If the default port for v9.6 is 5433, why does the utility pg_isready still default to searching for 5432?
The Ubuntu packages use 5433 if you already have something (either a different packaged version, or an unpackaged system) running on 5432. That does not change the compiled-in default, but it will make a shell wrapper around pg_isready which causes it to look at the right port, as long as you invoked the correct pg_isready (the one that came with the package)
