planner: fix group_concat function couldn't resolve the index of its order-by item

expression/integration_test/integration_test.go

@@ -2048,3 +2048,14 @@ func TestCompareBuiltin(t *testing.T) {
 	result = tk.MustQuery(`select row(1+3,2,3)<>row(1+3,2,3)`)
 	result.Check(testkit.Rows("0"))
 }
+
+func TestIssue41986(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+
+	tk.MustExec("use test")
+	tk.MustExec("CREATE TABLE poi_clearing_time_topic (effective_date datetime DEFAULT NULL , clearing_time int(11) DEFAULT NULL);")
+	tk.MustExec("insert into poi_clearing_time_topic values ('2023:08:25', 1)")
+	// shouldn't report they can't find column error and return the right result.
+	tk.MustQuery("SELECT GROUP_CONCAT(effective_date order by stlmnt_hour DESC) FROM ( SELECT (COALESCE(pct.clearing_time, 0)/3600000) AS stlmnt_hour ,COALESCE(pct.effective_date, '1970-01-01 08:00:00') AS effective_date FROM poi_clearing_time_topic pct ORDER BY pct.effective_date DESC ) a;").Check(testkit.Rows("2023-08-25 00:00:00"))
+}

planner/core/rule_aggregation_push_down.go

@@ -23,6 +23,7 @@ import (
 	"github.com/pingcap/tidb/expression/aggregation"
 	"github.com/pingcap/tidb/parser/ast"
 	"github.com/pingcap/tidb/parser/mysql"
+	"github.com/pingcap/tidb/planner/util"
 	"github.com/pingcap/tidb/sessionctx"
 	"github.com/pingcap/tidb/types"
 )
@@ -559,6 +560,8 @@ func (a *aggregationPushDownSolver) aggPushDown(p LogicalPlan, opt *logicalOptim
 				}
 				oldAggFuncsArgs := make([][]expression.Expression, 0, len(agg.AggFuncs))
 				newAggFuncsArgs := make([][]expression.Expression, 0, len(agg.AggFuncs))
+				oldAggOrderItems := make([][]*util.ByItems, 0, len(agg.AggFuncs))
+				newAggOrderItems := make([][]*util.ByItems, 0, len(agg.AggFuncs))
 				if noSideEffects {
 					for _, aggFunc := range agg.AggFuncs {
 						oldAggFuncsArgs = append(oldAggFuncsArgs, aggFunc.Args)
@@ -571,6 +574,27 @@ func (a *aggregationPushDownSolver) aggPushDown(p LogicalPlan, opt *logicalOptim
 							break
 						}
 						newAggFuncsArgs = append(newAggFuncsArgs, newArgs)
+						// for ordeByItems, treat it like agg func's args, if it can be substituted by underlying projection's expression recording them temporarily.
+						if len(aggFunc.OrderByItems) != 0 {
+							oldAggOrderItems = append(oldAggOrderItems, aggFunc.OrderByItems)
+							newOrderByItems := make([]expression.Expression, 0, len(aggFunc.OrderByItems))
+							for _, oby := range aggFunc.OrderByItems {
+								newOrderByItems = append(newOrderByItems, expression.ColumnSubstitute(oby.Expr, proj.schema, proj.Exprs))
+							}
+							if ExprsHasSideEffects(newOrderByItems) {
+								noSideEffects = false
+								break
+							}
+							oneAggOrderByItems := make([]*util.ByItems, 0, len(aggFunc.OrderByItems))
+							for i, obyExpr := range newOrderByItems {
+								oneAggOrderByItems = append(oneAggOrderByItems, &util.ByItems{Expr: obyExpr, Desc: aggFunc.OrderByItems[i].Desc})
+							}
+							newAggOrderItems = append(newAggOrderItems, oneAggOrderByItems)
+						} else {
+							// occupy the pos for convenience of subscript index
+							oldAggOrderItems = append(oldAggOrderItems, nil)
+							newAggOrderItems = append(newAggOrderItems, nil)
+						}
 					}
 				}
 				for i, funcsArgs := range oldAggFuncsArgs {
@@ -580,6 +604,16 @@ func (a *aggregationPushDownSolver) aggPushDown(p LogicalPlan, opt *logicalOptim
 							break
 						}
 					}
+					for j, item := range newAggOrderItems {
+						if item == nil {
+							continue
+						}
+						// substitution happened, check the eval type compatibility.
+						if oldAggOrderItems[i][j].Expr.GetType().EvalType() != newAggOrderItems[i][j].Expr.GetType().EvalType() {
+							noSideEffects = false
+							break
+						}
+					}
 					if !noSideEffects {
 						break
 					}
@@ -588,6 +622,7 @@ func (a *aggregationPushDownSolver) aggPushDown(p LogicalPlan, opt *logicalOptim
 					agg.GroupByItems = newGbyItems
 					for i, aggFunc := range agg.AggFuncs {
 						aggFunc.Args = newAggFuncsArgs[i]
+						aggFunc.OrderByItems = newAggOrderItems[i]
 					}
 					projChild := proj.children[0]
 					agg.SetChildren(projChild)