Fix explain for union and inheritance. Rename Append structure

Bruce Momjian <bruce@momjian.us>

Commit: 9e964f90fbfe40a71b5f0db5e7a8a76a01ea6573
Author: Bruce Momjian <bruce@momjian.us>
Date: 1998-07-15T14:54:39Z
Releases: 7.1.1
Fix explain for union and inheritance.  Rename Append structure
members to be clearer.  Fix cost computation for these.

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