Understanding Rollback Segments in Oracle Databases

Rollback segments are essential components of Oracle databases, providing a mechanism to reverse uncommitted transactions and maintain data consistency. This post presents a SQL query for inspecting rollback segments, as well as explanations and insights into their role.

Sample SQL Command

1select	segment_name
2,	status
3from	dba_rollback_segs
4/

Purpose

This SQL code serves to:

• List Rollback Segments: Generate a clear listing of all rollback segments existing within your Oracle database.
• Show Status: Display the current status of each rollback segment (e.g., 'ONLINE,' 'OFFLINE,' 'AVAILABLE', etc.).

Breakdown

• select segment_name, status: The core of the query, retrieving the names and statuses of the rollback segments.
• from dba_rollback_segs: This indicates that the data is obtained from the dba_rollback_segs data dictionary view, which holds information about rollback segments.

Key Points

• Transaction Management: Rollback segments store 'undo' information, allowing the database to roll back changes if a transaction fails or is aborted.
• Status Meanings:
  • 'ONLINE': Segment is active and in use.
  • 'OFFLINE': Segment is not currently in use.
  • 'AVAILABLE': Segment is currently not in use but ready to be used if needed.
  • 'NEEDS RECOVERY': Segment contains data that needs to be recovered.

Insights

• System Activity: The number and status of rollback segments can indicate database transaction load.
• Troubleshooting: Status information can be valuable when diagnosing transaction-related issues.
• Configuration: This query helps in assessing rollback segment configuration and sizing.

References

Oracle Database Documentation on Rollback Segments https://docs.oracle.com/en/database/oracle/oracle-database/21/refrn/ROLLBACK_SEGMENTS.html

Oracle Documentation on Data Dictionary Views: https://docs.oracle.com/en/database/oracle/oracle-database/21/refrn/toc.htm