Here is an updated patch with some merge conflicts resolved, to keep it
fresh. It's still pending in the commit fest from last time.
My focus right now is to work on the "psql - add SHOW_ALL_RESULTS
option" patch (https://commitfest.postgresql.org/33/2096/) first, which
is pretty much a prerequisite to this one. The attached patch set
contains a minimal variant of that patch in 0001 and 0002, just to get
this working, but disregard those for the purposes of code review.
The 0003 patch contains comprehensive documentation and test changes
that can explain the feature in its current form.