Oracle Database - UNDO (Rollback Segment)

What is UNDO ?

rollback journal in Oracle.

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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

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Reference/Documentation

• What is UNDO ?
• Why Undo when you have Redo ?