Undo

How to Resolve ORA-01466: unable to read data - table definition has changed

by Ed Chen
March 16, 2023March 21, 2023

ORA-01466 Tried to perform a flashback query, but it failed with ORA-01466. SQL> select count(*) cnt from t1 as of timestamp to_timestamp('2023-02-19 22:15:00', 'yyyy-mm-dd hh24:mi:ss');… Read More »

Truncate vs Delete in Oracle

by Ed Chen
May 27, 2020December 10, 2022

Truncate vs Delete Here I compare the differences between TRUNCATE and DELETE an entire table in the following chart. Question Truncate Table Delete From DDL… Read More »

How to Resolve ORA-01555 UNDO_RETENTION

by Ed Chen
April 8, 2016December 11, 2022

ORA-01555 UNDO_RETENTION Let's see the error ORA-01555 UNDO_RETENTION first: Description ORA-01555: snapshot too old: rollback segment number string with name "string" too small Cause rollback… Read More »

UNDO_RETENTION, How and Why

by Ed Chen
April 7, 2016December 11, 2022

UNDO_RETENTION At times, you may find out or feel that UNDO_RETENTION is useless at all. Raising the number of seconds cannot resolve ORA-01555. This is… Read More »

How to Resolve SMON: about to recover undo segment xx SMON: mark undo segment xx as available

by Ed Chen
July 12, 2015March 30, 2018

Repeated messages were found in the alert log after instance startup:...SMON: about to recover undo segment 32SMON: mark undo segment 32 as availableSMON: about to recover… Read More »

How to Resolve ORA-01562: failed to extend rollback segment number

by Ed Chen
May 13, 2015March 30, 2018

Found repeated errors in the alert log:...ORA-01562: failed to extend rollback segment number 96...It seemed that some transactions were doing DML, but they failed to… Read More »