Planner matching constants across tables in a join

I know this has been covered on one of the lists in the past, but I'm damned
if I can find the keywords to locate it.

If I join two tables with a comparison to a constant on one, why can't the
planner see that the comparison applies to both tables:

SELECT a.id FROM a JOIN b ON a.id=b.id WHERE a.id=1;

runs much slower than

SELECT a.id FROM a JOIN b ON a.id=b.id WHERE a.id=1 AND b.id=1;

It's not a real problem since it's easy to work around, but I was wondering
what the difficulties are for the planner in seeing that query 1 is the same
as query 2. Note that it doesn't seem related to JOIN forcing the planner's
hand, the same applies just using WHERE a.id=b.id

--
  Richard Huxton