PG won't use index on ORDER BY <expression>

Hi all!

I have problems getting PG to use an index when sorting.
I have a simple table

create table person(
id serial primary key,
firstname varchar,
lastname varchar
);

I create an index:
CREATE INDEX person_lowerfullname_idx ON
person((lower(COALESCE(firstname, '')) || lower(COALESCE(lastname, '')))
varchar_pattern_ops);

And this query refuses to use that index:
select id from person order by (lower(COALESCE(firstname, '')) ||
lower(COALESCE(lastname, ''))) ASC limit 1;

If I add an index:
CREATE INDEX person_lowerfirstname_idx ON person(lower(firstname));

The following query will use that index for sorting and cut-off:
select id from person order by (lower(firstname) ) ASC limit 1;

Any hints or explaination on why the "concat-index" won't be used?

PS: I have tried to issue a "set enable_seqscan to off;" to ensure that it
will use an index if one appropriate exists

--
Andreas Joseph Krogh <andreak(at)officenet(dot)no>
Senior Software Developer / Manager
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