Oracle – How to Make an Existing Column Auto Increment

two options: Use the "datatype" SERIAL or create a sequence and use this sequence as a default value for your integer:

CREATE SEQUENCE your_seq;
CREATE TABLE foo(
  id int default nextval('your_seq'::regclass),
  other_column TEXT
);
INSERT INTO foo(other_column) VALUES ('bar') RETURNING *;

Here's what I would do. I hope I did understand your question correctly. It's a workaround and perhaps not very elegant:

• create a random field, and then an AUTO_INCREMENT field ("ID") as the single field of a primary key, order by RND etc
• create a temporary table with MIN(ID) for each combination of Col1/2 (first, but randomly):

SELECT
  MIN(ID) AS MIN_ID, Col1, Col2 
FROM 
  table 
GROUP BY 
 Col1, Col2

• add a second INT field to be the first_index
• update the table and set first_index to be the difference between the minimum value and the running value of the autoincremented field:

UPDATE 
  table t, temp_table tmp
SET 
  t.first_index = (t.ID - tmp.MIN_ID) + 1
WHERE 
  t.Col1=tmp.Col1 AND t.Col2=tmp.Col2 

• optionally remove the random and ID field, and add a new PK with the three fields Col1/Col2/first_index (auto incremented) for following inserts