From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: Edmund Horner <ejrh00@gmail.com>,
        David Rowley <david.rowley@2ndquadrant.com>,
        tomas.vondra@2ndquadrant.com,
        PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Tid scan improvements
In-reply-to: <20181220223359.beygiujulocdbmas@alap3.anarazel.de>
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Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Thu, 20 Dec 2018 14:33:59 -0800"
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Date: Thu, 20 Dec 2018 18:06:41 -0500
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Andres Freund <andres@anarazel.de> writes:
> On 2018-12-20 17:21:07 -0500, Tom Lane wrote:
>> I'm having a hard time wrapping my mind around why you'd bother with
>> backwards TID scans.

> I've not followed this thread, but wouldn't that be quite useful to be
> able to move old tuples to free space earlier in the table?
> I've written multiple scripts that update the later pages in a table, to
> force reuse of earlier free pages (in my case by generating ctid = ANY()
> style queries with all possible tids for the last few pages, the most
> efficient way I could think of).

Sure, but wouldn't you now write those using something on the order of

      WHERE ctid >= '(cutoff_page_here, 1)'

?  I don't see that you'd want to write "ORDER BY ctid DESC LIMIT n"
because you wouldn't know what value of n to use to get all the
tuples on some-number-of-ending-pages.

			regards, tom lane