种类：flow
id：append
描述：这个step，从2个step读取数据，不过，只处理第二个step的数据（在第一个step完成以后）。
一般情况下，这2个step的数据行的格式是一模一样的：相同的长度，相同的字段类型，相同的索引等。
Important: If you don't care about the order in which the output rows occur, you can use any step to create a union of 2 or more data streams.
Options
| Option | Description |
|---|---|
| Step name | Name of the step. Note: This name has to be unique in a single transformation. |
| Head hop | The name of the step from which will be read from first. |
| Tail hop | The name of the step from which will be read from last. |