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In-Reply-To: <b96a16a5-02de-4cc2-afb4-fbccba43eb54@postgrespro.ru>
From: Alexander Korotkov <aekorotkov@gmail.com>
Date: Fri, 31 Oct 2025 01:48:50 +0200
Message-ID: 
 <CAPpHfduQwwa4ytK-dGjRGN0fTWfpWNNysYuUKrOetsyGioBMfA@mail.gmail.com>
Subject: Re: Add SPLIT PARTITION/MERGE PARTITIONS commands
To: Dmitry Koval <d.koval@postgrespro.ru>
Cc: pgsql-hackers@lists.postgresql.org
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On Tue, Oct 28, 2025 at 1:12=E2=80=AFAM Dmitry Koval <d.koval@postgrespro.r=
u> wrote:
> 1.
>  > The second CommandCounterIncrement() is needed to make the renamed
>  > relation visible within our transaction.  Why do we need the first
>  >one?  I see tests pass without it.
>
> It's strange. If I comment the first "CommandCounterIncrement();", in blo=
ck
> -----------------------------------------------------------------------
> /*
>   * We must bump the command counter to make the split partition tuple
>   * visible for renaming.
>   */
> CommandCounterIncrement();
> /* Rename partition. */
> sprintf(tmpRelName, "split-%u-%X-tmp", RelationGetRelid(rel), ...);
> RenameRelationInternal(splitRelOid, tmpRelName, true, false);
> /*
>   * We must bump the command counter to make the split partition tuple
>   * visible after renaming.
>   */
> CommandCounterIncrement();
> -----------------------------------------------------------------------
> , I got the error "ERROR: tuple already updated by self" in the
> partition_split.sql test (Ubuntu). If I comment the second
> "CommandCounterIncrement();", I got the error "ERROR:  relation
> "sales_others" already exists" in the same test.

Sorry, actually it fails.  It appears that the first
CommandCounterIncrement() is needed to see the result of
detachPartitionTable().

> 2.
>  >The branch handling null value in the outer loop, uses null2 flag from
>  >the inner loop.  I think for the null value of the outer loop we still
>  >need to run inner loop to search for the matching null value.
>
> This code looks a little confusing, but it probably works correctly.
> This can be verified using two typical examples:
> -----------------------------------------------------------------------
> list1: (NULL, 1)
> list2: (2, NULL)
>
> (1) isnull1 =3D true, (2) "if (isnull1) lappend(NULL)"
> -----------------------------------------------------------------------
> list1: (1, NULL)
> list2: (NULL, 2)
>
> (1) isnull2 =3D true, (2) "if (isnull2) lappend(NULL)"
> -----------------------------------------------------------------------
> In both cases, we return from the function immediately after lappend.
> This works because we need to find exactly one repeating value.

How this function could handle this case?

list1: (NULL)
list2: whatever containing NULL

The outer loop will just quit after iterating the only element of
list1 without even iterating list2.

------
Regards,
Alexander Korotkov
Supabase