Gotcha, done and now have an implementation that passes all our regression tests.
Very cool! So the next step, I guess, is to address your original problem by cranking up the cost estimates for these functions --- have you tried that yet? In principle you should be able to do that and not have any bad planning side-effects, but this is all pretty new territory so maybe some problems remain to be ironed out.
BTW, if you'd like me to review the code you added for this, I'd be happy to do so. I've never looked at PostGIS' innards, but probably I can make sense of the code for this despite that.
I would be ecstatic for a review, I’m sure I’ve left a million loose threads dangling.
