planner: choose TableScan when use an empty index hint

planner/core/logical_plans.go

@@ -392,6 +392,16 @@ 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",

planner/core/planbuilder.go

@@ -558,6 +558,19 @@ func (b *PlanBuilder) getPossibleAccessPaths(indexHints []*ast.IndexHint, tblInf
 		}
 
 		hasScanHint = true
+		if hint.IndexNames == nil {
+			// If indexNames is nil, choose TablePath.
+			// For example: `select a from t use index()`.
+			if path := getTablePath(publicPaths); path != nil {
+				if hint.HintType == ast.HintIgnore {
+					ignored = append(ignored, path)
+				} else {
+					hasUseOrForce = true
+					path.forced = true
+					available = append(available, path)
+				}
+			}
+		}
 		for _, idxName := range hint.IndexNames {
 			path := getPathByIndexName(publicPaths, idxName, tblInfo)
 			if path == nil {