Finding the Size on disk of each table in a postgres (PostGreSQL) database:

Postgres (PostGreSQL) refers to tables and indexes as relationships or relations.  This is helpful to know when reading the command:

SELECT nspname || '.' || relname AS "relation",
    pg_size_pretty(pg_relation_size(C.oid)) AS "size"
  FROM pg_class C
  LEFT JOIN pg_namespace N ON (N.oid = C.relnamespace)
  WHERE nspname NOT IN ('pg_catalog', 'information_schema')
  ORDER BY pg_relation_size(C.oid) DESC
  LIMIT 20;

This will list the top 20 tables or index structures (or any other large relationship) in your database.