类型转换函数

类型转换函数

• cast(x as y)

描述：类型转换函数，将x转换成y指定的类型。如果目标类型为char类型，在dolphin.b_compatibility_mode=on的情况下，实际转换为varchar类型；否则仍为char类型。

示例：

``` 1. openGauss=# set dolphin.b_compatibility_mode=on;

1. openGauss=# SELECT cast('abc' as char(10));
2. varchar

3. ---

4. abc

5. (1 row)

6. openGauss=# SELECT dolphin.b_compatibility_mode=off;

7. openGauss=# SELECT cast('abc' as char(10));

8. bpchar

9. ---

10. abc
11. (1 row) ```

在后续开发中，扩展的CAST转换功能有money到unsigned和timestamp到unsigned的转换。

1. CREATE CAST (timestamp AS uint8) WITH FUNCTION timestamp_uint8(timestamp) AS ASSIGNMENT;
2. CREATE CAST (money AS uint8) WITH FUNCTION cash_uint(money) AS ASSIGNMENT;

兼容性

CREATE CAST指令符合SQL标准，除了SQL没有为二进制可强制转换类型或者实现函数的额外参数来实现功能。

1. openGauss=# SELECT CAST('$2'::money as unsigned);
2. uint8
3. -------
4. 2
5. (1 row)
6. openGauss=# SELECT CAST(CURRENT_TIMESTAMP::TIMESTAMP AS UNSIGNED);
7. current_timestamp
8. -------------------
9. 20230103023621
10. (1 row)

```