🧩 How to Fix ORA-01555: Snapshot Too Old Error in Oracle

🧩 Introduction:

The ORA-01555 error occurs when Oracle cannot retrieve a consistent read due to insufficient undo tablespace. This usually happens during large queries, long-running operations, or when undo tablespace is small. Understanding and resolving this issue is critical to ensure smooth database operations.

Step 1: Identify the Query Causing the Error

Review the SQL statements being executed. ORA-01555 typically occurs during large SELECT statements or UPDATE operations with complex joins:

SELECT * 

FROM big_table t1

JOIN another_table t2 ON t1.id = t2.id

WHERE t1.status = 'ACTIVE';

Step 2: Check Undo Tablespace Usage

Check the size and usage of undo tablespaces:

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

FROM dba_undo_extents;

Step 3: Increase Undo Tablespace

If undo tablespace is insufficient, add a new datafile:

ALTER DATABASE ADD UNDO TABLESPACE undotbs2 DATAFILE '/u01/app/oracle/oradata/undotbs02.dbf' SIZE 200M AUTOEXTEND ON;

Step 4: Optimize Long Queries

Break large queries into smaller parts.

Avoid unnecessary joins or large full-table scans.

Use COMMIT more frequently in long DML operations to free undo space.

Step 5: Example Scenario

A user executes:

UPDATE big_table SET status='CLOSED' WHERE condition = 'XYZ';

If the undo tablespace is too small for the operation, ORA-01555 occurs. Adding undo space and committing periodically resolves the error.

Step 6: Preventive Measures

Monitor undo tablespace regularly.

Enable autoextend on undo datafiles.

Analyze and optimize long-running queries or transactions.

Conclusion:

ORA-01555 errors indicate insufficient undo space for consistent reads. By monitoring undo tablespaces, optimizing queries, and increasing available space, DBAs can prevent snapshot too old errors and ensure reliable database operations.