cherry pick #21213 to release-4.0 (#21380)

Signed-off-by: ti-srebot <ti-srebot@pingcap.com>
Co-authored-by: Kenan Yao <cauchy1992@gmail.com>

planner/core/integration_test.go

@@ -1571,6 +1571,17 @@ func (s *testIntegrationSerialSuite) TestIssue20710(c *C) {
 	}
 }
 
+func (s *testIntegrationSuite) TestQueryBlockTableAliasInHint(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+
+	tk.MustExec("use test")
+	c.Assert(tk.HasPlan("select /*+ HASH_JOIN(@sel_1 t2) */ * FROM (select 1) t1 NATURAL LEFT JOIN (select 2) t2", "HashJoin"), IsTrue)
+	tk.MustQuery("select /*+ HASH_JOIN(@sel_1 t2) */ * FROM (select 1) t1 NATURAL LEFT JOIN (select 2) t2").Check(testkit.Rows(
+		"1 2",
+	))
+	c.Assert(tk.Se.GetSessionVars().StmtCtx.GetWarnings(), HasLen, 0)
+}
+
 func (s *testIntegrationSuite) TestIssue10448(c *C) {
 	tk := testkit.NewTestKit(c, s.store)
 	tk.MustExec("use test")

planner/core/logical_plan_builder.go

@@ -484,7 +484,11 @@ func extractTableAlias(p Plan, parentOffset int) *hintTableInfo {
 		if blockOffset != parentOffset && blockAsNames != nil && blockAsNames[blockOffset].TableName.L != "" {
 			blockOffset = parentOffset
 		}
-		return &hintTableInfo{dbName: firstName.DBName, tblName: firstName.TblName, selectOffset: blockOffset}
+		dbName := firstName.DBName
+		if dbName.L == "" {
+			dbName = model.NewCIStr(p.SCtx().GetSessionVars().CurrentDB)
+		}
+		return &hintTableInfo{dbName: dbName, tblName: firstName.TblName, selectOffset: blockOffset}
 	}
 	return nil
 }