Oracle Database: Understanding Lock Conflicts with dba_lock and Column Formatting

Purpose

This Oracle SQL query is designed to investigate and understand lock conflicts within your database, particularly those causing performance bottlenecks. It leverages the dba_lock view to expose critical lock information in a human-readable format, thanks to explicit column formatting.

Sample SQL Command

 1column lock_type format a12
 2column mode_held format a10
 3column mode_requested format a10
 4column blocking_others format a20
 5column username format a10
 6SELECT	session_id
 7,	lock_type
 8,	mode_held
 9,	mode_requested
10,	blocking_others
11,	lock_id1
12FROM	dba_lock l
13WHERE 	lock_type NOT IN ('Media Recovery', 'Redo Thread')
14/

Code Breakdown

• column lock_type format a12
• column mode_held format a10
• column mode_requested format a10
• column blocking_others format a20
• column username format a10 [

These lines are not part of the core query. They are SQL*Plus (or similar client) commands that set the display width and format for specific columns, making the output more visually organized.

SQL

SELECT  session_id
,       lock_type
,       mode_held
,       mode_requested
,       blocking_others
,       lock_id1
FROM    dba_lock l
WHERE   lock_type NOT IN ('Media Recovery', 'Redo Thread')    1.  www.dbaref.com www.dbaref.com
/

• SELECT session_id, lock_type, mode_held, mode_requested, blocking_others, lock_id1: This is the heart of the query, retrieving key columns from the dba_lock view.
• FROM dba_lock l: Specifies the data source. The alias 'l' is optional but can improve readability in more complex queries.
• WHERE lock_type NOT IN ('Media Recovery', 'Redo Thread'): This filter excludes locks related to internal database operations, focusing on user- or application-level lock conflicts.

Key Points and Insights

• session_id: Identifies the session holding or requesting the lock.
• lock_type: The type of lock (e.g., 'TM' for table lock, 'TX' for transaction lock).
• mode_held: The lock mode currently held by the session.
• mode_requested: The lock mode the session is trying to acquire.
• blocking_others: 'YES' if this lock is blocking other sessions, otherwise 'NO'.
• lock_id1: A unique identifier for the locked object (e.g., table, row).

Explanations

This query helps you pinpoint:

• Which sessions are involved in lock conflicts.
• The nature of those conflicts (lock types and modes).
• Whether a particular session is blocking others.

Armed with this information, you can take corrective actions, such as:

• Optimizing application code to minimize lock contention.
• Killing blocking sessions (with caution!) if necessary.
• Considering database configuration changes to improve concurrency.

Remember: Lock conflicts are a normal part of database operation, but excessive or prolonged conflicts can severely impact performance. This query is a valuable tool in your arsenal for diagnosing and resolving such issues.

References

• Oracle Documentation on dba_lock: https://docs.oracle.com/en/database/oracle/oracle-database/21/refrn/DBA_LOCK.html