Remember when MySql for Rails on Mac was easier than PostgreSQL? (02 dec 16)
This, and many other pages in stackoverflow.com, were needed to solve the problem of even starting MySql in the latest version of the Mac O/S (Ver 14.14 Distrib 5.7.16, for osx10.12 x86_64).
$ sudo chown -R _mysql:mysql /usr/local/var/mysql
For PostgreSQL, you just go to this page, and download the one-click Mac installer: https://www.postgresql.org/download/macosx/