🧩 ORA-01555: Snapshot Too Old Error in Oracle Database

🧩 Introduction

The ORA-01555 error, commonly known as the Snapshot Too Old error, occurs when a query tries to access older undo information that is no longer available in the UNDO tablespace. This is a frequent issue when running long queries or batch processes, and it indicates that Oracle was unable to maintain enough undo data for consistent reads.

Step 1: Identify the Cause

Check the alert log or trace files to confirm the error.

SHOW ALERT;

Usually, the problem arises due to:

Long-running queries.

Insufficient undo tablespace size.

High DML activity during query execution.

Step 2: Monitor Undo Tablespace Usage

Query to check current undo tablespace size and usage:

SELECT tablespace_name, file_name, bytes/1024/1024 AS size_mb

FROM dba_data_files

WHERE tablespace_name LIKE 'UNDO%';

Step 3: Increase Undo Tablespace

If the undo space is too small, add more space:

ALTER DATABASE DATAFILE '/u01/oradata/ORA/undotbs01.dbf' RESIZE 1G;

Or add a new datafile:

ALTER DATABASE ADD DATAFILE '/u01/oradata/ORA/undotbs02.dbf' SIZE 500M AUTOEXTEND ON;

Step 4: Tune the Query

Break long-running queries into smaller batches.

Avoid large joins without indexes.

Ensure statistics are up to date.

Step 5: Preventive Measures

Use a properly sized undo tablespace.

Monitor undo usage regularly.

Optimize queries to reduce undo consumption.

Conclusion

The ORA-01555 error is one of the most common issues Oracle DBAs face. It is often a sign of insufficient undo space or poorly tuned queries. By enlarging the undo tablespace and improving query design, you can prevent this error and maintain smooth database operations.