Also, rename it to plan_union_chidren, so the old name wasn't
very descriptive.  This results in a small net reduction in code,
seems at least to me to be easier to understand, and saves
space on the process stack.

Patch by me, reviewed and tested by Ashutosh Bapat and Rajkumar
Raghuwanshi.

Discussion: http://postgr.es/m/CA+TgmoaLRAOqHmMZx=ESM3VDEPceg+-XXZsRXQ8GtFJO_zbMSw@mail.gmail.com