Re: BUG #19355: Attempt to insert data unexpectedly during concurrent update

On Mon, 22 Dec 2025 at 14:51, Bh W <wangbihua(dot)cn(at)gmail(dot)com> wrote:
>
> The issue is that the MERGE INTO match condition is not updated.
> In the MATCHED path of MERGE INTO, when the target row satisfies the match condition and the condition itself has not changed, the system should still be able to handle concurrent updates to the same target row by relying on EvalPlanQual (EPQ) to refetch the latest version of the tuple, and then proceed with the intended update.
> However, in the current implementation, even though the concurrent update does not modify any columns relevant to the ON condition, the EPQ recheck unexpectedly results in a match condition failure, causing the update path that should remain MATCHED to be treated as NOT MATCHED.

I spent a little time looking at this, and managed to reduce the
reproducer test case down to this:

-- Setup
drop table if exists t1,t2;
create table t1(a int primary key, b int);
create table t2(a int, b int);

insert into t1 values(1,0),(2,0);
insert into t2 values(1,1),(2,2);

-- Session 1
begin;
update t1 set b = b+1;

-- Session 2
merge into t1 using (values(1,1),(2,2)) as t3(a,b) on (t1.a = t3.a)
when matched then
update set b = t1.b + 1
when not matched then
insert (a,b) values (1,1);

-- Session 1
commit;

This works fine in PG17, but fails with a PK violation in PG18.
Git-bisecting points to this commit:

cbc127917e04a978a788b8bc9d35a70244396d5b is the first bad commit
commit cbc127917e04a978a788b8bc9d35a70244396d5b
Author: Amit Langote <amitlan(at)postgresql(dot)org>
Date: Fri Feb 7 17:15:09 2025 +0900

Track unpruned relids to avoid processing pruned relations

Doing a little more debugging, it looks like the problem might be this
change in InitPlan():

- /* ignore "parent" rowmarks; they are irrelevant at runtime */
- if (rc->isParent)
+ /*
+ * Ignore "parent" rowmarks, because they are irrelevant at
+ * runtime. Also ignore the rowmarks belonging to child tables
+ * that have been pruned in ExecDoInitialPruning().
+ */
+ if (rc->isParent ||
+ !bms_is_member(rc->rti, estate->es_unpruned_relids))
continue;

which seems to cause it to incorrectly skip a rowmark, which I suspect
is what is causing EvalPlanQual() to return the wrong result.

Regards,
Dean