The definitions of tables are held within PostgreSQL, and can be accessed using the Information Schema catalog views.
There might be valid reasons why the definitions differ. We've excluded PostgreSQL's own internal tables because there are similar names between the two catalogs: PostgreSQL's implementation of the SQL Standard Information Schema and PostgreSQL's own internal pg_catalog schema.
Those queries are fairly complex. In fact, there is even more complexity that we could add to those queries to compare all sorts of things such as default values or constraints. The basic idea can be extended in various directions from here.
