Date: Tue, 7 Apr 2020 23:25:57 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Andrew Gierth <andrew@tao11.riddles.org.uk>
Cc: Surafel Temesgen <surafel3000@gmail.com>,
	Tomas Vondra <tomas.vondra@2ndquadrant.com>,
	PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: FETCH FIRST clause WITH TIES option
Message-ID: <20200408032557.GA29002@alvherre.pgsql>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <87a73mn4cl.fsf@news-spur.riddles.org.uk>
User-Agent: Mutt/1.10.1 (2018-07-13)
Precedence: bulk

On 2020-Apr-08, Andrew Gierth wrote:

> >>>>> "Alvaro" == Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> 
>  Alvaro> It turns out that the SQL standard is much more limited in what
>  Alvaro> it will accept there. But our grammar (what we'll accept for
>  Alvaro> the ancient LIMIT clause) is very lenient -- it'll take just
>  Alvaro> any expression. I thought about reducing that to NumericOnly
>  Alvaro> for FETCH FIRST .. WITH TIES, but then I have to pick: 1)
>  Alvaro> gram.y fails to compile because of a reduce/reduce conflict, or
>  Alvaro> 2) also restricting FETCH FIRST .. ONLY to NumericOnly. Neither
>  Alvaro> of those seemed very palatable.
> 
> FETCH FIRST ... ONLY was made _too_ restrictive initially, such that it
> didn't allow parameters (which are allowed by the spec); see 1da162e1f.

Hmm, oh, I see.

> (That change didn't present a problem for ruleutils, because FETCH FIRST
> ... ONLY is output as a LIMIT clause instead.)

Right, I noticed that and kept it unchanged.

> This needs to be fixed in ruleutils, IMO, not by changing what the
> grammar accepts.

Fair.  I didn't change what the grammar accepts.  I ended up only
throwing an error in parse analysis when a bare NULL const is seen.
I guess we could fix ruleutils to add parens when NULL is seen, but
I'm not sure it's necessary or useful.  (LIMIT uses a null to represent
the LIMIT ALL case.)

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services