planner: allow TableDual to match the required property if the row co…

…unt is 0 (#16927) (#17014)

planner/core/find_best_task.go

@@ -66,7 +66,10 @@ func getPropByOrderByItems(items []*ByItems) (*property.PhysicalProperty, bool)
 }
 
 func (p *LogicalTableDual) findBestTask(prop *property.PhysicalProperty) (task, error) {
-	if !prop.IsEmpty() {
+	// If the required property is not empty and the row count > 1,
+	// we cannot ensure this required property.
+	// But if the row count is 0 or 1, we don't need to care about the property.
+	if !prop.IsEmpty() && p.RowCount > 1 {
 		return invalidTask, nil
 	}
 	dual := PhysicalTableDual{

planner/core/integration_test.go

@@ -375,3 +375,13 @@ partition p2 values less than (unix_timestamp('2020-04-15 00:00:00')))`)
 	tk.MustQuery("select count(*) from floor_unix_timestamp where ts <= '2020-04-05 23:00:00'").Check(testkit.Rows("4"))
 	tk.MustQuery("select * from floor_unix_timestamp partition(p1, p2) where ts > '2020-04-14 00:00:00'").Check(testkit.Rows("2020-04-14 00:00:42.000"))
 }
+
+func (s *testIntegrationSuite) TestTableDualWithRequiredProperty(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t1, t2;")
+	tk.MustExec("create table t1 (a int, b int) partition by range(a) " +
+		"(partition p0 values less than(10), partition p1 values less than MAXVALUE)")
+	tk.MustExec("create table t2 (a int, b int)")
+	tk.MustExec("select /*+ MERGE_JOIN(t1, t2) */ * from t1 partition (p0), t2  where t1.a > 100 and t1.a = t2.a")
+}