ORA-01555 for auto extendable UNDO segment

Question

I have an undo tablespace with the AUTOEXTEND option enabled. But sometimes I receive error: ORA-01555 snapshot too old: rollback segment number 5 with name "_SYSSMU234124$" too small... But how it can be? when space is low, instead of overwriting unexpired undo information, the tablespace auto-extends.

Answer 1

    show parameter undo_retention;
    ALTER SYSTEM SET UNDO_RETENTION = 3600;
  show parameter undo_retention;