If the table being attached had different attribute numbers than the
parent, the old code could incorrectly decide it needed to be scanned.

Amit Langote, reviewed by Ashutosh Bapat

Discussion: http://postgr.es/m/CA+TgmobexgbBr2+Utw-pOMw9uxaBRKRjMW_-mmzKKx9PejPLMg@mail.gmail.com