From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andrew Dunstan <andrew@dunslane.net>
cc: Alvaro Herrera <alvherre@alvh.no-ip.org>, Bruce Momjian <bruce@momjian.us>,
        Shay Rojansky <roji@roji.org>,
        pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Missing docs on AT TIME ZONE precedence?
In-reply-to: <9df52975-686d-c945-36bd-559c930f2dc7@dunslane.net>
References: <202311271732.hn567fcmvy37@alvherre.pgsql> <4006949.1701117264@sss.pgh.pa.us> <9df52975-686d-c945-36bd-559c930f2dc7@dunslane.net>
Comments: In-reply-to Andrew Dunstan <andrew@dunslane.net>
	message dated "Mon, 27 Nov 2023 16:09:10 -0500"
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Date: Tue, 28 Nov 2023 10:27:26 -0500
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Precedence: bulk

Andrew Dunstan <andrew@dunslane.net> writes:
> Looks good. Perhaps the comments above the UNBOUNDED precedence setting 
> (esp. the first paragraph) need strengthening, with a stern injunction 
> to avoid different precedence for non-reserved keywords if at all possible.

OK.  How about rewriting that first para like this?

 * Sometimes it is necessary to assign precedence to keywords that are not
 * really part of the operator hierarchy, in order to resolve grammar
 * ambiguities.  It's best to avoid doing so whenever possible, because such
 * assignments have global effect and may hide ambiguities besides the one
 * you intended to solve.  (Attaching a precedence to a single rule with
 * %prec is far safer and should be preferred.)  If you must give precedence
 * to a new keyword, try very hard to give it the same precedence as IDENT.
 * If the keyword has IDENT's precedence then it clearly acts the same as
 * non-keywords and other similar keywords, thus reducing the risk of
 * unexpected precedence effects.
 * 
 * We used to need to assign IDENT an explicit precedence just less than Op,
 * to support target_el without AS.  While that's not really necessary since
 * we removed postfix operators, we continue to do so because it provides a
 * reference point for a precedence level that we can assign to other
 * keywords that lack a natural precedence level.

			regards, tom lane