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## 常序列生成函数

常序列生成函数用于生成所有数据点的值都相同的时间序列。

常序列生成函数接受一个或者多个时间序列输入，其输出的数据点的时间戳集合是这些输入序列时间戳集合的并集。

目前 IoTDB 支持如下常序列生成函数：

| 函数名 | 必要的属性参数                                               | 输出序列类型               | 功能描述                                                     |
| ------ | ------------------------------------------------------------ | -------------------------- | ------------------------------------------------------------ |
| CONST  | `value`: 输出的数据点的值 <br />`type`: 输出的数据点的类型，只能是 INT32 / INT64 / FLOAT / DOUBLE / BOOLEAN / TEXT | 由输入属性参数 `type` 决定 | 根据输入属性 `value` 和 `type` 输出用户指定的常序列。        |
| PI     | 无                                                           | DOUBLE                     | 常序列的值：`π` 的 `double` 值，圆的周长与其直径的比值，即圆周率，等于 *Java标准库* 中的`Math.PI`。 |
| E      | 无                                                           | DOUBLE                     | 常序列的值：`e` 的 `double` 值，自然对数的底，它等于 *Java 标准库*  中的 `Math.E`。 |

例如：

```   sql
select s1, s2, const(s1, 'value'='1024', 'type'='INT64'), pi(s2), e(s1, s2) from root.sg1.d1; 
```

结果：

```
select s1, s2, const(s1, 'value'='1024', 'type'='INT64'), pi(s2), e(s1, s2) from root.sg1.d1; 
+-----------------------------+--------------+--------------+-----------------------------------------------------+------------------+---------------------------------+
|                         Time|root.sg1.d1.s1|root.sg1.d1.s2|const(root.sg1.d1.s1, "value"="1024", "type"="INT64")|pi(root.sg1.d1.s2)|e(root.sg1.d1.s1, root.sg1.d1.s2)|
+-----------------------------+--------------+--------------+-----------------------------------------------------+------------------+---------------------------------+
|1970-01-01T08:00:00.000+08:00|           0.0|           0.0|                                                 1024| 3.141592653589793|                2.718281828459045|
|1970-01-01T08:00:00.001+08:00|           1.0|          null|                                                 1024|              null|                2.718281828459045|
|1970-01-01T08:00:00.002+08:00|           2.0|          null|                                                 1024|              null|                2.718281828459045|
|1970-01-01T08:00:00.003+08:00|          null|           3.0|                                                 null| 3.141592653589793|                2.718281828459045|
|1970-01-01T08:00:00.004+08:00|          null|           4.0|                                                 null| 3.141592653589793|                2.718281828459045|
+-----------------------------+--------------+--------------+-----------------------------------------------------+------------------+---------------------------------+
Total line number = 5
It costs 0.005s
```