Oracle CHAR Comparison Not Working in Function - oracle

Question

Could someone please explain to me the difference between the below two Oracle queries? I know they look very similar but the first one returns results and the second one does not. My implementation of the function can be seen below as well.
--Returns results
SELECT *
FROM <TABLE_NAME>
WHERE ID = CAST(<UserID> AS CHAR(2000)); --ID is defined as CHAR(8) in the DB.
--Does not return results
SELECT *
FROM <TABLE_NAME>
WHERE ID = CAST_TO_CHAR(<UserID>); --ID is defined as CHAR(8) in the DB.
--Function definition
CREATE OR REPLACE FUNCTION CAST_TO_CHAR(varToPad IN VARCHAR2)
RETURN CHAR IS returnVal CHAR(2000);
BEGIN
SELECT CAST(varToPad AS CHAR(2000))
INTO returnVal
FROM DUAL;
RETURN returnVal;
END;
/
It almost seems to me that the type is not persisting when the value is retrieved from the database. From what I understand from CHAR comparisons in Oracle, it will take the smaller of the two fields and truncate the larger one so that the sizes match (that is why I am casting the second variable to length 2000).
The reason that I need to achieve something like this is because a vendor tool that we are upgrading from DB2 to Oracle defined all of the columns in the Oracle database as CHAR instead of VARCHAR2. They did this to make their legacy code more easily portable to a distributed environment. This is causing big issues in our web applications because compares are now being done against fixed length CHAR fields.
I thought about using TRIM() but these queries will be accessed a lot and I do not want them to do a full table scan each time. I also considered RPAD(, ) but I don't really want to hard code lengths in the application as these may change in the future.
Does anyone have any thoughts about this? Thank you in advance for your help!

Answer 1

I have similar problem. It turned out that these are the rules of implicit data conversion. Oracle Database automatically converts a value from one datatype to another when such a conversion makes sense.
If you change your select:
SELECT *
FROM <TABLE_NAME>
WHERE CAST(ID as CHAR(2000)) = CAST_TO_CHAR(<UserID>);
You will see that's works properly.
And here's another test script showing that the function works correctly:
SET SERVEROUTPUT ON --for DBMS_OUTPUT.PUT_LINE.
DECLARE
test_string_c CHAR(8);
test_string_v VARCHAR2(8);
BEGIN
--Assign the same value to each string.
test_string_c := 'string';
test_string_v := 'string';
--Test the strings for equality.
IF test_string_c = CAST_TO_CHAR(test_string_v) THEN
DBMS_OUTPUT.PUT_LINE('The names are the same');
ELSE
DBMS_OUTPUT.PUT_LINE('The names are NOT the same');
END IF;
END;
/
anonymous block completed
The names are the same
Here are some rules govern the direction in which Oracle Database makes implicit datatype conversions:
During INSERT and UPDATE operations, Oracle converts the value to
the datatype of the affected column.
During SELECT FROM operations, Oracle converts the data from the
column to the type of the target variable.
When comparing a character value with a numeric value, Oracle
converts the character data to a numeric value.
When comparing a character value with a DATE value, Oracle converts
the character data to DATE.
When making assignments, Oracle converts the value on the right side
of the equal sign (=) to the datatype of the target of the assignment
on the left side.
When you use a SQL function or operator with an argument of a
datatype other than the one it accepts, Oracle converts the argument
to the accepted datatype.
Complete list of datatype comparison rules you can explore here