Wait Events

What is Oracle wait events? 

Oracle Wait Events are conditions where a session is waiting for something to happen.  A wait event can be caused by many things, from slow read/write speeds on the disk, to locking situations caused by the architecture, to various kinds of Oracle contentions.  Waits are either system-level or session-level.  A session-level wait event is an event that affects a single user activity within the database.  System-level wait events affect the entire database system.   

When Oracle executes an SQL statement, it is not constantly executing. Sometimes it has to wait for a specific event to happen befor it can proceed.

For example, if Oracle (or the SQL statement) wants to modify data, and the corresponding database block is not currently in the SGA, Oracle waits for this block to be available for modification.

All possible wait events can be found in v$event_name. In Oracle 10g R1, there are some 806 different wait events.

What Oracle waits for and how long it has totally waited for these events can be monitored through the following views:

• v$session_event
• v$session_wait
• v$system_event

Oracle Wait Events

Before looking into wait events, let us understand various state of user process.
Oracle user process is typically in one of the three states:

a. Idle wait. e.g. 'SQL*Net message from client'

b. Running code - Either on CPU or on a run queue. Oracle itself does not know if it is on-CPU or just on a run queue.

c. Waiting

i. for some resource to become available. e.g. enqueue (lock) or a latch
ii. for an activity to complete that it has requested. Like an IO read request.

Oracle has a set of 'Wait Events' for activities in 'a' and 'c', and record CPU utilization for 'b'.

This is best illustrated with a simplified example of few seconds in the life of an Oracle shadow process:

State Notes...

 IDLE : Waiting for 'SQL*Net message from client'. Receives a SQL*Net packet requesting 'parse/execute' of a statement

ON CPU : decodes the SQL*Net packet.

WAITING : Waits for 'latch free' to obtain the a 'library cache' latch Gets the latch.

ON CPU : Scans for the SQL statement in the shared pool, finds a match, frees latch , sets up links to the shared cursor etc.. & begins to execute.

WAITING : Waits for 'db file sequential read' as we need a block which is not in the buffer cache. Ie: Waiting for an IO to complete.

ON CPU : Block read has completed so execution can continue. Constructs a SQL*Net packet to send back to the user containing the first row of data.

WAITING : Waits on 'SQL*Net message to client' for an acknowledgement that the SQL*Net packet was reliably delivered.

IDLE : Waits on 'SQL*Net message from client' for the next thing to do.

Most common wait events are..

Buffer Busy waits/Cache Buffers Chains Latch waits

•This wait happens when a session wants to access a database block in the buffer cache but it cannot as the buffer is "busy". The two main cases where this can occur are:

1.Another session is reading the block into the buffer
2.Another session holds the buffer in an incompatible mode to our request

Cache Buffers Chains Latch waits are caused by contention where multiple sessions waiting to read the same block.

Typical solutions are:-

Look at the execution plan for the SQL being run and try to reduce the gets per executions which will minimise the number of blocks being accessed and therefore reduce the chances of multiple sessions contending for the same block.

Increase the PCTFREE for the table storage parameter. This will result in less rows per block.

Consider implementing reverse key indexes. (if range scans aren't commonly used against the segment)

In v$session_wait, the P1, P2, and P3 columns identify the file number, block number, and buffer busy reason codes, respectively.

"Read By Other Session" wait event.

When user sessions request for data, Oracle will first read the data from disk into the database buffer cache. If two or more sessions request the same data, the first session will read the data into the buffer cache while other sessions wait. In previous versions, this wait was classified under the "buffer busy waits" event. However, in Oracle 10g and higher, this wait time is now broken out into the "read by other session" wait event.

Excessive waits for this event are typically due to several processes repeatedly reading the same blocks, e.g. many sessions scanning the same index or performing full table scans on the same table. Tuning this issue is a matter of finding and eliminating this contention.

