What is UNDO ?

rollback journal in Oracle.

Support

ORA-01555 - snapshot too old

ORA-01555: snapshot too old: rollback segment number 12 with name "_SYSSMU12_253434260$" too small '

  • The UNDO space was too small. The SELECT statement was killed letting the DML operations continue.
  • Of the Minimum Undo Retention Period was achieved Minimum Undo Retention

See AskTom

Reference/Documentation