How to debug ORA-42016 with DBMS_REDEFINITION?

I am writing a number of scripts to transform some tables using DBMS_REDEFINITION. Is there an easy way to debug the “ORA-42016: shape of interim table does not match specified column mapping” error?

I am generating the column map dynamically with PL/SQL similar to this:

CREATE TABLE my_table_redef AS SELECT * FROM my_table WHERE 1=0;

ALTER TABLE my_tabl_redef ADD (
    column_b INT,
    column_id INT NOT NULL
);

DECLARE
    col_map VARCHAR2(32767);
    CURSOR col_cursor IS
        SELECT column_name
        FROM   user_tab_columns
        WHERE  table_name = 'MY_TABLE'
        ORDER BY column_id;
BEGIN
    FOR col IN col_cursor LOOP
        col_map := col_map || col.column_name || ' ' || col.column_name || ', ';
    END LOOP;

    col_map := col_map || 'my_function(column_a) column_b, '
                       || 'my_sequence.NEXTVAL   column_id';

    DBMS_REDEFINITION.START_REDEF_TABLE('user', 'my_table', 'my_table_redef', col_map, DBMS_REDEFINITION.CONS_USE_ROWID);
END;
/

The function my_function works for other DBMS_REDEFINITIONs, so I know that’s not the problem.

I’ve tried outputting col_map and doing:

INSERT INTO my_table_redef
SELECT <col_map>
FROM   my_table;

with success, so I know that there are no columns wrong.

What else can I try/do to figure out what’s causing the problem?

Edit: In this case, the problem turned out to be typos in the generated column map, which obviously would not be caught by my insert/select test. To that end, I will add a parsing out the destination column names and doing a select from the redef table to my repertoire of debug techniques.