find index key.sql

       SELECT
          pg_attribute.attname,
          format_type(pg_attribute.atttypid, pg_attribute.atttypmod)
        FROM
          pg_index, pg_class, pg_attribute, pg_namespace
       WHERE
          nspname = 'public' AND
          relname = 'orders' AND
          indrelid = pg_class.oid AND
          pg_class.relnamespace = pg_namespace.oid AND
          pg_attribute.attrelid = pg_class.oid AND
          pg_attribute.attnum = any(pg_index.indkey) AND
          indisprimary;
attname | format_type
---------+-------------
 id      | bigint