Feedback-ID: ie3de48e3:Fastmail
Date: Thu, 10 Sep 2026 18:05:05 +0200
From: Alvaro Herrera <alvherre@kurilemu.de>
To: Chao Li <li.evan.chao@gmail.com>
Cc: Antonin Houska <ah@cybertec.at>,
	Kirill Reshke <reshkekirill@gmail.com>,
 PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: Fix REPACK with WITHOUT OVERLAPS replica identity indexes
Message-ID: <aqLTreG8ePby8ud4@alvherre.pgsql>
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On 2026-May-13, Chao Li wrote:

> After some investigation, I think I see the mismatch:
> 
> * get_relation_info(): non-ordered GiST cannot provide sort order.
> That is expected.
> * copy_table_data() only uses plan_cluster_use_sort() for btree. For
> any other clusterable index, it sets use_sort = false and does a raw
> index scan.
> * The docs say REPACK can re-sort using index scan “if the index is a
> b-tree” or seqscan+sort, which does not describe what the code
> actually does for GiST.
> 
> I am not sure whether we should change the behavior in PG19. Alvaro
> may have a better idea about that. But I agree that we can at least
> clarify the code comment and documentation. The attached patch
> attempts to do that.

I am sure that we should not change the behavior for 19.  We could
change it in 20, perhaps.  But I think the really gross problem is that
somehow I made the documentation say exactly the opposite of truth, in
the rework for commit ac58465e0618.  I fixed that just now with by
pushing 07aef52eba3c, which includes your (Chao's) proposed change; but
also, while reading and rereading the existing docs and your patch, it
struck me that the preexisting text was not super nice either: there was
one introductory text, then some technical details, then some more
discussion, then more technicalities.  That was weird and probably just
historical accident.  I reordered the text and made some other wording
changes.  I *think* result is reasonable, or at least it contains no
egregious errors; but maybe it could be further improved, if anybody is
interested.

-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/
"El Maquinismo fue proscrito so pena de cosquilleo hasta la muerte"
(Ijon Tichy en Viajes, Stanislaw Lem)