Ora-01555 : Snapshot too old error
By this time you might have come accross this ora-01555 error lot of times. Before learning about the causes of the Ora-01555 error, you need to understand about Read Consistency and dirty reads in Oracle.
Read Consistency : Let us take an example of Reading a 1 million records from a table emp through a cursor.
While fetching the data from the cursor in session A, if another session B updates the data simultaneously in the table emp by which 10000 records has been affected in the set of 1 million records. if these 10000 rows are not yet read by the cursor in session A, then
Whether the Oracle would read the updated records to Session A ? NO.