On-disk Bitmap Index patch

Hi,

I have attached a patch for the on-disk bitmap index to the CVS Head. If
this can get in 8.2, that would be great. I still need to add the following
things:

(1) README file in src/backend/access/bitmap.
(2) Bitmap index documentation
(3) Hiding the internal btree

Please let me know if you find any problems. Any comments and suggestions
are welcome.

To create a bitmap index, use the following command:

create index <name> on <table_name> using bitmap(<att>);

Thanks,
Jie