[wiredfool]

I don't believe that the Primary Key affects the physical ordering of records in PostgreSQL.

Primary Key is implemented as a unique index coupled with a not null constraint. It's essentially syntactic sugar.

[mortehu]

To elaborate, even if you do call CLUSTER[1] to enforce a physical ordering, the primary key is not the default (there is no default).

1. http://www.postgresql.org/docs/9.5/static/sql-cluster.html