pg_relation_size on partitioned table

Hi, hackers

When I try to get total size of partition tables though partitioned table
name using pg_relation_size(), it always returns zero.  I can use the
following SQL to get total size of partition tables, however, it is a bit
complex.

    SELECT
        pg_size_pretty(sum(pg_relation_size(i.inhrelid)))
    FROM
        pg_class c JOIN pg_inherits i ON c.oid = i.inhparent
    WHERE
        relname = 'parent';

Could we provide a function to get the total size of the partition table
though the partitioned table name?  Maybe we can extend
the pg_relation_size() to get the total size of partition tables through
the partitioned table name.

--
Regrads,
Japin Li.
ChengDu WenWu Information Technology Co.,Ltd.