Functional index performance question

Let's assume I have a table like so:

CREATE TABLE employees (
employeeid text not null,
name text not null
);

CREATE INDEX i_employees ON employees(lower(name));

Let's also assume that the lower() function is computationally
expensive. Now if I have a query like:

SELECT lower(name)
FROM employees
WHERE lower(name) = 'mike'

will PostgreSQL re-evaluate lower(name)? Is it necessary?

Mike Mascari
mascarm(at)mascari(dot)com