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

planner/core/logical_plan_builder.go

@@ -1973,7 +1973,7 @@ func (b *PlanBuilder) pushTableHints(hints []*ast.TableOptimizerHint, nodeType n
 		case HintStreamAgg:
 			preferAggType |= preferStreamAgg
 		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

@@ -397,6 +397,16 @@ type accessPath struct {
 	partialIndexPaths []*accessPath
 }
 
+// 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, conds []expression.Expression) (bool, error) {

planner/core/physical_plan_test.go

@@ -65,6 +65,10 @@ func (s *testPlanSuite) TestDAGPlanBuilderSimpleCase(c *C) {
 			sql:  "select * from t t1 use index(c_d_e)",
 			best: "IndexLookUp(Index(t.c_d_e)[[NULL,+inf]], Table(t))",
 		},
+		{
+			sql:  "select f from t use index() where f = 1",
+			best: "TableReader(Table(t)->Sel([eq(test.t.f, 1)]))",
+		},
 		// Test ts + Sort vs. DoubleRead + filter.
 		{
 			sql:  "select a from t where a between 1 and 2 order by c",
@@ -1772,6 +1776,12 @@ 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",

planner/core/planbuilder.go

@@ -558,6 +558,17 @@ func (b *PlanBuilder) getPossibleAccessPaths(indexHints []*ast.IndexHint, tblInf
 		}
 
 		hasScanHint = true
+
+		// It is syntactically valid to omit index_list for USE INDEX, which means “use no indexes”.
+		// Omitting index_list for FORCE INDEX or IGNORE INDEX is a syntax error.
+		// See https://dev.mysql.com/doc/refman/8.0/en/index-hints.html.
+		if hint.IndexNames == nil && hint.HintType != ast.HintIgnore {
+			if path := getTablePath(publicPaths); path != nil {
+				hasUseOrForce = true
+				path.forced = true
+				available = append(available, path)
+			}
+		}
 		for _, idxName := range hint.IndexNames {
 			path := getPathByIndexName(publicPaths, idxName, tblInfo)
 			if path == nil {