We already do this for Gather, but it got overlooked for Gather Merge.

Amit Kapila, with review and minor revisions by Rushabh Lathia
and by me.

Discussion: http://postgr.es/m/CAA4eK1KUC5Uyu7qaifxrjpHxbSeoQh3yzwN3bThnJsmJcZ-qtA@mail.gmail.com