From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Andrew Dunstan <andrew(at)dunslane(dot)net> |
Cc: | Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, Bruce Momjian <bruce(at)momjian(dot)us>, Shay Rojansky <roji(at)roji(dot)org>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Missing docs on AT TIME ZONE precedence? |
Date: | 2023-11-28 15:27:26 |
Message-ID: | 4146714.1701185246@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Andrew Dunstan <andrew(at)dunslane(dot)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
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