indexing with lower(...) -> queries are not optimised very well - Please Help

Hi,

I am using PostgreSQL 7.4, but I did have the same problem with the
last version.

I indexed the column word (defined as varchar(64)) using lower(word).
If I use the following query, everything is fine, the index is used and
the query is executed very quickly:

select * from token where lower(word) = 'saxophone';

However, with EXPLAIN you get the following:

                                        QUERY PLAN
------------------------------------------------------------------------
----------------
  Index Scan using word_lower_idx on token  (cost=0.00..98814.08
rows=25382 width=16)
    Index Cond: (lower((word)::text) = 'saxophone'::text)


I indexed the same column without the use of lower(...). Now

explain select * from token where word = 'saxophone';

results in:
                                  QUERY PLAN
------------------------------------------------------------------------
-----
  Index Scan using word_idx on token  (cost=0.00..6579.99 rows=1676
width=16)
    Index Cond: ((word)::text = 'saxophone'::text)

Please note the difference in the estimated cost! Why is there such a
huge difference? Both queries almost exactly need the same time to
execute (all instances of 'saxophone' in the table are lower-case (this
is a coincidence)).

The Problem is, if I use this query as part of a more complicated query
the optimiser chooses a *very* bad query plan.

Please help me. What am I doing wrong? I would appreciate any help an
this very much.

Regards,
Martin.