在Join中指定第一个表的字段

Question

我试图优化此连接，因为两个表都很大（＃rows，＃records）：

Update Table1 as T1
Inner Join Table2 as T2
On T1.X=T2.Y
Set T1.A=T2.B;

结束了性能提升：

Update Table1 as T1
Inner Join (Select T2.Y,T2.B from Table2) as T2
On T1.X=T2.Y
Set T1.A=T2.B;

因此我想对Table1做同样的事情。然而，当我尝试：

Update (Select T1.X, T1.A from Table1) as T1
Inner Join (Select T2.Y,T2.B from Table2 )as T2
On T1.X=T2.Y
Set T1.A=T2.B;

我收到T1无法更新的错误。我怎么知道限制Table1加载的字段？