From: Antonin Houska <ah@cybertec.at>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
cc: Mihail Nikalayeu <mihailnikalayeu@gmail.com>,
    Srinath Reddy Sadipiralla <srinath2133@gmail.com>,
    Matthias van de Meent <boekewurm+postgres@gmail.com>,
    Pg Hackers <pgsql-hackers@lists.postgresql.org>,
    Robert Treat <rob@xzilla.net>
Subject: Re: Adding REPACK [concurrently]
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Comments: In-reply-to Alvaro Herrera <alvherre@alvh.no-ip.org>
   message dated "Tue, 24 Mar 2026 23:32:17 +0100."
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Date: Wed, 25 Mar 2026 10:00:49 +0100
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Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:

> - 0008 to 0010 are as posted by Antonin; they are unchanged, except for
>   fixes for the problems pointed out by Mihail.  Antonin, I would
>   appreciate it if you want to change the "reform" bit in 0007 as
>   discussed.

I've taken a look, but not sure if the tuple slots help here. In
heapam_relation_copy_for_cluster(), both table_scan_getnextslot() and
index_getnext_slot() call ExecStoreBufferHeapTuple() ->
tts_buffer_heap_store_tuple(), which AFAICS do not deform the tuple. Then
ExecFetchSlotHeapTuple() is used to retrieve the tuple, but again, the
underlying slot (TTSOpsBufferHeapTuple) handles it by copying rather than
deforming / forming. Thus I think the explicit "reforming" currently does not
add any performance overhead.

Of course, we can still use the slots, and do the following: 1) enforce tuple
deforming (by calling slot_getallattrs()), 2) set the dropped attributes to
NULL, 3) use ExecStoreVirtualTuple() to store the tuple into another slot and
4) get the heap tuple from the other slot. Should I do that? I'm asking
because I wasn't sure if you're concerned about performance or coding (or
both).


Whatever approach we take, I see two more opportunities for better
performance:

1. Do the "reforming" only if there are some dropped columns. (AFAICS even the
old CLUSTER / VACUUM FULL did not check this.)

2. Get rid of the values of dropped columns earlier, so that the dropped
values are not put into the tuplestore (likewise, I think that CLUSTER /
VACUUM FULL did not care.)


Besides that, I think that heap_form_tuple() should set the values of dropped
columns to NULL by default, or do I miss something? Anyway, this should be
addressed by a separate patch.

-- 
Antonin Houska
Web: https://www.cybertec-postgresql.com