planner: choose TableScan when use an empty index hint (pingcap#12037)

planner/core/logical_plan_builder.go

@@ -1968,7 +1968,7 @@ func (b *PlanBuilder) pushTableHints(hints []*ast.TableOptimizerHint, nodeType n
 		case HintAggToCop:
 			aggHints.preferAggToCop = true
 		case HintIndex:
-			if len(hint.Tables) != 0 && len(hint.Indexes) != 0 {
+			if len(hint.Tables) != 0 {
 				indexHintList = append(indexHintList, indexHintInfo{
 					tblName: hint.Tables[0].TableName,
 					indexHint: &ast.IndexHint{

planner/core/logical_plans.go

@@ -387,6 +387,16 @@ func getTablePath(paths []*accessPath) *accessPath {
 	return nil
 }
 
+// getTablePath finds the TablePath from a group of accessPaths.
+func getTablePath(paths []*accessPath) *accessPath {
+	for _, path := range paths {
+		if path.isTablePath {
+			return path
+		}
+	}
+	return nil
+}
+
 // deriveTablePathStats will fulfill the information that the accessPath need.
 // And it will check whether the primary key is covered only by point query.
 func (ds *DataSource) deriveTablePathStats(path *accessPath) (bool, error) {

planner/core/physical_plan_test.go

@@ -1820,6 +1820,12 @@ func (s *testPlanSuite) TestIndexHint(c *C) {
 			best:    "IndexLookUp(Index(t.f)[[NULL,+inf]], Table(t))",
 			hasWarn: false,
 		},
+		// use TablePath when the hint only contains table.
+		{
+			sql:     "select /*+ INDEX(t) */ f from t where f > 10",
+			best:    "TableReader(Table(t)->Sel([gt(test.t.f, 10)]))",
+			hasWarn: false,
+		},
 		// there will be a warning instead of error when index not exist
 		{
 			sql:     "select /*+ INDEX(t, no_such_index) */ * from t",