When a session is waiting on this event, an entry will be seen in the v$session_wait system view giving more information on the blocks being waited for:

SELECT p1 "file#", p2 "block#"
FROM v$session_wait WHERE event = 'read by other session';

If information collected from the above query repeatedly shows that the same block (or range of blocks) is experiencing waits, this indicates a "hot" block or object.

The following query will give the name and type of the object:

SELECT owner, segment_name, segment_type
FROM dba_extents WHERE file_id = &file
AND &block BETWEEN block_id AND block_id + blocks - 1

Log File Sync waits

Log file sync waits occur when sessions wait for redo data to be written to disk.
Typically this is caused by slow writes or committing too frequently in the application.

  DB file sequential read

Wait for an I/O read request to complete. A sequential read is usually a single-block read. This differs from "db file scattered read" in that a sequential read reads data into contiguous memory (whilst a scattered read reads multiple blocks and scatters them into different buffers in the SGA).

  DB file scattered read

This wait happens when a session is waiting for a multiblock IO to complete. This typically occurs during full table scans or index fast full scans. Oracle reads up to DB_FILE_MULTIBLOCK_READ_COUNT consecutive blocks at a time and scatters them into buffers in the buffer cache.

Direct path read

Direct path reads are generally used by Oracle when reading directly into PGA memory (as opposed to into the buffer cache).

This style of read request is typically used for:

Sort I/Os when memory Sort areas are exhausted and temporary tablespaces are used to perform the sort
Parallel Query slaves.

Direct path write

This wait is seen for:

Direct load operations (eg: Create Table as Select (CTAS) may use this)
Parallel DML operations
Sort IO (when a sort does not fit in memory)

DB file parallel write

DBW waits on "db file parallel write" when waiting for a parallel write to files and blocks to complete.
The db file parallel write occurs when the process, typically DBWR, has issued multiple I/O requests in parallel to write dirty blocks from the buffer cache to disk, and is waiting for all requests to complete.

V$Session_wait_history

From Oracle Database 10g a new view V$Session_wait_history will allow us to see the last few wait events a session waited on.

The last 10 wait events that a session experienced can be displayed using the v$session_wait_history view. The session has to be currently active. Once the session ends this information is not available.

We can use the seq# column to sort the wait events into the order in which the wait events occurred for the session.

Oracle Wait Events

Queries pass through hundreds of internal database processes called Oracle Wait Events.

Commonly encountered wait events include:

Block Change Tracking Buffer—Related to the sizing / usage of the CTWR dba buffer in the Large Pool memory structure.

Buffer Busy Wait—This wait event happens when a session tries to access a block in the buffer cache but it can't because the buffer is busy, that is another session is modifying the block and the contents of the block are in flux.

Control File Reads and Writes—The three different wait events of 'control file sequential read', 'control file single write', and 'control file parallel write' all contribute to the amount of time Oracle takes to keep the control file current. Through normal operation the control file is continuously hammered with reads and writes as it is being updated.

Db File Sequential Read—There are two things you can do to these waits: optimize the SQL statement or reduce the average wait time.

Direct Path Read Temp—When a session reads buffers from disk directly into the PGA, the wait is on direct path read temp. This is closely related to the direct path read wait.
 
Enq: TX - row lock contention—A wait for the Oracle TX enqueue in mode 6 (row lock contention) is a common enqueue wait, and occurs when a transaction tries to update or delete rows that are currently locked by another transaction.

Enq: TM - index contention—Waits on enq: TM - contention in Oracle indicate there are unindexed foreign key constraints. In this article, I examine how foreign key constraints relate to this wait event and how to tune for this event.

File Open—This wait event happens when the database needs to open a file. Wait time is recorded starting just prior to when the open request is issued and extends until the time the request is returned, having succeeded or failed, from the operating system.

Index Block Split—As applications, users or sessions request rows from a table, Oracle may determine which index access path is best for finding rows in a table. If another session is inserting or updating data during this index lookup process, which in turn causes updates to that index and requires an index block split, the first session must wait on that index block split until finished. This is the index block split wait event.

