Functional Index

When building an index over the function f() of a
field a, what does postgres store internally in the
index pages: a, or f(a)?

It could be done either way, but it makes a difference
in some tradeoffs that affect me. In particular it
affects space usage, and it affects whether

SELECT a from T WHERE f(a) < foobar

can be answered using purely the index without
touching the underlying heapfile. I couldn't quite
tell what postgres actually does from the docs at

http://postgresql.crimelabs.net/users-lounge/docs/7.1/postgres/indices-functional.html

Thanks in advance
Taher

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