Question

I’m looking for the best way to change a data type of a column in a populated table. Oracle only allows changing of data type in colums with null values.

My solution, so far, is a PLSQL statement which stores the data of the column to be modified in a collection, alters the table and then iterates over the collection, restoring the original data with data type converted.

-- Before: my_table ( id NUMBER, my_value VARCHAR2(255))
-- After: my_table (id NUMBER, my_value NUMBER)

DECLARE
  TYPE record_type IS RECORD ( id NUMBER, my_value VARCHAR2(255));
  TYPE nested_type IS TABLE OF record_type;
  foo nested_type;
BEGIN
  SELECT id, my_value BULK COLLECT INTO foo FROM my_table;
  UPDATE my_table SET my_value = NULL;
  EXECUTE IMMEDIATE 'ALTER TABLE my_table MODIFY my_value NUMBER';
  FOR i IN foo.FIRST .. foo.LAST
  LOOP
    UPDATE my_table 
        SET  = TO_NUMBER(foo(i).my_value) 
        WHERE my_table.id = foo(i).id;
  END LOOP;
END;
/

I’m looking for a more experienced way to do that.

Answer 1

The solution is wrong. The alter table statement does an implicit commit. So the solution has the following problems:

• You cannot rollback after alter the alter table statement and if the database crashes after the alter table statement you will loose data
• Between the select and the update users can make changes to the data

Instead you should have a look at oracle online redefinition.