During looking stuff up for a discussion on -general, I realized that

During looking stuff up for a discussion on -general, I realized that
I'd placed the check for newly created matching pk rows for on update no
action earlier than it needed to be so that it'd check even when the key
values hadn't changed.  This patch moves it to after checking for NULLs
in the old row and comparing the values since the select's probably more
expensive.

Stephan Szabo