I don't think this is really a bug, more of a feature request /
optimization. However, I do agree this is an important one. I don't see
offhand why we can't append the where list to our existing list of
important columns. There would be a few downsides, but none that would be
worse than going full replica identity or creating a new index. I'll see
about making a proof of concept patch and throwing it on -hackers. Will cc
you on that.
Cheers,
Greg