SELECT oid, 
       datname, 
       pg_size_pretty(pg_database_size(datname))as size_pretty, 
       pg_database_size(datname) as size,
       (SELECT pg_size_pretty (SUM( pg_database_size(datname))::bigint)  
          FROM pg_database)  AS total,
       ((pg_database_size(datname) / (SELECT SUM( pg_database_size(datname))  
                                       FROM pg_database) ) * 100)::numeric(6,3) AS pct
  FROM pg_database 
  ORDER BY datname;