ORA-1555 error

The ORA-1555 error means that a consistent get on a particular database block has failed.

When a transaction or query begins, the current SCN is recorded. That SCN serves as the snapshot SCN for the query or transaction. This term is derived from the requirement that the transaction or query must see a consistent snapshot of the database at that time.
Every block used to select rows for the query or transaction must reflect the state of the database at the snapshot SCN. This applies to the selection of rows to be updated or deleted, as much as it does to the selection of rows for a query. If a block has to be changed, then those changes will be applied to the current version of that block. However, the selection of the rows to be changed must be based on a version of the block consistent with the snapshot SCN. The temporary reconstruction of a version of the block consistent with the snapshot SCN is called a consistent get.
This error can be the result of there being insufficient rollback segments. A query may not be able to create the snapshot because the rollback data is not available. This can happen when there are many transactions that are modifying data, and performing commits and rollbacks. Rollback data is overwritten when the rollback segments are too small for the size and number of changes that are being performed.
To correct this problem, make more larger rollback segments available. Your rollback data for completed transactions will be kept longer.

This error can be the result of programs not closing cursors after repeated FETCH and UPDATE statements. To correct this problem, make sure that you are closing cursors when you no longer require them.

This error can occur if a FETCH is executed after a COMMIT is issued. The number of rollback records created since the last CLOSE of your cursor will fill the rollback segments and you will begin overwriting earlier records.