Simple query not using index: why?

[William Garrison <postgres@xxxxxxxxxxxx>]

I am looking for records with duplicate keys, so I am running this query:

SELECT
   fileid, COUNT(*)
FROM
   file
GROUP BY
   fileid
HAVING
   COUNT(*)>1

The table has an index on fileid (non-unique index) so I am surprised 
that postgres is doing a table scan.  This database is >15GB, and there 
are a number of fairly large string columns in the table.  I am very 
surprised that scanning the index is not faster than scanning the 
table.  Any thoughts on that?  Is scanning the table faster than 
scanning the index?  Is there a reason that it needs anything other than 
the index?