Schema size in PostgreSQL DB

SELECT pg_size_pretty(sum(table_size)::bigint) as "disk_space"
FROM (
     SELECT n.nspname as schema_name,
            pg_relation_size(c.oid) as table_size
     FROM   pg_catalog.pg_class c  
         JOIN pg_catalog.pg_namespace n
             ON c.relnamespace = n.oid 
     WHERE n.nspname = $$SCHEMANAME
) t