Oracle 23c

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create function f_apply_transform(p in varchar2)
  return varchar2
is
begin
  dbms_output.put(p);
  return p || '_converted';
end;
/
create package pkg_transform as
  
  function describe(tab in out DBMS_TF.TABLE_T, col in DBMS_TF.COLUMNS_T)
    return DBMS_TF.DESCRIBE_T
  ;

  procedure fetch_rows;

end pkg_transform;
/
select *
from user_errors
create package body pkg_transform as


  function describe(tab in out DBMS_TF.TABLE_T, col in DBMS_TF.COLUMNS_T)
    return DBMS_TF.DESCRIBE_T
  is
    new_cols DBMS_TF.COLUMNS_NEW_T;
    new_id pls_integer := 1;
  begin
    for i in 1..tab.column.count loop

      for j in 1..col.count loop
        if tab.column(i).description.name = col(j) then
          /*Add column to modification list and remove from pass through*/
          tab.column(i).for_read := true;
          tab.column(i).pass_through := false;
          new_cols(new_id) := tab.column(i).description;
          new_cols(new_id).type := DBMS_TF.TYPE_VARCHAR2;
          new_cols(new_id).max_len := 1000;
          new_id := new_id + 1;
          exit;
        end if;
      end loop;
    end loop;

    return DBMS_TF.DESCRIBE_T(new_columns => new_cols);
  end;

  procedure fetch_rows
  is
    env DBMS_TF.ENV_T := DBMS_TF.get_env();
    rowset DBMS_TF.ROW_SET_T;
    rowcount pls_integer;
    converted_col DBMS_TF.TAB_VARCHAR2_T;
  begin
    /*Get input*/
    DBMS_TF.get_row_set(rowset => rowset, row_count => rowcount);
    /*Process each column*/
    for i in 1..rowset.count loop
      /*Process each row in column vector*/
      for j in 1..rowcount loop
         /*Modify a value*/
         converted_col(j) := f_apply_transform(rowset(i).tab_varchar2(j));
      end loop;
      /*Put modified vector in place of old column*/
      DBMS_TF.put_col(i, converted_col);
    end loop;
  end;

end pkg_transform;
/
select *
from user_errors
create function f_convert(tab table, col columns)
  return table
pipelined row polymorphic
using pkg_transform;
/
with sample(a,b,c) as (
  select dbms_random.string('U',15), dummy, lpad(level, 3, '0')
  from dual
  connect by level <= 10
)
select *
from f_convert(sample, columns(a, b))


C
A
B


001
ACNQONWIMSNBHHU_converted
X_converted

002
OOJBAXPGQKKIKMR_converted
X_converted

003
YLXBSKURCTWERXY_converted
X_converted

004
IZPZTVUAYHKNWBA_converted
X_converted

005
FUTKTGMCYXIKOTY_converted
X_converted

006
WYFFKEEFNQFWKXU_converted
X_converted

007
LZLWDPKEZXGHZNN_converted
X_converted

008
TQYEMVRAGOWAQUQ_converted
X_converted

009
AYCKTLAZXYTEMEY_converted
X_converted

010
SXWMUSRCXBZVLMY_converted
X_converted

C	A	B
001	ACNQONWIMSNBHHU_converted	X_converted
002	OOJBAXPGQKKIKMR_converted	X_converted
003	YLXBSKURCTWERXY_converted	X_converted
004	IZPZTVUAYHKNWBA_converted	X_converted
005	FUTKTGMCYXIKOTY_converted	X_converted
006	WYFFKEEFNQFWKXU_converted	X_converted
007	LZLWDPKEZXGHZNN_converted	X_converted
008	TQYEMVRAGOWAQUQ_converted	X_converted
009	AYCKTLAZXYTEMEY_converted	X_converted
010	SXWMUSRCXBZVLMY_converted	X_converted