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：

  MogDB=# set dolphin.b_compatibility_mode=on;
  
  MogDB=# SELECT cast('abc' as char(10));
   varchar
  ---------
   abc
  (1 row)
  
  MogDB=# SELECT dolphin.b_compatibility_mode=off;
  
  MogDB=# SELECT cast('abc' as char(10));
     bpchar
  ------------
   abc
  (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。

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