Multicolumn hash tables space complexity.

Hi,
I hope its not off topic.
I have an algorithm to implement where it needs
to hold the tuples (complete tuples - all columns)
either in a b+tree or a hashtable.
I am concerned about the space such an index will require.
What is the difference in percentages from the size of all
the data not indexed vs. holding it in a b+tree or a hashtable.

also what is the difference when those indices are half-full?

e.g. I understand a b+tree half full in the worse case can take
space as if it was full. I am guessing it's the same with hashtables.

I understand that the HASH indice in postgresql does not support
a multicolumn. What does it take to upgrade it to do that?

Regards,
tzahi.

WARNING TO SPAMMERS: see at
http://members.lycos.co.uk/my2nis/spamwarning.html