session: fix dml_batch_size doesn't load the global variable

session/session.go

@@ -2652,6 +2652,7 @@ var builtinGlobalVariable = []string{
 	variable.TiDBAllowFallbackToTiKV,
 	variable.TiDBEnableDynamicPrivileges,
 	variable.CTEMaxRecursionDepth,
+	variable.TiDBDMLBatchSize,
 }
 
 // loadCommonGlobalVariablesIfNeeded loads and applies commonly used global variables for the session.

session/session_test.go

@@ -4476,3 +4476,13 @@ func (s *testTxnStateSuite) TestRollbacking(c *C) {
 	c.Assert(tk.Se.TxnInfo().State, Equals, txninfo.TxnRollingBack)
 	<-ch
 }
+
+func (s *testSessionSuite) TestReadDMLBatchSize(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("set global tidb_dml_batch_size=1000")
+	se, err := session.CreateSession(s.store)
+	c.Assert(err, IsNil)
+	// `select 1` to load the global variables.
+	_, _ = se.Execute(context.TODO(), "select 1")
+	c.Assert(se.GetSessionVars().DMLBatchSize, Equals, 1000)
+}