From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
cc: Andres Freund <andres@anarazel.de>, Mark Dilger <hornschnorter@gmail.com>,
        Regina Obe <lr@pcorp.us>,
        PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: PostgreSQL 10 changes in exclusion constraints - did something
 change? CASE WHEN behavior oddity
In-reply-to: <20170610180419.2gref4t7oipjcri5@alvherre.pgsql>
References: <20170610180419.2gref4t7oipjcri5@alvherre.pgsql>
Comments: In-reply-to Alvaro Herrera <alvherre@2ndquadrant.com>
	message dated "Sat, 10 Jun 2017 15:04:19 -0300"
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Date: Sun, 11 Jun 2017 14:18:41 -0400
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Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> Interesting stuff.  Here's a small recommendation for a couple of those
> new messages.

Hm.  I don't object to folding those two messages into one, but now that
I look at it, the text needs some more work anyway, perhaps.  What we're
actually checking is not so much whether the IS DISTINCT FROM construct
returns a set as whether the underlying equality operator does.  If we
want to be pedantic about it, we'd end up writing something like

	"equality operator used by %s must not return a set"

But perhaps it's okay to fuzz the distinction and just write

	"%s must not return a set"

You could justify that on the reasoning that if we were to allow this
then an underlying "=" that returned a set would presumably cause
IS DISTINCT FROM or NULLIF to also return a set.

I'm kind of thinking that the second wording is preferable, but there's
room to argue that the first is more precise.  Opinions?

			regards, tom lane


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