Re: BUG #19802: MERGE into a partitioned table fails with "SubPlan found with no parent plan"

PG Bug reporting form <noreply(at)postgresql(dot)org> 于2026年10月29日周四 00:25写道：
> MERGE into a partitioned table fails with an internal error when the source
> is a
> subquery that is not pulled up, an UPDATE SET clause contains a scalar
> subquery
> that references the source, and the statement has both WHEN MATCHED UPDATE
> and
> WHEN NOT MATCHED INSERT:
>
> CREATE TABLE mp (a int, b text) PARTITION BY RANGE (a);
> CREATE TABLE mp1 PARTITION OF mp FOR VALUES FROM (0) TO (10);
> MERGE INTO mp t
> USING (SELECT true AS c OFFSET 0) s ON t.a IS NULL
> WHEN MATCHED THEN UPDATE SET b = (SELECT NULL WHERE s.c)
> WHEN NOT MATCHED THEN INSERT VALUES (1, 'x');
>
> ERROR: SubPlan found with no parent plan
>
Hi,

Thanks for the report and the nicely reduced test case.

The problem is in ExecInitPartitionInfo(). When tuple routing creates a
ResultRelInfo for a partition during MERGE, it also builds
partition-specific state for the MERGE actions. For an UPDATE action, it
calls ExecBuildUpdateProjection() with a NULL parent PlanState.

If the UPDATE target list contains a SubPlan, ExecInitSubPlanExpr() needs
the parent PlanState to initialize the SubPlan and add the resulting
SubPlanState to the parent's subPlan list. Because no parent was supplied,
executor initialization fails with:

ERROR: SubPlan found with no parent plan

The corresponding code in ExecInitMerge() passes &mtstate->ps when
building the UPDATE projection. There is also another call to
ExecBuildUpdateProjection() earlier in ExecInitPartitionInfo(), for
ON CONFLICT DO UPDATE, that already passes &mtstate->ps. Only the
partition-specific MERGE UPDATE path passes NULL.

Passing the same parent in that path fixes the problem:

- NULL);
+ &mtstate->ps);

Attached is a patch with the fix. I have run the regression tests successfully.

--
Thanks,
Tender Wang