Latch Free—This wait event occurs when a session needs a latch, tries to get the latch, but fails because someone else has it. When this happens, the session sleeps with a wait on latch free, wakes up and tries again. The time it was asleep is the wait time for "latch free."

Locking and Enqueue Waits—Oracle enqueue waits indicate that a seesion is waiting for a lock that is held by another user (or sessions). For Oracle 9i, there is a specific Enqueue wait event. For Oracle 10g and later, the Enqueue wait event has been separated into more than 200 unique wait events, which each include more specific information about the related lock type.

Log Buffer Space—This wait event occurs when server processes write data into the log buffer faster than the LGWR process can write it out.

Log File Switch Completion—As data is manipulated and altered in an Oracle database, time must be spent recording and archiving these changes through redo log generation. The subsequent wait is known as Oracle log file switch completion.The performance impact of switching too many redo logs is the culprit.

Log File Sync—This wait event is triggered when a user session issues a commit (or a rollback). The user session will signal or post the LGWR to write the log buffer to the redo log file. When the LGWR has finished writing, it will post the user session.

Oracle RAC Wait Events— Monitoring an Oracle RAC database often means monitoring this situation and the amount of requests going back and forth over the RAC interconnect. The most common wait events related to this are gc cr request and gc buffer busy.

PL/SQL Lock Timer—This wait event represents the amount of time a user or application has "slept" through the USER_LOCK.SLEEP or DBMS_LOCK.SLEEP procedures. The PL/SQL lock timer event is worth watching because it can indicate issues with application response, throughput and possibly coding.

Ready by Other Session—When a session waits on the "read by other session" event, it indicates a wait for another session to read the data from disk into the Oracle buffer cache. If this happens too often the performance of the query or the entire database can suffer.

RO Enqueue—This wait event is used to synchronize the work required between a foreground process and background process such as DBWR or CKPT. It is most often seen when dropping objects or truncating tables.

Row Cache Lock Wait—In order for DDL (Data Definition Language) to execute, it must acquire a row cache lock to lock the Data Dictionary information. The shared pool contains a cache of rows from the data dictionary that helps reduce physical I/O to the data dictionary tables and allows locking of individual data dictionary rows.

Wait events Oracle 11g2 (RAC) – Enqueues

Enqueues – are shared memory structures (locks) that serialize access to database resources. They can be associated with a session or transaction. Enqueue names are displayed in the LOCK_TYPE column of the DBA_LOCK and DBA_LOCK_INTERNAL data dictionary views.

In case your session is slow you can always check what it’s waiting for in v$session table.

This query just shows sessions waiting for enqueue resources

SELECT sid, serial#, event, wait_class FROM v$session WHERE event LIKE 'enq:%';

To get description of an enqueue resource use following query.

SELECT eq_name "Enqueue", ev.name "Enqueue Type", eq.req_description "Description" FROM v$enqueue_statistics eq, v$event_name ev WHERE eq.event#=ev.event# ORDER BY ev.name;

