From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
cc: Surafel Temesgen <surafel3000@gmail.com>, Erik Rijkers <er@xs4all.nl>,
        Thomas Munro <thomas.munro@gmail.com>,
        Tomas Vondra <tomas.vondra@2ndquadrant.com>,
        David Steele <david@pgmasters.net>,
        Michael Paquier <michael@paquier.xyz>,
        Robert Haas <robertmhaas@gmail.com>,
        Andrew Gierth <andrew@tao11.riddles.org.uk>,
        PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: FETCH FIRST clause WITH TIES option
In-reply-to: <20190924154827.GA26751@alvherre.pgsql>
References: <20190924154827.GA26751@alvherre.pgsql>
Comments: In-reply-to Alvaro Herrera <alvherre@2ndquadrant.com>
	message dated "Tue, 24 Sep 2019 12:48:27 -0300"
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Date: Tue, 24 Sep 2019 18:57:53 -0400
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Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> create table w (a point);

> # select * from w order by a fetch first 2 rows with ties;
> ERROR:  could not identify an ordering operator for type point
> LINE 1: select * from w order by a fetch first 2 rows with ties;
>                                   ^
> HINT:  Use an explicit ordering operator or modify the query.

> I'm not sure that the HINT is useful here.

That's not new to this patch, HEAD does the same:

regression=# create table w (a point);
CREATE TABLE
regression=# select * from w order by a ;
ERROR:  could not identify an ordering operator for type point
LINE 1: select * from w order by a ;
                                 ^
HINT:  Use an explicit ordering operator or modify the query.

It is a meaningful hint IMO, since (in theory) you could add
something like "USING <<" to the ORDER BY to specify a
particular ordering operator.  The fact that no suitable
operator is actually available in core doesn't seem like
a reason not to give the hint.

			regards, tom lane