expression: fix a bug caused by cast function signature wrong in aggregation push down

expression/integration_test.go

@@ -10393,3 +10393,25 @@ func (s *testIntegrationSuite) TestIdentity(c *C) {
 	tk.MustExec(`INSERT INTO identity VALUES (NULL);`)
 	tk.MustQuery("SELECT @@identity, LAST_INSERT_ID()").Check(testkit.Rows("3 3"))
 }
+
+func (s *testIntegrationSuite) TestIssue28804(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists perf_offline_day;")
+	tk.MustExec(`CREATE TABLE perf_offline_day (
+uuid varchar(50),
+ts timestamp NOT NULL,
+user_id varchar(50) COLLATE utf8mb4_general_ci DEFAULT NULL,
+platform varchar(50) COLLATE utf8mb4_general_ci DEFAULT NULL,
+host_id bigint(20) DEFAULT NULL,
+PRIMARY KEY (uuid,ts) /*T![clustered_index] NONCLUSTERED */
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci
+PARTITION BY RANGE ( UNIX_TIMESTAMP(ts) ) (
+PARTITION p20210906 VALUES LESS THAN (1630944000),
+PARTITION p20210907 VALUES LESS THAN (1631030400),
+PARTITION p20210908 VALUES LESS THAN (1631116800),
+PARTITION p20210909 VALUES LESS THAN (1631203200)
+);`)
+	tk.MustExec("INSERT INTO `perf_offline_day` VALUES ('dd082c8a-3bab-4431-943a-348fe0592abd','2021-09-08 13:00:07','Xg9C8zq81jGNbugM', 'pc', 12345);")
+	tk.MustQuery("SELECT cast(floor(hour(ts) / 4) as char) as win_start FROM perf_offline_day partition (p20210907, p20210908) GROUP BY win_start;").Check(testkit.Rows("3"))
+}

expression/util.go

@@ -225,15 +225,20 @@ func ColumnSubstituteImpl(expr Expression, schema *Schema, newExprs []Expression
 		newExpr.SetCoercibility(v.Coercibility())
 		return true, newExpr
 	case *ScalarFunction:
+		substituted := false
 		if v.FuncName.L == ast.Cast {
 			newFunc := v.Clone().(*ScalarFunction)
-			_, newFunc.GetArgs()[0] = ColumnSubstituteImpl(newFunc.GetArgs()[0], schema, newExprs)
+			substituted, newFunc.GetArgs()[0] = ColumnSubstituteImpl(newFunc.GetArgs()[0], schema, newExprs)
+			if substituted {
+				// Workaround for issue https://github.com/pingcap/tidb/issues/28804
+				refExprArr := cowExprRef{v.GetArgs(), nil}
+				return true, NewFunctionInternal(v.GetCtx(), v.FuncName.L, v.RetType, refExprArr.Result()...)
+			}
 			return true, newFunc
 		}
 		// cowExprRef is a copy-on-write util, args array allocation happens only
 		// when expr in args is changed
 		refExprArr := cowExprRef{v.GetArgs(), nil}
-		substituted := false
 		_, coll := DeriveCollationFromExprs(v.GetCtx(), v.GetArgs()...)
 		for idx, arg := range v.GetArgs() {
 			changed, newFuncExpr := ColumnSubstituteImpl(arg, schema, newExprs)