[SQL] Cannot CREATE INDEX that contains a function

[Roberto Bertolusso]

I am testing Postgresql-7.0.3

I would like to have an index all in lowercase to speed up this select:
SELECT * FROM test WHERE lower(username) = 'max';

Please tell me what am I doing wrong:
In psql...
testdb=# CREATE TABLE test (username varchar(50));
CREATE
testdb=# CREATE UNIQUE INDEX test_index ON test (lower(username));
ERROR: DefineIndex: function 'lower(varchar)' does not exist

The same happens using upper, initcap,...

Thanks
Roberto Bertolusso