🧩 How to Fix ORA-01555: Snapshot Too Old

🧩 Introduction

The ORA-01555 error occurs when Oracle cannot maintain a consistent read due to undo tablespace limitations. This “Snapshot Too Old” error typically happens during long-running queries, large updates, or complex joins. Understanding how to troubleshoot this error is crucial for DBAs to ensure smooth query execution and database performance.

Step 1: Identify the Problem Query

Check Oracle alert logs or user session reports to find which query triggered ORA-01555.

Long-running queries or queries that process large datasets are the usual culprits.

Step 2: Analyze Undo Tablespace

Verify the size and usage of the undo tablespace:

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

FROM dba_data_files

WHERE tablespace_name = 'UNDOTBS1';

Consider increasing the size of the undo tablespace if it is frequently full.

Step 3: Optimize Queries

Break large queries into smaller, manageable chunks.

Avoid unnecessary complex joins or subqueries that generate huge undo data.

Use temporary tables or hints to reduce undo consumption.

Step 4: Configure Undo Retention

Check current undo retention settings:

SHOW PARAMETER undo_retention;

Increase UNDO_RETENTION to allow longer transactions without triggering the error.

Step 5: Monitor Undo Usage

Monitor undo tablespace usage regularly to anticipate problems:

SELECT tablespace_name, file_id, bytes/1024/1024 AS size_mb, free_space/1024/1024 AS free_mb

FROM dba_undo_extents;

This helps prevent ORA-01555 errors before they impact users.

Step 6: Prevent Future Occurrences

Optimize queries to reduce undo generation.

Allocate sufficient undo tablespace for large transactions.

Regularly monitor undo tablespace and adjust retention parameters.

Conclusion

ORA-01555 errors (“Snapshot Too Old”) are often caused by long-running queries or insufficient undo tablespace. By identifying problematic queries, optimizing their execution, adjusting undo tablespace size, and monitoring usage, DBAs can resolve this error and maintain database stability.

---