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From: Alvaro Herrera <[email protected]>
To: Dean Rasheed <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Subject: Re: Incorrect command tag row count for MERGE with a cross-partition update
Date: Tue, 21 Feb 2023 10:34:11 +0100
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAEZATCWRMG7XX2QEsVL1LswmNo2d_YG8tKTLkpD3=Lp644S7rg@mail.gmail.com>

On 2023-Feb-20, Dean Rasheed wrote:

> Playing around with MERGE some more, I noticed that the command tag
> row count is wrong if it does a cross-partition update:
> 
> CREATE TABLE target (a int, b int) PARTITION BY LIST (b);
> CREATE TABLE target_p1 PARTITION OF target FOR VALUES IN (1);
> CREATE TABLE target_p2 PARTITION OF target FOR VALUES IN (2);
> INSERT INTO target VALUES (1,1);
> 
> MERGE INTO target t USING (VALUES (1)) v(a) ON t.a = v.a
>   WHEN MATCHED THEN UPDATE SET b = 2;
> 
> which returns "MERGE 2" when only 1 row was updated, because
> ExecUpdateAct() will update estate->es_processed for a cross-partition
> update (but not for a normal update), and then ExecMergeMatched() will
> update it again.

Hah.

> I think the best fix is to have ExecMergeMatched() pass canSetTag =
> false to ExecUpdateAct(), so that ExecMergeMatched() takes
> responsibility for updating estate->es_processed in all cases.

Sounds sensible.


-- 
Álvaro Herrera        Breisgau, Deutschland  —  https://www.EnterpriseDB.com/
"Las mujeres son como hondas:  mientras más resistencia tienen,
 más lejos puedes llegar con ellas"  (Jonas Nightingale, Leap of Faith)

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