ORA-01555 Snapshot Too Old Error in Oracle – Complete Solution Guide

ORA-01555: Snapshot Too Old Error in Oracle – Complete Fix

The ORA-01555: snapshot too old error is one of the most common and frustrating issues faced by Oracle Database users. It usually occurs during long-running queries when Oracle cannot retrieve the required old data from the undo tablespace.

In this detailed guide, we will explain the causes, solutions, and best practices to fix and prevent this error in Oracle 11g, 12c, and later versions.

Quick Solution: Increase undo tablespace size and set UNDO_RETENTION to a higher value (e.g., 1800 seconds) to prevent ORA-01555 error.

What is ORA-01555 Snapshot Too Old Error?

ORA-01555 snapshot too old is an Oracle Database error that occurs when a query cannot access the required undo data because it has been overwritten. This usually happens due to a small undo tablespace, low undo retention, or long-running queries.

If you are also facing recovery-related issues like ORA-01194 file needs more recovery to be consistent, check our detailed guide.

Error Message

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

Why does ORA-01555 occur?

This error occurs when Oracle cannot access the required undo data needed to maintain read consistency for a query.

Common causes include:

• Undo tablespace is too small
• Undo retention period is too low
• Long-running queries
• Frequent commits in loops
• High DML activity (INSERT, UPDATE, DELETE)

Understanding the Problem

Oracle uses undo data to provide a consistent snapshot of data. If the undo data is overwritten before a query finishes, Oracle throws the ORA-01555 error.

How to prevent ORA-01555 in Oracle?

1. Increase UNDO tablespace size
2. Increase UNDO_RETENTION parameter
3. Avoid frequent commits in loops
4. Optimize long-running queries
5. Use RETENTION GUARANTEE (Optional)
6. Monitor undo usage regularly

Step-by-Step Solutions

1. Increase UNDO Tablespace Size

The most effective solution is to increase the size of the undo tablespace.

ALTER DATABASE DATAFILE '/path/undo01.dbf' RESIZE 2G;

Or add a new datafile:

ALTER TABLESPACE UNDOTBS1 
ADD DATAFILE '/path/undo02.dbf' SIZE 1G AUTOEXTEND ON;

2. Increase UNDO_RETENTION

Set a higher undo retention value to retain undo data for a longer period.

ALTER SYSTEM SET UNDO_RETENTION = 1800;

(Value is in seconds)

3. Avoid Frequent Commits in Loops

Frequent commits can overwrite undo data quickly.

❌ Bad Practice:

FOR i IN 1..10000 LOOP
   UPDATE table_name SET column=value;
   COMMIT;
END LOOP;

✅ Good Practice:

FOR i IN 1..10000 LOOP
   UPDATE table_name SET column=value;
END LOOP;
COMMIT;

4. Optimize Long-Running Queries

Try to reduce query execution time by:

• Using proper indexes
• Avoiding full table scans
• Breaking large queries into smaller parts

5. Use RETENTION GUARANTEE (Optional)

This ensures undo data is not overwritten before retention time.

ALTER TABLESPACE UNDOTBS1 RETENTION GUARANTEE;

Note: Use carefully as it may cause space issues.

6. Monitor Undo Usage

Check undo usage using:

SELECT tablespace_name, status, contents FROM dba_tablespaces;

Real-World Scenario

A report query was running for 30 minutes while heavy updates were happening in the database. The undo tablespace was small, and undo data got overwritten.

This resulted in ORA-01555 error.

Solution applied:

• Increased undo tablespace
• Set UNDO_RETENTION to higher value

Issue resolved successfully.

Common Mistakes

• Keeping undo tablespace too small
• Frequent commits in loops
• Ignoring long-running queries
• Not monitoring undo usage

Authentication issues can also cause database problems. Learn how to fix ORA-01017 invalid username/password error.

Best Practices

• Always size undo tablespace properly
• Set appropriate UNDO_RETENTION
• Avoid unnecessary commits
• Optimize queries regularly
• Monitor database performance

ORA-01555 Quick Fix Summary

Problem	Solution
Small Undo Tablespace	Increase size
Low Undo Retention	Increase UNDO_RETENTION
Frequent Commits	Avoid commits in loops
Long Queries	Optimize queries

Frequently Asked Questions

What causes ORA-01555?
It is caused by insufficient undo data due to small tablespace or long-running queries.

How do I fix ORA-01555?
Increase undo tablespace size, retention, and optimize queries.

Can ORA-01555 be prevented?
Yes, by proper database tuning and avoiding frequent commits.

Conclusion

The ORA-01555 error occurs due to insufficient undo data for long-running queries. By increasing undo tablespace, adjusting retention, and optimizing queries, you can effectively resolve and prevent this issue.

If you are facing connection issues, check our guide on ORA-12541 TNS no listener error.

Proper database management and monitoring are key to avoiding such errors in the future.

Related Posts

• ORA-01194 File Needs More Recovery – Solution
• ORA-01017 Invalid Username/Password – Fix
• ORA-12541 TNS No Listener – Solution