select sum(a.mb) from (select c.relname, isdirty,  count(*)*8/1024.0 as mb
 from pg_class c inner join pg_buffercache b on (b.relfilenode = c.relfilenode)
 inner join pg_database d on (b.reldatabase=d.oid and d.datname = current_database())
group by c.relname, isdirty order by 3 desc) a