Databases

Query To Check Concurrency In Oracle

By on Monday, May 5, 2025

Concurrency control is a crucial aspect of database management ensuring that multiple transactions can run simultaneously without causing data inconsistencies. In Oracle Database concurrency management is handled through locks isolation levels and transaction control mechanisms.

This guide explores how to check concurrency in Oracle using SQL queries analyze locking issues and optimize database performance for multi-user environments.

Understanding Concurrency in Oracle

Concurrency refers to the ability of multiple transactions to access and modify data simultaneously. While concurrency improves system efficiency it can also lead to conflicts such as deadlocks blocking sessions and inconsistent reads.

Oracle ensures concurrency through:

  • Multiversion Concurrency Control (MVCC) – Allows read consistency without locking rows.
  • Locks and Latches – Prevents data corruption by controlling access.
  • Isolation Levels – Defines how transactions interact with each other.

Understanding these mechanisms helps in writing efficient queries to monitor and troubleshoot concurrency issues.

Checking Active Sessions in Oracle

To identify active database sessions that might be affecting concurrency use the following query:

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

Explanation:

  • SID and SERIAL# – Identify the session uniquely.
  • USERNAME – Shows the database user.
  • STATUS – Indicates whether the session is ACTIVE or INACTIVE.
  • OSUSER and MACHINE – Provide system-level details.

This query helps track active users and detect potential concurrency issues.

Checking Blocking Sessions

A blocking session occurs when one session holds a lock on a resource that another session needs. To identify blocking sessions run:

SELECT BLOCKING_SESSION SID SERIAL# EVENT WAIT_CLASS 
FROM V$SESSION 
WHERE BLOCKING_SESSION IS NOT NULL;

Explanation:

  • BLOCKING_SESSION – The SID of the session holding the lock.
  • SID SERIAL# – Identify blocked sessions.
  • EVENT – Shows what the blocked session is waiting for.
  • WAIT_CLASS – Categorizes the wait event.

If multiple sessions are blocked the issue might be a deadlock or long-running transaction.

Checking Locked Objects

To see which objects are locked and by whom use:

SELECT O.OBJECT_NAME O.OBJECT_TYPE S.SID S.SERIAL# L.LOCKED_MODE
FROM V$LOCK L 
JOIN DBA_OBJECTS O ON L.ID1 = O.OBJECT_ID
JOIN V$SESSION S ON L.SID = S.SID;

Explanation:

  • OBJECT_NAME – Shows the locked table or index.
  • OBJECT_TYPE – Specifies if it’s a table view or index.
  • SID SERIAL# – Identify the session holding the lock.
  • LOCKED_MODE – Displays the lock mode (e.g. exclusive share).

Understanding lock modes helps determine if a session is causing excessive blocking.

Detecting Deadlocks in Oracle

Deadlocks occur when two sessions hold locks that the other needs leading to a circular wait. Oracle automatically resolves deadlocks but you can check them using:

SELECT * FROM V$LOCK WHERE REQUEST > 0;

For detailed deadlock analysis view Oracle’s trace files in the alert.log.

To forcefully resolve a deadlock kill the offending session:

ALTER SYSTEM KILL SESSION 'SIDSERIAL#' IMMEDIATE;

Replace SID and SERIAL# with the values from V$SESSION.

Monitoring Long-Running Transactions

Transactions that run too long can hold locks for extended periods affecting concurrency. To check for long-running transactions:

SELECT S.SID S.SERIAL# T.START_TIME T.STATUS 
FROM V$TRANSACTION T 
JOIN V$SESSION S ON T.SES_ADDR = S.SADDR
ORDER BY T.START_TIME ASC;

Explanation:

  • START_TIME – Shows when the transaction began.
  • STATUS – Indicates whether the transaction is active or pending.

If a transaction runs longer than expected consider reviewing its SQL statements.

Best Practices for Improving Concurrency

1. Use Proper Indexing

  • Indexes reduce the need for full table scans minimizing lock contention.

2. Commit Transactions Promptly

  • Long-running transactions increase lock duration so commit frequently.

3. Use the Appropriate Isolation Level

  • For high concurrency use READ COMMITTED instead of SERIALIZABLE to reduce locking.
ALTER SESSION SET ISOLATION_LEVEL = READ COMMITTED;

4. Avoid Unnecessary Locks

  • Use SELECT ... FOR UPDATE only when required to prevent excessive locking.

5. Monitor and Tune Queries Regularly

  • Identify slow queries using the Oracle SQL Performance Analyzer.

Concurrency control in Oracle is essential for ensuring smooth multi-user access to databases. By monitoring active sessions detecting blocking issues and optimizing queries you can prevent deadlocks and performance slowdowns.

Using the SQL queries provided in this guide database administrators can track locks resolve conflicts and enhance database efficiency. Regular monitoring and tuning ensure that concurrent transactions run without conflicts leading to a more stable system.

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