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From: Andrew Dunstan <[email protected]>
To: Tom Lane <[email protected]>
To: 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: Mon, 27 Nov 2023 16:09:10 -0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>


On 2023-11-27 Mo 15:34, Tom Lane wrote:
> Alvaro Herrera <[email protected]> writes:
>> (TBH I don't think the added comments really explain the problems fully.
>> That's most likely because I don't actually understand what the problems
>> are.)
> The actual problem is that nobody has applied a cluestick to the SQL
> committee about writing an unambiguous grammar :-(.  But I digress.
>
> I don't like the existing coding for more reasons than just
> underdocumentation.  Global assignment of precedence is a really,
> really dangerous tool for solving ambiguous-grammar problems, because
> it can mask problems unrelated to the one you think you are solving:
> basically, it eliminates bison's complaints about grammar ambiguities
> related to the token you mark.  (Commits 12b716457 and 28a61fc6c are
> relevant here.)  Attaching precedence to individual productions is
> far safer, because it won't have any effect that extends beyond that
> production.  You still need a precedence attached to the lookahead
> token; but I think we should try very hard to not assign a precedence
> different from IDENT's to any unreserved keywords.
>
> After a bit of fooling around I found a patch that seems to meet
> that criterion; attached.
>
> 			



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.


cheers


andrew

--
Andrew Dunstan
EDB: https://www.enterprisedb.com

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