txn: produce warning for Pipelined DML when falling back because of t…

…idb_constraint_check_in_place (#51923)

ref #50215

pkg/session/session.go

@@ -4413,6 +4413,12 @@ func (s *session) usePipelinedDmlOrWarn() bool {
 	}
 	if s.GetSessionVars().ConstraintCheckInPlace {
 		// we enforce that pipelined DML must lazily check key.
+		stmtCtx.AppendWarning(
+			errors.New(
+				"Pipelined DML can not be used when tidb_constraint_check_in_place=ON. " +
+					"Fallback to standard mode",
+			),
+		)
 		return false
 	}
 	is, ok := s.GetDomainInfoSchema().(infoschema.InfoSchema)

tests/realtikvtest/pipelineddmltest/pipelineddml_test.go

@@ -209,6 +209,12 @@ func TestPipelinedDMLNegative(t *testing.T) {
 	tk.MustExec("insert into t values(10, 10)")
 	tk.MustQuery("show warnings").CheckContain("Pipelined DML can not be used without Metadata Lock. Fallback to standard mode")
 	tk.MustExec("set global tidb_enable_metadata_lock = on")
+
+	// tidb_constraint_check_in_place = ON
+	tk.MustExec("set @@tidb_constraint_check_in_place = 1")
+	tk.MustExec("insert into t values(11, 11)")
+	tk.MustQuery("show warnings").CheckContain("Pipelined DML can not be used when tidb_constraint_check_in_place=ON. Fallback to standard mode")
+	tk.MustExec("set @@tidb_constraint_check_in_place = 0")
 }
 
 func compareTables(t *testing.T, tk *testkit.TestKit, t1, t2 string) {