PostgreSQL 11 introduces compress method for SP-GiST opclasses.  That
was mistakenly interpreted as compression support for SP-GiST while
actually that allows lossy representation of leaf keys.

Author: Alexander Korotkov, based on proposal by Darafei Praliaskouski
Discussion: https://postgr.es/m/CAC8Q8tKbYmNdiyWr7hE4GfMY4fbqHKkFziKgrUuWHH6HJQs3og%40mail.gmail.com