Enqueue name	Enqueue type	Description
Auto BMR	enq: AB – ABMR process initialized	Lock held to ensure that ABMR process is initialized
Auto BMR	enq: AB – ABMR process start/stop	Lock held to ensure that only one ABMR is started in the cluster
ASM Disk AU Lock	enq: AD – allocate AU	Synchronizes accesses to a specific ASM disk AU
ASM Disk AU Lock	enq: AD – deallocate AU	Synchronizes accesses to a specific ASM disk AU
ASM Disk AU Lock	enq: AD – relocate AU	Synchronizes accesses to a specific ASM disk AU
Edition Lock	enq: AE – lock	Prevent Dropping an edition in use
Advisor Framework	enq: AF – task serialization	This enqueue is used to serialize access to an advisor task
Analytic Workspace Generation	enq: AG – contention	Synchronizes generation use of a particular workspace
ASM Enqueue	enq: AM – ASM ACD Relocation	Block ASM cache freeze
ASM Enqueue	enq: AM – ASM Amdu Dump	Allow only one AMDU dump when block read failure
ASM Enqueue	enq: AM – ASM File Destroy	Prevent same file deletion race
ASM Enqueue	enq: AM – ASM Grow ACD	Serializes growing ASM ACD
ASM Enqueue	enq: AM – ASM Password File Update	Allow one ASM password file update per cluster at a time
ASM Enqueue	enq: AM – ASM User	Prevents a user from being dropped if it owns any open files
ASM Enqueue	enq: AM – ASM cache freeze	Start ASM cache freeze
ASM Enqueue	enq: AM – ASM disk based alloc/dealloc	Synchronizes disk based allocations/deallocations
ASM Enqueue	enq: AM – ASM file descriptor	Serializes access to ASM file descriptors
ASM Enqueue	enq: AM – ASM file relocation	Serializes file relocation shrink and drop
ASM Enqueue	enq: AM – ASM reserved	Check id1 of call for specific purpose
ASM Enqueue	enq: AM – background COD reservation	Reserve a background COD entry
ASM Enqueue	enq: AM – block repair	Serializes block repairs
ASM Enqueue	enq: AM – client registration	Registers DB instance to ASM client state object hash
ASM Enqueue	enq: AM – disk offline	Synchronizes disk offlines
ASM Enqueue	enq: AM – group block	ASM group block
ASM Enqueue	enq: AM – group use	Client group use
ASM Enqueue	enq: AM – rollback COD reservation	Reserve a rollback COD entry
ASM Enqueue	enq: AM – shutdown	Prevent DB instance registration during ASM instance shutdown
MultiWriter Object Access	enq: AO – contention	Synchornizes access to objects and scalar variables
SPM Autopurge	enq: AP – contention	This enqueue is used to serialize the purging of SQL plan baselines
Service Operations	enq: AS – service activation	Synchronizes new service activation
Alter Tablespace	enq: AT – contention	Serializes ‘alter tablespace’ operations
ASM volume locks	enq: AV – AVD client registration	Serialize inst reg and first DG use
ASM volume locks	enq: AV – add/enable first volume in DG	Serialize taking the AVD DG enqueue
ASM volume locks	enq: AV – persistent DG number	prevent DG number collisions
ASM volume locks	enq: AV – volume relocate	Serialize relocating volume extents
Analytic Workspace	enq: AW – AW generation lock	In-use generation state for a particular workspace
Analytic Workspace	enq: AW – AW state lock	Row lock synchronization for the AW$ table
Analytic Workspace	enq: AW – AW$ table lock	Global access synchronization to the AW$ table
Analytic Workspace	enq: AW – user access for AW	Synchronizes user accesses to a particular workspace
KSXA Test Affinity Dictionary	enq: AY – contention	Affinity Dictionary test affinity synchronization
Global Transaction Branch	enq: BB – 2PC across RAC instances	2PC distributed transaction branch across RAC instances
BLOOM FILTER	enq: BF – PMON Join Filter cleanup	PMON bloom filter recovery
BLOOM FILTER	enq: BF – allocation contention	Allocate a bloom filter in a parallel statement
clonedb bitmap file access	enq: BM – clonedb bitmap file write	synchronizes clonedb bitmap file operations
Backup/Restore	enq: BR – file shrink	Lock held to prevent file from decreasing in physical size during RMAN backup
Backup/Restore	enq: BR – multi-section restore header	Lock held to serialize file header access during multi-section restore
Backup/Restore	enq: BR – multi-section restore section	Lock held to serialize section access during multi-section restore
Backup/Restore	enq: BR – perform autobackup	Lock held to perform a new controlfile autobackup
Backup/Restore	enq: BR – proxy-copy	Lock held to allow cleanup from backup mode during an RMAN proxy-copy backup
