* Consider compressing indexes by storing key values duplicated in
  several rows as a single index entry
>
>   http://archives.postgresql.org/pgsql-hackers/2006-12/msg00341.php
>   http://archives.postgresql.org/pgsql-hackers/2007-02/msg01264.php
>   http://archives.postgresql.org/pgsql-hackers/2007-03/msg00465.php
>