Compare two similar Postgres databases for differences I occasionally download publicly available data sets in the form of Postgres dBs.   These datasets are updated/modified/expanded over time by the repository host.  

Is there a Postgres command or tool (ideally FOSS) that can show the differences between an OLD and NEW Postgres database ?  (A working assumption is that 95% of the entries are unchanged and that the tables and relationships also do not change).
Have a look at the Stack Overflow Q &amp; A, How to check difference between two databases in PostgreSQL?; in particular Another PostgreSQL Diff Tool (apgdiff) (schema comparison only). Apgdiff is recommended by Alexander Kuznetsov - a published database book author and bit of a guru.

Maybe also look at Liquibase or Compare Database Table Data.

There are also commercial products - e.g. Altova Database Differencing with DiffDog