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。当dolphin.b_compatibility_mode=on，转换为signed类型实现了mysql的兼容性，即超过signed类型表示范围的溢出规则如下：大于int64最大值的正数转换为数位表示对应的负整数，小于int64最小值的负数转换为数位表示对应的正整数，当源类型为numeric和浮点类型时，溢出的正数截断为int64的最大值，溢出的负数截断为int64的最小值。
示例：
```
openGauss=# set dolphin.b_compatibility_mode=on;

openGauss=# SELECT cast('abc' as char(10));
 varchar
---------
 abc
(1 row)

openGauss=# SELECT dolphin.b_compatibility_mode=off;

openGauss=# SELECT cast('abc' as char(10));
   bpchar
------------
 abc
(1 row)
```

转换为signed类型。

--- 超出范围，严格模式下报错，宽松模式下告警
openGauss=# set dolphin.sql_mode ='';
SET
--- 默认为dolphin.b_compatibility_mode=off
openGauss=# select cast('9223372036854775808' as signed);
WARNING:  value "9223372036854775808" is out of range for type bigint
LINE 1: select cast('9223372036854775808' as signed);
                ^
CONTEXT:  referenced column: int8
        int8         
---------------------
 9223372036854775807
(1 row)

--- 打开参数后应用mysql兼容性表现
openGauss=# set dolphin.b_compatibility_mode = on;
SET
openGauss=# select cast('9223372036854775808' as signed);
WARNING:  bigint out of range
CONTEXT:  referenced column: int8
         int8         
----------------------
-9223372036854775808
(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。

CREATE CAST (timestamp AS uint8) WITH FUNCTION timestamp_uint8(timestamp) AS ASSIGNMENT;
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。

 openGauss=# SELECT CAST('$2'::money as unsigned);
  uint8
  -------
  2
  (1 row)
 openGauss=# SELECT CAST(CURRENT_TIMESTAMP::TIMESTAMP AS UNSIGNED);
  current_timestamp
  -------------------
  20230103023621
  (1 row)