planner: choose TableScan when use an empty index hint (#12100)

Author: francis0407

planner/core/logical_plans.go

@@ -372,6 +372,16 @@ type accessPath struct {
 	forced bool
 }
 
+// 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

@@ -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

@@ -478,6 +478,17 @@ func getPossibleAccessPaths(indexHints []*ast.IndexHint, tblInfo *model.TableInf
 		}
 
 		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 {