How to Identify Oracle Database Orphan Sessions

 In the world of database management, particularly with Oracle databases, "orphan sessions" are a common issue that can affect performance and resource utilization. 

In Oracle databases, an orphan session, sometimes known as a "zombie session," is a session that remains in the database even though its corresponding client process has terminated. These sessions no longer have a user actively interacting with them, yet they consume system resources and can hold locks, leading to performance degradation and blocking issues.

Orphan sessions can occur due to various reasons such as:

• Network issues that disrupt the connection between the client and the server.
• Application or client crashes that terminate the session abnormally.
• Database bugs or misconfigurations.

Queries to Identify Orphan Sessions:

SELECT s.sid, s.serial#, p.spid, s.username, s.program

FROM v$session s

JOIN v$process p ON p.addr = s.paddr

WHERE s.type != 'BACKGROUND';

This query lists active sessions, excluding background processes. It provides session identifiers (sid, serial#), the operating system process identifier (spid), and the username and program name. Orphan sessions often show NULL or unusual entries in the program column.

SELECT s.sid, s.serial#, p.spid, s.username, s.program

FROM v$session s

JOIN v$process p ON p.addr = s.paddr

WHERE s.type != 'BACKGROUND'

AND NOT EXISTS (SELECT NULL FROM v$process WHERE spid = s.process);

This query filters the sessions where the client process (spid) associated with the session does not exist in the v$process view, indicating a potential orphan.

SELECT s.sid, s.serial#, l.object_id, o.object_name, o.object_type

FROM v$session s

JOIN dba_objects o ON o.object_id = l.object_id

JOIN v$lock l ON s.sid = l.sid

WHERE s.sid IN (SELECT sid FROM v$session WHERE ... /* Conditions from above queries */);

This query identifies locks held by sessions suspected to be orphans, which is useful for understanding the impact of these sessions on the database.

How to Manage Orphan Sessions:

Manual Termination: Using the ALTER SYSTEM KILL SESSION command to terminate the identified orphan sessions. Or Kill at OS level with kill -9 spid command.

Automated Monitoring and Cleanup: Implementing automated scripts or database jobs to periodically identify and clean up orphan sessions.

Prevention: Addressing the root causes, such as network stability and application robustness, can reduce the occurrence of orphan sessions.