Functions and Indexes

Hi folks,
I'm creating indexes for some tables, and I came across a doubt.

If a column appears in the WHERE clause (and so it should be placed in
index), in case it is "processed" in a function (see below), is it
possible to insert this function to further narrow down things?

Common index:
SELECT foo1, foo2 FROM bar WHERE foo1 = 2
CREATE index bar1_idx ON bar USING btree(foo1);

What about if query becomes
SELECT foo1, foo2 FROM bar WHERE (POSITION(foo1 IN 'blah blah') >0)

Second question: I 've seen contrasting opinions about putting JOIN
parameters (ON a.field1 = b.field2) in an index and I'd like to know
your thoughts.

Thanks,
Moreno.