clause with an alias is a <subquery> and therefore hides table references
appearing within it, according to the spec.  This is the same as the
preliminary patch I posted to pgsql-patches yesterday, plus some really
grotty code in ruleutils.c to reverse-list a query tree with the correct
alias name depending on context.  I'd rather not have done that, but unless
we want to force another initdb for 7.1, there's no other way for now.