How to resolve ORA-01555 Snapshot Too Old?
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The ORA-01555 error, Snapshot Too Old occurs in Oracle databases when a transaction requires data from a consistent point in time (a snapshot), but the necessary data has been overwritten or aged out from the undo tablespace.

Here are some steps you can take to address the ORA-01555 error:

1. Increase the UNDO retention: By increasing the UNDO retention time , you can extend the duration for which Oracle keeps undo data in the undo tablespace. This allows queries or transactions that require older data to access the necessary information. You can modify the UNDO_RETENTION parameter in the initialization file or using the ALTER SYSTEM command.
Note that increasing UNDO_RETENTION may require additional space in the undo tablespace. 
alter system set undo_retention=3600 scope=both;  #default:900

2. Adjust the size of the undo tablespace: If the undo tablespace is too small to retain the required amount of undo data, you may encounter the ORA-01555 error. Monitor the space usage of the undo tablespace and consider increasing its size to accommodate the workload and retention requirements. 
# To extend an exsting UNDO datafile
alter database datafile 'D:\ORACLE_DB\TESTDB\TESTDB\UNDOTBS01.DBF' resize 1500M;

# To add a new datafile to UNDO tablespace
alter tablespace UNDOTBS1 ADD DATAFILE 'D:\ORACLE_DB\TESTDB\TESTDB\UNDOTBS02.DBF' size 2000M;
3. Optimize long-running queries or transactions: Long-running queries or transactions that access a large amount of data can contribute to the ORA-01555 error. Review and optimize such operations to reduce their impact on the undo tablespace. Strategies may include reducing the amount of data accessed, optimizing indexes, rewriting queries, or breaking down transactions into smaller units.

4. Implement Automatic Undo Management (AUM): Automatic Undo Management simplifies undo space management by allowing Oracle to manage the undo tablespace automatically. If you are not already using AUM, consider enabling it. AUM ensures that the undo tablespace is efficiently utilized and can help mitigate the ORA-01555 error. 

#To check current setting
SHOW PARAMETER UNDO_MANAGEMENT

#Static Parameters - to change the parameter value
ALTER SYSTEM SET UNDO_MANAGEMENT=AUTO SCOPE=SPFILE;

5. Adjust the transaction's isolation level: In some cases, changing the isolation level of the transaction can resolve the ORA-01555 error. For example, reducing the isolation level to READ COMMITTED can help avoid reading consistent data from previous snapshots.

6. Enable Flashback Query: If your Oracle version supports it, consider using Flashback Query to retrieve past versions of data. Flashback Query allows you to query data as it appeared at specific points in time, which can help avoid the need for accessing older undo data.

It's crucial to carefully evaluate the impact of any changes to your database configuration, especially when modifying retention times or increasing the size of the undo tablespace. Consider the space requirements, performance implications, and overall system stability when implementing solutions.


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