BUG #4690: an select query is not using the index

The following bug has been logged online:

Bug reference: 4690
Logged by: vikas
Email address: vikas(dot)dubey(at)newgen(dot)co(dot)in
PostgreSQL version: PostgreSQL 7.3
Operating system: i686-pc-linux-gnu
Description: an select query is not using the index
Details:

hi

there is a table PDBFolder whose one column is Parentfolderindex on which an
index 'idx_folder_parentfolderindex' is created.

when i execute the following query

Select ParentFolderIndex From PDBFolder Where ParentFolderIndex In (
Select FolderId From FolderTree Where Leaf = 1::int4 )

it takes approximately 10 minutes and when i see the query execution plan of
this query it shows sequence scan on pdbfolder(which contain 50,000 rows).
can you suggest there is any way to use index idx_folder_parentfolderindex
in above query

regards
Vikas Dubey