Count the number of tuples in the index honestly, instead of assuming
that it's the same as the number of tuples in the heap.  (It might be
different if the index is partial.)

Back-patch to all supported versions.

Tomas Vondra

Discussion: https://postgr.es/m/3b3d8eac-c709-0d25-088e-b98339a1b28a@2ndquadrant.com