ORA-01555 Snapshot Too Old - Causes, Solutions & Best Practices

Introduction
Causes of ORA-01555
Implications of ORA-01555
Solutions to ORA-01555
Conclusion

Introduction

The ORA-01555 error, also known as "Snapshot Too Old," is a common issue in Oracle databases that occurs when a query attempts to read data using a consistent read mechanism but finds that the required undo data has been overwritten or aged out of the undo tablespace.

Causes of ORA-01555

Several factors can contribute to the occurrence of ORA-01555:

  • Short Undo Retention: If the undo retention period is not long enough, Oracle may reuse undo space needed for consistent read operations.
  • High Transaction Rates: Continuous updates, deletes, or long-running transactions can cause older undo information to be overwritten quickly.
  • Small Undo Tablespace: Insufficient space allocated to the undo tablespace can lead to premature reuse of undo data.
  • Large Data Modifications: Operations such as bulk inserts or updates may generate a large amount of undo data, increasing the likelihood of undo space reuse.
  • Insufficient Database Tuning: Poorly tuned databases may not effectively manage undo retention or space usage, exacerbating the ORA-01555 error.

Implications of ORA-01555

Encountering ORA-01555 can have several consequences:

  • Query Failures: Queries attempting to read consistent data may fail or return incomplete or inconsistent results.
  • Application Issues: Inconsistencies in data may impact applications relying on accurate and up-to-date information.
  • Performance Degradation: Addressing ORA-01555 may require rolling back transactions or increasing undo retention, potentially impacting database performance.

Solutions to ORA-01555

To mitigate and resolve ORA-01555 errors, consider implementing the following solutions:


1. Adjust Undo Retention: Increase the undo retention period to ensure that Oracle retains old enough undo data for consistent reads.
For example: 

ALTER SYSTEM SET undo_retention = 3600 scope=both; -- Set retention to 1 hour (in seconds)

2. Optimize SQL Queries: Rewrite SQL queries to minimize the need for consistent read operations or reduce undo usage. For example, use Flashback Query instead of relying on undo: 

SELECT * FROM table AS OF TIMESTAMP (SYSTIMESTAMP - INTERVAL '1' HOUR);

3. Monitor Undo Tablespace: Regularly monitor and resize the undo tablespace to accommodate the database workload and avoid space constraints: 

SELECT tablespace_name, round(sum(bytes)/1024/1024, 2) AS "Size (MB)" 
FROM dba_data_files 
WHERE tablespace_name = 'UNDOTBS1' 
GROUP BY tablespace_name;

4. Database Tuning: Implement database tuning practices to optimize undo usage and reduce transaction rates. For example, analyze and tune SQL statements using Oracle AWR reports or SQL Tuning Advisor.
5. Use Flashback Technology: Leverage Oracle Flashback Query or Flashback Version Query to retrieve consistent data versions without relying on undo information: 

SELECT * FROM table AS OF SCN 12345; -- Specify System Change Number (SCN) to query data

Conclusion

ORA-01555: Snapshot Too Old is a critical error in Oracle databases that affects query consistency and performance. By understanding its causes, implications, and applying appropriate solutions such as adjusting undo retention, optimizing SQL queries, and monitoring undo space, database administrators can effectively manage and prevent this error. Implementing proactive database tuning and utilizing Oracle's flashback technologies can further enhance data consistency and optimize database performance in handling ORA-01555 errors.



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Last updated in July, 2024

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