Incorrect command tag row count for MERGE with a cross-partition update

[Dean Rasheed]

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.

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.

Regards,
Dean

diff --git a/src/backend/executor/nodeModifyTable.c b/src/backend/executor/nodeModifyTable.c
new file mode 100644
index a94d7f8..6f0543a
--- a/src/backend/executor/nodeModifyTable.c
+++ b/src/backend/executor/nodeModifyTable.c
@@ -2878,7 +2878,7 @@ lmerge_matched:
 				}
 				ExecUpdatePrepareSlot(resultRelInfo, newslot, context->estate);
 				result = ExecUpdateAct(context, resultRelInfo, tupleid, NULL,
-									   newslot, mtstate->canSetTag, &updateCxt);
+									   newslot, false, &updateCxt);
 				if (result == TM_Ok && updateCxt.updated)
 				{
 					ExecUpdateEpilogue(context, &updateCxt, resultRelInfo,