Create the implementation package to_doc_p containing the DESCRIBE function and FETCH_ROWS procedure for the to_doc polymorphic table function (PTF).

CREATE PACKAGE to_doc_p AS 
   FUNCTION describe(tab      IN OUT DBMS_TF.table_t, 
		     cols     IN     DBMS_TF.columns_t DEFAULT NULL) 
		     RETURN DBMS_TF.describe_t; 
    
   PROCEDURE fetch_rows; 
END to_doc_p; 

Create the package containing the DESCRIBE function and FETCH_ROWS procedure.

CREATE PACKAGE BODY to_doc_p AS 
   FUNCTION describe(tab      IN OUT DBMS_TF.table_t, 
		     cols     IN     DBMS_TF.columns_t DEFAULT NULL) 
		     RETURN DBMS_TF.describe_t AS 
   BEGIN 
      FOR i IN 1 .. tab.column.count LOOP  
	 CONTINUE WHEN NOT DBMS_TF.supported_type(tab.column(i).description.TYPE); 
	  
	 IF cols IS NULL THEN 
	    tab.column(i).for_read     := TRUE; 
	    tab.column(i).pass_through := FALSE; 
	    continue; 
	 END IF; 
	  
	 FOR j IN 1 .. cols.count LOOP 
	    IF (tab.column(i).description.name = cols(j)) THEN 
	       tab.column(i).for_read     := TRUE; 
	       tab.column(i).pass_through := FALSE; 
	    END IF; 
	 END LOOP; 
	  
      END LOOP; 
       
      RETURN DBMS_TF.describe_t(new_columns => 
	DBMS_TF.columns_new_t(1 => 
	DBMS_TF.column_metadata_t(name =>'DOCUMENT'))); 
       
   END; 
    
   PROCEDURE fetch_rows AS  
      rst DBMS_TF.row_set_t; 
      col DBMS_TF.tab_varchar2_t; 
      rct PLS_INTEGER; 
   BEGIN 
      DBMS_TF.get_row_set(rst, row_count => rct); 
      FOR rid IN 1 .. rct LOOP  
	 col(rid) := DBMS_TF.row_to_char(rst, rid);  
      END LOOP; 
      DBMS_TF.put_col(1, col); 
   END;  
    
END to_doc_p; 

Create the standalone to_doc PTF.

CREATE FUNCTION to_doc( 
       tab TABLE,  
       cols columns DEFAULT NULL)  
    RETURN TABLE PIPELINED ROW POLYMORPHIC USING to_doc_p; 

SELECT * FROM to_doc(scott.dept)

For all employees in departments 10 and 30, display the DEPTNO, ENAME and DOCUMENT columns ordered by DEPTNO and ENAME.

SELECT deptno, ename, document  
FROM   to_doc(scott.emp, columns(empno,job,mgr,hiredate,sal,comm)) 
WHERE  deptno IN (10, 30)  
ORDER BY 1, 2

WITH e AS ( 
   SELECT ename name, sal, deptno, loc  
   FROM scott.emp NATURAL JOIN scott.dept  
   WHERE job = 'CLERK') 
SELECT ROWNUM doc_id, t.* 
FROM   to_doc(e) t

WITH t(c1,c2,c3)  AS ( 
   SELECT NULL, NULL, NULL  
   FROM dual  
   UNION ALL 
   SELECT 1, NULL, NULL  
   FROM dual  
   UNION ALL 
   SELECT NULL, 2, NULL  
   FROM dual  
   UNION ALL 
   SELECT 0, NULL, 3  
   FROM dual) 
SELECT *  
FROM to_doc(t)

For all employees in department 30, display the values of the member with property names ENAME and COMM.

SELECT JSON_VALUE(document, '$.ENAME') ename,  
       JSON_VALUE(document, '$.COMM')  comm  
FROM   to_doc(scott.emp) 
WHERE  JSON_VALUE(document, '$.DEPTNO') = 30