Answer / guest

The most common reason is that the rollback segments are
too small
http://www.cryer.co.uk/brian/oracle/ORA01555.htm

Answer / bezalel

ORA-01555:
-----------
snapshot too old: rollback segment number string
with name "string" too small

Cause:
-------
rollback records needed by a reader for
consistent read are overwritten by other writers

Action:
-------
If in Automatic Undo Management mode, increase
undo_retention setting. Otherwise, use larger rollback segments

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