ORA-01555: snapshot too old error when deleting with transactions - oracle

Question

I hope someone can explain me why I'm seeing ORA-01555. I have a function and a procedure to perform a cleanup in a huge table:
-- Small cleanup in separate transaction.
FUNCTION clean_single(ts_until IN TIMESTAMP, datapoint_id IN NUMBER) RETURN NUMBER IS
PRAGMA AUTONOMOUS_TRANSACTION;
BEGIN
DELETE FROM VALUES WHERE DATAPOINT_ID = datapoint_id AND TS < ts_until;
COMMIT;
RETURN sql%rowcount;
END clean_single;
PROCEDURE prc_clean IS
count_all_deleted_vals NUMBER(10) := 0;
BEGIN
BEGIN
FOR dps IN (
SELECT x AS dpid, y as tsUntil FROM Z where some conditions
LOOP
count_all_deleted_vals := count_all_deleted_vals + clean_single(dps.tsUntil, dps.dpid);
END LOOP;
DBMS_OUTPUT.PUT_LINE('Removed ' || count_all_deleted_vals || ' values');
END;
END prc_clean;
The idea is to run prc_clean() from a job and after the relevant datapoint ids are selected, the deletion per datapoint-id is done in a single transaction to avoid having one huge transaction.
But when I run this it runs for a while and then fails with ORA-01555.
In detail i do not understand why this is happening. Why does the PRAGMA AUTONOMOUS_TRANSACTION; in the function not prevent this?
What can I do to prevent it?

Answer 1

As far as I can tell, cause of ORA-01555 is often committing within a loop - and that's exactly what you're doing.
Skip the function altogether (doing DML in it is usually wrong anyway) and use only procedure, e.g.
PROCEDURE prc_clean
IS
count_all_deleted_vals NUMBER (10) := 0;
BEGIN
FOR dps IN (SELECT x AS dpid, y AS tsUntil
FROM Z
WHERE some conditions)
LOOP
DELETE FROM VALUES
WHERE DATAPOINT_ID = dps.dpid
AND TS < dps.tsuntil;
count_all_deleted_vals := count_all_deleted_vals + SQL%ROWCOUNT;
END LOOP;
COMMIT;
DBMS_OUTPUT.PUT_LINE ('Removed ' || count_all_deleted_vals || ' values');
END;