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.
