . Maybe some hardware savings could even be achieved.