我有一张外桌：

CREATE TABLE PFun ( 
  idF   VARCHAR(255),
  dt   DATE
)
ORGANIZATION EXTERNAL
  (
    TYPE oracle_loader
  DEFAULT DIRECTORY ext_dep    
  ACCESS PARAMETERS
  ( 
    RECORDS DELIMITED BY '\n' 
    BADFILE     CHARACTER
      DISCARDFILE CHARACTER
      LOGFILE     CHARACTER
      FIELDS TERMINATED BY ','
      OPTIONALLY ENCLOSED BY "'"
      MISSING FIELD VALUES ARE NULL     
      (
        idF     CHAR(255),
        dt      CHAR(255) date_format DATE mask 'YYYY/MM/DD'
      )  
  )
  LOCATION ('prfun.csv')
)
REJECT LIMIT UNLIMITED;

我创建这个函数是为了返回一年中的周数

/
CREATE OR REPLACE FUNCTION TestFunction(dtIn IN DATE)  
RETURN NUMBER   
IS res NUMBER;
BEGIN  
    SELECT CEIL((TO_DATE(dtIn, 'yyyy/mm/dd') - TO_DATE('2020/12/31', 'yyyy/mm/dd'))/7)
    INTO res
    FROM PFun
    WHERE dtIn >= '2020-01-01';
    RETURN res;
END;
/
SELECT idF, TestFunction(dt)
FROM PFun;

如果我只运行这个函数，它会告诉我所有的东西都已经编译好了。但是当我运行select时，它会给我以下错误：

ORA-29913: Error executing ODCIEXTTABLEFETCH callout
ORA-01861: the value does not match the format string
ORA-06512: a "SYSTEM.TESTFUNCTION", line 5
29913.0000 - "error in executing% s callout"

* Cause: The execution of the specified callout caused an error.
* Action: Examine the error messages take appropriate action.

[我这样输入日期：'2020/01/01'，但是sql developer在控制查询中向我显示的日期是'01-jan-20'。我不知道这些信息是否重要]
我该怎么解决？