Session Enqueue Wait Analysis (vsessenq.sql)

Use at Your Own Risk

This script is for educational purposes only. Always test in development environments before production use. See our disclaimer for full terms.

What This Script Does

This script queries Oracle views to provide database information via the vsessenq.sql script.

The Script

rem  vsessenq.sql
rem
set linesize 132
rem
ttitle 'Session Waits - Enqueues'
rem
col event format a29 heading 'EVENT'
col sid format 9999 heading 'SID'
col seq# format 99999 heading 'SEQ'
col wait_time format b999 heading 'WAIT|TIME'
col seconds_in_wait format b999999 heading 'SECONDS|IN WAIT'
col state format a17 heading 'STATE'
col parm1 format a20 heading 'PARM1'
col parm2 format a20 heading 'PARM2'
col parm3 format a20 heading 'PARM3'
col parameters format a59 heading 'PARAMETERS'
rem
select w.sid,
       seq#,
       wait_time,
/*
select event,
       seconds_in_wait,
       state,
       p1text, p1, p1raw,
       p2text, p2, p2raw,
       p3text, p3, p3raw
       p1text || ' = ' || p1 parm1,
       p2text || ' = ' || p2 parm2,
       p3text || ' = ' || p3 parm3
*/
       decode( p1text, null, null,
               p1text || ' = ' || p1 ) ||
       decode( p2text, null, null,
               ', ' || p2text || ' = ' || p2 ) ||
       decode( p3text, null, null,
               ', ' || p3text || ' = ' || p3 )
       parameters,
       row_wait_obj#,
       row_wait_file#,
       row_wait_block#,
       row_wait_row#
  from v$session s,
       v$session_wait w
 where w.sid = s.sid
   and w.event = 'enqueue'
 order by event, p1, p2, p3;
rem
set linesize 80

What This Script Does

The vsessenq.sql script monitors and analyzes session-level enqueue waits in Oracle databases. It identifies sessions that are currently waiting on enqueue locks, providing detailed information about the specific lock type, parameters, and row-level wait information. This is crucial for diagnosing blocking and deadlock scenarios.

Usage

SQL> @vsessenq.sql

No parameters required - The script automatically identifies all sessions currently waiting on enqueue events.

Required Privileges

• SELECT privilege on V$SESSION and V$SESSION_WAIT (usually requires DBA role or specific grants)

Sample Output

Session Waits - Enqueues

  SID   SEQ WAIT PARAMETERS                                          ROW_WAIT_OBJ# ROW_WAIT_FILE# ROW_WAIT_BLOCK# ROW_WAIT_ROW#
----- ----- ---- --------------------------------------------------------- ------------- -------------- --------------- -------------
  156   423    0 name|mode = 1415053318, id1 = 65537, id2 = 25            75234               4            12845            0
  234   156    0 name|mode = 1415053318, id1 = 65537, id2 = 25            75234               4            12845            0
  445   789    0 name|mode = 1414812756, id1 = 131073, id2 = 0           102456               5            23451            2

Key Output Columns

• SID: Session ID experiencing the enqueue wait
• SEQ: Sequence number of the wait event
• WAIT TIME: Time since the wait began (0 = currently waiting)
• PARAMETERS: Decoded enqueue parameters showing lock type and identifiers
• ROW_WAIT_OBJ#: Object ID of the object being waited on
• ROW_WAIT_FILE#: File number containing the waited-on row
• ROW_WAIT_BLOCK#: Block number containing the waited-on row
• ROW_WAIT_ROW#: Specific row number being waited on

Understanding Enqueue Parameters

Common Enqueue Types

The name|mode parameter encodes the enqueue type and mode:

• TX (Transaction Lock): Row-level locking conflicts
• TM (Table Lock): Table-level locking issues
• DX (Distributed Transaction): Distributed transaction locks
• UL (User Lock): Application-defined locks

Parameter Breakdown

• id1/id2: Additional identifiers specific to the enqueue type
  • For TX locks: usually USN and slot numbers
  • For TM locks: object ID and table lock mode
  • For other locks: varies by lock type

Common Use Cases

1. Blocking Session Analysis

   • Identify sessions waiting for locks held by other sessions
   • Determine the scope and impact of blocking scenarios

2. Deadlock Investigation

   • Analyze enqueue waits that may lead to deadlock situations
   • Identify circular wait dependencies

3. Row-Level Lock Monitoring

   • Monitor specific row-level locking conflicts
   • Identify hot blocks or rows causing contention

4. Application Troubleshooting

   • Diagnose application-level locking issues
   • Identify poorly designed transaction patterns

Performance Analysis

Wait Time Interpretation

• wait_time = 0: Session is currently waiting
• wait_time > 0: Time since the wait completed (historical)

Row Wait Information

• Non-zero row_wait values: Specific row-level waits
• Zero values: Table or object-level waits

Troubleshooting Guide

High Enqueue Waits

1. Identify Blocking Sessions:

   SELECT blocking_session, sid, seconds_in_wait
   FROM v$session
   WHERE blocking_session IS NOT NULL;

2. Check Lock Holdings:

   SELECT * FROM v$lock
   WHERE sid IN (blocking_session_list);

Common Enqueue Issues

• TX Locks: Often indicate row-level blocking or deadlocks
• TM Locks: Usually table-level operations conflicts
• Long-running waits: May indicate hung transactions

Related Scripts

• gvlock.sql - Comprehensive lock analysis across all instances
• gvlockb.sql - Blocking lock analysis
• gvsess.sql - General session analysis

Script Features

• Real-time Monitoring: Shows current enqueue waits as they occur
• Detailed Parameters: Decodes enqueue parameters for easy interpretation
• Row-level Details: Provides specific row wait information
• Formatted Output: Clean, readable display of complex wait information

Limitations

• Only shows sessions currently waiting on enqueue events
• Historical wait information is limited
• Does not show the blocking sessions (use additional queries for complete analysis)

Best Practices

1. Regular Monitoring: Run during peak hours to identify recurring issues
2. Combine with Other Views: Use with V$LOCK and V$SESSION for complete analysis
3. Document Patterns: Track common enqueue types for your application
4. Proactive Resolution: Address long-running enqueue waits promptly to prevent cascading issues