Re: Extract numeric filed in JSONB more effectively

Adding this comment via the CF app so it isn't lost, while an improperly-interpreted-DKIM-headers issue is still preventing me from mailing directly to -hackers.

It was my view that the patch was getting close by the end of the last commitfest, but still contained a bit of a logic wart made necessary by a questionable choice of error message wording, such that in my view it would be better to determine whether a different error message would better conform to ISO SQL in the first place, and obviate the need for the logic wart.

There seemed to be some progress possible on that when petere had time to weigh in on the standard shortly after the last CF ended.

So, it would not have been my choice to assign RfC status before getting to a resolution on that.

Also, it is possible for a JsonbValue to hold a timestamp (as a result of a jsonpath evaluation, I don't think that can happen any other way), and if such a jsonpath evaluation were to be the source expression of a cast to SQL timestamp, that situation seems exactly analogous to the other situations being optimized here and would require only a few more lines in the exact pattern here introduced. While that could be called out of scope when this patch's title refers to "numeric field" specifically, it might be worth considering for completeness. The patch does, after all, handle boolean already, as well as numeric.