This is NOT a "save only what has been modified patch". All columns of every record will still be updated on a save. This patch merely checks to see if anything has been modified in the record, and if not, the record will not be saved.
This patch was inspired by a particularly nasty issue that arises when you have after_create hooks that (as a side effect) cause the record to be saved again. This causes the "@new_record_before_save" flag to be reset, and means that subsequent after_create/after_save hooks will misinterpret the record as being "not new", which can wreak havoc in (among other places) the after_save hook for habtm collections.
I did some naive benchmarking on this patch (on my 1.5ghz Powerbook, using MySQL). Without the patch, 1000 finds took 6.04s. When using Object#hash to snapshot the attributes hash, it took anywhere from 6.04s to 6.06s for 1000 finds. (The problem with Object#hash, though, is the danger of collisions, which can result in false negatives in Base#dirty? and result in records not being saved when they ought to.) Using MD5 hashing, 1000 finds took about 6.11s.
I did not benchmark #save calls, though they will suffer the same performance penalty (because the digest has to be computed in order to determine whether it differs from the original), but save is not typically called as often as find.
