Fix MERGE command tag for actions blocked by BEFORE ROW triggers.

This ensures that the row count in the command tag for a MERGE is
correctly computed in the case where UPDATEs or DELETEs are skipped
due to a BEFORE ROW trigger returning NULL (the INSERT case was
already handled correctly by ExecMergeNotMatched() calling
ExecInsert()).

Back-patch to v15, where MERGE was introduced.

Discussion: https://postgr.es/m/CAEZATCU8XEmR0JWKDtyb7iZ%3DqCffxS9uyJt0iOZ4TV4RT%2Bow1w%40mail.gmail.com

src/backend/executor/nodeModifyTable.c

@@ -2883,8 +2883,9 @@ lmerge_matched:
 				if (!ExecUpdatePrologue(context, resultRelInfo,
 										tupleid, NULL, newslot, &result))
 				{
-					/* Blocked by trigger, or concurrent update/delete */
-					break;
+					if (result == TM_Ok)
+						return true;	/* "do nothing" */
+					break;		/* concurrent update/delete */
 				}
 				result = ExecUpdateAct(context, resultRelInfo, tupleid, NULL,
 									   newslot, false, &updateCxt);
@@ -2901,8 +2902,9 @@ lmerge_matched:
 				if (!ExecDeletePrologue(context, resultRelInfo, tupleid,
 										NULL, NULL, &result))
 				{
-					/* Blocked by trigger, or concurrent update/delete */
-					break;
+					if (result == TM_Ok)
+						return true;	/* "do nothing" */
+					break;		/* concurrent update/delete */
 				}
 				result = ExecDeleteAct(context, resultRelInfo, tupleid, false);
 				if (result == TM_Ok)

src/test/regress/expected/merge.out

@@ -942,12 +942,25 @@ SELECT * FROM target full outer join source on (sid = tid);
 
 create trigger merge_skip BEFORE INSERT OR UPDATE or DELETE
   ON target FOR EACH ROW EXECUTE FUNCTION skip_merge_op();
+DO $$
+DECLARE
+  result integer;
+BEGIN
 MERGE INTO target t
 USING source AS s
 ON t.tid = s.sid
 WHEN MATCHED AND s.sid = 3 THEN UPDATE SET balance = t.balance + s.delta
 WHEN MATCHED THEN DELETE
 WHEN NOT MATCHED THEN INSERT VALUES (sid, delta);
+IF FOUND THEN
+  RAISE NOTICE 'Found';
+ELSE
+  RAISE NOTICE 'Not found';
+END IF;
+GET DIAGNOSTICS result := ROW_COUNT;
+RAISE NOTICE 'ROW_COUNT = %', result;
+END;
+$$;
 NOTICE:  BEFORE INSERT STATEMENT trigger
 NOTICE:  BEFORE UPDATE STATEMENT trigger
 NOTICE:  BEFORE DELETE STATEMENT trigger
@@ -957,6 +970,8 @@ NOTICE:  BEFORE INSERT ROW trigger row: (4,40)
 NOTICE:  AFTER DELETE STATEMENT trigger
 NOTICE:  AFTER UPDATE STATEMENT trigger
 NOTICE:  AFTER INSERT STATEMENT trigger
+NOTICE:  Not found
+NOTICE:  ROW_COUNT = 0
 SELECT * FROM target FULL OUTER JOIN source ON (sid = tid);
  tid | balance | sid | delta 
 -----+---------+-----+-------

src/test/regress/sql/merge.sql

@@ -636,12 +636,25 @@ $$;
 SELECT * FROM target full outer join source on (sid = tid);
 create trigger merge_skip BEFORE INSERT OR UPDATE or DELETE
   ON target FOR EACH ROW EXECUTE FUNCTION skip_merge_op();
+DO $$
+DECLARE
+  result integer;
+BEGIN
 MERGE INTO target t
 USING source AS s
 ON t.tid = s.sid
 WHEN MATCHED AND s.sid = 3 THEN UPDATE SET balance = t.balance + s.delta
 WHEN MATCHED THEN DELETE
 WHEN NOT MATCHED THEN INSERT VALUES (sid, delta);
+IF FOUND THEN
+  RAISE NOTICE 'Found';
+ELSE
+  RAISE NOTICE 'Not found';
+END IF;
+GET DIAGNOSTICS result := ROW_COUNT;
+RAISE NOTICE 'ROW_COUNT = %', result;
+END;
+$$;
 SELECT * FROM target FULL OUTER JOIN source ON (sid = tid);
 DROP TRIGGER merge_skip ON target;
 DROP FUNCTION skip_merge_op();