I wrote:
> I thought of a better way, as attached.
Pushed. Ben, could you confirm that the committed patch fixes your
original use-case? The 9.5 version of the patch is at
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=13d2ed921035f2d88adf87d796373e920bdd56ee
regards, tom lane