planner: outer join elimination doesn't consider the ORDER BY items f…

…rom the agg func

planner/core/integration_test.go

@@ -7675,3 +7675,17 @@ func TestIssue38295(t *testing.T) {
 	tk.MustGetErrCode("SELECT t0.c1, t0.c2 FROM t0 GROUP BY MOD(t0.c0, DEFAULT(t0.c2));", errno.ErrFieldNotInGroupBy)
 	tk.MustExec("UPDATE t0 SET c2=1413;")
 }
+
+func TestOuterJoinEliminationForIssue18216(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t1, t2;")
+	tk.MustExec("create table t1 (a int, c int);")
+	tk.MustExec("insert into t1 values (1, 1), (1, 2), (2, 3), (2, 4);")
+	tk.MustExec("create table t2 (a int, c int);")
+	tk.MustExec("insert into t2 values (1, 1), (1, 2), (2, 3), (2, 4);")
+	// The output might be unstable.
+	tk.MustExec("select group_concat(c order by (select group_concat(c order by a) from t2 where a=t1.a)) from t1; ")
+	tk.MustQuery("select group_concat(c order by (select group_concat(c order by c) from t2 where a=t1.a), c desc) from t1;").Check(testkit.Rows("2,1,4,3"))
+}

planner/core/rule_join_elimination.go

@@ -211,6 +211,9 @@ func (o *outerJoinEliminator) doOptimize(p LogicalPlan, aggCols []*expression.Co
 			for _, expr := range aggDesc.Args {
 				parentCols = append(parentCols, expression.ExtractColumns(expr)...)
 			}
+			for _, byItem := range aggDesc.OrderByItems {
+				parentCols = append(parentCols, expression.ExtractColumns(byItem.Expr)...)
+			}
 		}
 	default:
 		parentCols = append(parentCols[:0], p.Schema().Columns...)