SQL & PL/SQL :: Reducing Function Computation When Called From Statement?

In the below SQL, I am calling a function "listagg_nm(deptno)", and function is to get all the employee name for a particular department with comma separated

output is:
-------------------------------------------------
DEPTNO SAL LISTAGG_NM(DEPTNO)
10 5000 KING,CLARK,MILLER
10 1300 KING,CLARK,MILLER
10 2450 KING,CLARK,MILLER
20 2975 JONES,SCOTT,FORD,SMITH,ADAMS

[code]....

below are the code

select deptno,sal,listagg_nm(deptno) from emp order by deptno;
FUNCTION listagg_nm (p_deptno NUMBER)
RETURN CLOB
IS
v_str CLOB;

[code]....

My doubt/question is for every row in the SQL the function is being called. t means that for 1st row deptno=10, the function is called and return all concatenated name for department 10, then again for 2nd row deptno=10, it again do the same, similarly for all other department.

Can we avoid this, I mean, if function is already returned a concatenated string of ename for deptno=10, then in the next function call it will checks if it is for same department (i.e. deptno=10) then the function will not go into for loop and it will directly return the concatenated string of ename for deptno=10 which we already done for the first row and so on for other department number ( something like storing the value in variable. Is it possible to achive this and does this is good for performance as we are not going into loop inside the function to return the concatenated string of ename for the same deptno everytime ( I have to return more than 10000 char., so i used clob in function and similarly the numbers of row return by SQL is more than 1000)and also how to code this, I tried but unable to achive.