Yeah, grouped views are pretty flaky --- they work OK in isolation,
but not when you combine them with others. In general the current
implementation of views can't do anything you couldn't do by writing
out the complete computation as a single SQL query. Here that can't
work because you'd need to group/aggregate over different sets of
tuples for the two views. The rewriter not only can't handle this,
but is not bright enough to realize that it's emitting an incorrect
translation.
We hope to fix this and related problems for 7.2. It will take a
wholesale redesign of querytrees to fix all the problems ...
regards, tom lane