> BUT after clustering triples according to an index on att1:
>
>
>
> select relname, relpages from pg_class ;
> relname | relpages
> ---------------------------------+----------
> triples | 142 (8KB/buffer)
>
> 142 * 8 * 1024 = 1,163,264 Bytes
>
>
> Is there any compression or what?
Yes, there is:
http://www.postgresql.org/docs/8.0/interactive/storage-toast.html
Chris