How to get started hacking on pgsql

I have an idea for what I think may be a very simple optimization for postgres
to make. I would like to try my hand at implementing it, but the last time I
tried I apparently started off in the wrong direction.

In the following query, the sort step is completely unnecessary. The order is
already guaranteed by the index:

test=# create table test (a integer,b integer);
CREATE TABLE
test=# create index test_i on test(a,b);
CREATE INDEX
test=# explain select * from test where a=1 order by b;
QUERY PLAN
-------------------------------------------------------------------------
Sort (cost=5.95..5.96 rows=6 width=8)
Sort Key: b
-> Index Scan using test_i on test (cost=0.00..5.87 rows=6 width=8)
Index Cond: (a = 1)
(4 rows)

At what point in the process would it make sense to check for this?
Where should I be looking in the code?

--
greg