Словарь данных и системные каталоги — Запросы через information_schema

SELECT
    rc.constraint_name,
    kcu.column_name,
    ccu.table_schema AS foreign_schema,
    ccu.table_name AS foreign_table,
    ccu.column_name AS foreign_column
FROM information_schema.referential_constraints rc
JOIN information_schema.key_column_usage kcu
  ON rc.constraint_name = kcu.constraint_name
  AND rc.constraint_schema = kcu.constraint_schema
JOIN information_schema.constraint_column_usage ccu
  ON rc.unique_constraint_name = ccu.constraint_name
  AND rc.unique_constraint_schema = ccu.constraint_schema
WHERE kcu.table_schema = 'public'
  AND kcu.table_name = 'order_items';