Andrey Lepikhov <a.lepik...@postgrespro.ru> writes: > It is possible that custom_scan_tlist is designed too nontrivially, and > it is possible that it needs some comments describing in more detail how > to use it.
I totally buy the argument that the custom scan stuff is underdocumented :-(. FWIW, if we did have a use-case showing that somebody would like to make custom scans that can't project, the way to do it would be to add a flag bit showing whether a particular CustomPath/CustomScan could project or not. Not to assume that they all can't. This wouldn't be that much code really, but I'd still like to see a plausible use-case before adding it, because it'd be a small API break for existing CustomPath providers. regards, tom lane