On-disk bitmap index patch

Hi,

I have posted a patch to the CVS head for on-disk bitmap index to
pgsql-patches. If this can get in 8.2, that would be great. Any comments and
suggestions are welcome.

I still need to add several items:

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

Also, I have disabled the multi-column index support because there is a
known problem. Assume that there is a bitmap index on a and b. When a query
predicate has only a, the current code may generate a wrong result. That's
because the current code assumes that b is null. The ultimate problem is
because the search code only handles one bitmap vector now. I need a fix to
support manipulating multiple bitmap vectors.

If you find any other problems, please let me know.

Thanks,
Jie