I have to say thanks to IBM for PH11539/UI63324.
We have a Hibernate application that has several queryies with a mixture of inner and outer joins (many of the outer joins that actually should have originally been written as left or inner joins). When we initially went to version 12, several of these got considerably better but some were left with their former bad performance (even though they could be manually rewritten in a better way). Once we installed PH11539/UI63324, we saw a half dozen or so queries that are now running 1000x faster! The CPU savings was considerable!
It will not improve every query in your shop, but the queries in the sweet spot for this are really sweet!