Pushed. I added one comment to explain xmin_frozen also, which otherwise seemed a bit mysterious. I did not backpatch, though, so 9.6-11 are a bit different, but I'm not sure it's a good idea at this point, though it should be pretty innocuous.
-- Álvaro Herrera https://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
