PL/SQL V$SESSION View

The V$SESSION view in Oracle Database is an essential tool for database administrators and developers. It provides real-time information about active sessions, helping to monitor user activity, diagnose performance issues, and manage resources effectively. This article explores the structure, common uses, and queries related to V$SESSION.

What is V$SESSION?

V$SESSION is a dynamic performance view (also known as a fixed view) that contains information about each current database session. A session represents a connection between a user and the database, whether it is active or idle.

Key Columns in V$SESSION

Some of the most important columns in V$SESSION include:

SID: Session identifier, unique for each session.

SERIAL#: A unique number assigned to a session that helps identify and kill sessions.

USERNAME: The database user associated with the session.

STATUS: Indicates whether the session is ACTIVE, INACTIVE, KILLED, etc.

SCHEMANAME: The schema in which the session operates.

MACHINE: The client machine from which the session originated.

PROGRAM: The application using the session.

SQL_ID: The SQL statement currently being executed.

EVENT: The wait event the session is experiencing.

Common Uses of V$SESSION

1. Monitoring Active Sessions

To see all active sessions:

SELECT SID, SERIAL#, USERNAME, STATUS, MACHINE, PROGRAM
FROM V$SESSION
WHERE STATUS = 'ACTIVE';

This query helps in identifying users currently executing queries or transactions.

2. Identifying Long-Running Queries

To find sessions running SQL statements for a long time:

SELECT SID, SERIAL#, USERNAME, SQL_ID, EVENT, STATUS
FROM V$SESSION
WHERE STATUS = 'ACTIVE'
AND LAST_CALL_ET > 300;

LAST_CALL_ET (elapsed time in seconds) helps in identifying queries running for more than 5 minutes (300 seconds).

3. Checking Session Wait Events

To diagnose session wait events:

SELECT SID, SERIAL#, EVENT, WAIT_TIME, STATE
FROM V$SESSION
WHERE WAIT_TIME > 0;

This query helps understand if a session is waiting on a resource and what it is waiting for.

4. Terminating a Session

If a session is causing issues, you can terminate it using:

ALTER SYSTEM KILL SESSION 'SID,SERIAL#';

For example, to kill a session with SID 123 and SERIAL# 4567:

ALTER SYSTEM KILL SESSION '123,4567';

Best Practices

Regularly monitor sessions to identify bottlenecks and optimize performance.

Use caution when killing sessions, as it can lead to rollback operations affecting performance.

Combine V$SESSION with V$SQL and V$SESSION_WAIT for deeper query analysis.

Limit user session durations to prevent long-running idle connections that waste resources.

Conclusion

The V$SESSION view is a crucial tool for monitoring database activity, diagnosing performance issues, and managing sessions effectively. By understanding and utilizing its key columns and queries, database administrators can maintain optimal database performance and stability.