From: Tom Lane <tgl@sss.pgh.pa.us>
To: Heikki Linnakangas <hlinnaka@iki.fi>
cc: Xiaoran Wang <fanfuxiaoran@gmail.com>,
        pgsql-hackers <pgsql-hackers@postgresql.org>,
        Andrew Kane <andrew@ankane.org>,
        Matthias van de Meent <boekewurm+postgres@gmail.com>
Subject: Re: Avoid computing ORDER BY junk columns unnecessarily
In-reply-to: <02c142a8-051d-4252-a3a5-9d87be1c6e6b@iki.fi>
References: <2ca5865b-4693-40e5-8f78-f3b45d5378fb@iki.fi>
 <CAGjhLkPyROkuJYpKfQxax3fBEB_0vLN_CuQEPcfB5A8+GD85dg@mail.gmail.com>
 <dd7eed69-4e99-4da2-87a8-2a94b35309fa@iki.fi>
 <1052850.1703258655@sss.pgh.pa.us>
 <02c142a8-051d-4252-a3a5-9d87be1c6e6b@iki.fi>
Comments: In-reply-to Heikki Linnakangas <hlinnaka@iki.fi>
	message dated "Fri, 22 Dec 2023 18:55:00 +0200"
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <1067707.1703266312.1@sss.pgh.pa.us>
Date: Fri, 22 Dec 2023 12:31:52 -0500
Message-ID: <1067708.1703266312@sss.pgh.pa.us>
Archived-At: 
 <https://www.postgresql.org/message-id/1067708.1703266312%40sss.pgh.pa.us>
Precedence: bulk

Heikki Linnakangas <hlinnaka@iki.fi> writes:
> On 22/12/2023 17:24, Tom Lane wrote:
>> How much of your patchset still makes sense if we assume that we
>> can always extract the ORDER BY column values from the index?

> That would make it much less interesting. But I don't think that's a 
> good assumption. Especially in the kNN case, the ORDER BY value would 
> not be stored in the index. Most likely the index needs to calculate it 
> in some form, but it might take shortcuts like avoiding the sqrt().

Yeah, fair point.  I'll try to take a look at your patchset after
the holidays.

			regards, tom lane