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From: Tom Lane <[email protected]>
To: Andrew Dunstan <[email protected]>
Cc: Alvaro Herrera <[email protected]>
Cc: Bruce Momjian <[email protected]>
Cc: Shay Rojansky <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: Missing docs on AT TIME ZONE precedence?
Date: Tue, 28 Nov 2023 10:27:26 -0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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Andrew Dunstan <[email protected]> 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

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