Backup/Restore	enq: BR – request autobackup	Lock held to request controlfile autobackups
Backup/Restore	enq: BR – space info datafile hdr update	Lock held to prevent multiple process to update the headers at the same time
Calibration	enq: CA – contention	Synchronizes various IO calibration runs
Controlfile Transaction	enq: CF – contention	Synchronizes accesses to the controlfile
Cross-Instance Call Invocation	enq: CI – contention	Coordinates cross-instance function invocations
Label Security cache	enq: CL – compare labels	Synchronizes accesses to label cache for label comparison
Label Security cache	enq: CL – drop label	Synchronizes accesses to label cache when dropping a label
ASM Instance Enqueue	enq: CM – diskgroup dismount	serialize asm diskgroup dismount
ASM Instance Enqueue	enq: CM – gate	serialize access to instance enqueue
ASM Instance Enqueue	enq: CM – instance	indicate ASM diskgroup is mounted
KTCN REG enq	enq: CN – race with init	during descriptor initialization
KTCN REG enq	enq: CN – race with reg	during transaction commit to see concurrent registrations
KTCN REG enq	enq: CN – race with txn	during registration
KTUCLO Master Slave enq	enq: CO – master slave det	enqueue held be Master in Cleanout Optim
Cleanup querycache registrations	enq: CQ – contention	Serializes access to cleanup client query cache registrations
Reuse Block Range	enq: CR – block range reuse ckpt	Coordinates fast block range reuse ckpt
Block Change Tracking	enq: CT – CTWR process start/stop	Lock held to ensure that only one CTWR process is started in a single instance
Block Change Tracking	enq: CT – change stream ownership	Lock held by one instance while change tracking is enabled, to guarantee access to thread-specific resources
Block Change Tracking	enq: CT – global space management	Lock held during change tracking space management operations that affect the entire change tracking file
Block Change Tracking	enq: CT – local space management	Lock held during change tracking space management operations that affect just the data for one thread
Block Change Tracking	enq: CT – reading	Lock held to ensure that change tracking data remains in existence until a reader is done with it
Block Change Tracking	enq: CT – state	Lock held while enabling or disabling change tracking, to ensure that it is only enabled or disabled by one user at a time
Block Change Tracking	enq: CT – state change gate 1	Lock held while enabling or disabling change tracking in RAC
Block Change Tracking	enq: CT – state change gate 2	Lock held while enabling or disabling change tracking in RAC
Cursor	enq: CU – contention	Recovers cursors in case of death while compiling
TEXT: Index Specific Lock	enq: CX – TEXT: Index Specific Lock	Index Specific Lock on CTX index
DbsDriver	enq: DB – contention	Synchronizes modification of database wide supplementallogging attributes
ASM Local Disk Group	enq: DD – contention	Synchronizes local accesses to ASM disk groups
Datafile Online in RAC	enq: DF – contention	Enqueue held by foreground or DBWR when a datafile is brought online in RAC
ASM Disk Group Modification	enq: DG – contention	Synchronizes accesses to ASM disk groups
Direct Loader Index Creation	enq: DL – contention	Lock to prevent index DDL during direct load
Database Mount/Open	enq: DM – contention	Enqueue held by foreground or DBWR to syncrhonize database mount/open with other operations
Diskgroup number generator	enq: DN – contention	Serializes group number generations
ASM Disk Online Lock	enq: DO – Staleness Registry create	Synchronizes Staleness Registry creation
ASM Disk Online Lock	enq: DO – disk online	Synchronizes disk onlines and their recovery
ASM Disk Online Lock	enq: DO – disk online operation	Represents an active disk online operation
ASM Disk Online Lock	enq: DO – disk online recovery	Synchronizes disk onlines and their recovery
ASM Disk Online Lock	enq: DO – startup of MARK process	Synchronizes startup of MARK process
LDAP Parameter	enq: DP – contention	Synchronizes access to LDAP parameters
Distributed Recovery	enq: DR – contention	Serializes the active distributed recovery operation
Database Suspend	enq: DS – contention	Prevents a database suspend during LMON reconfiguration
Default Temporary Tablespace	enq: DT – contention	Serializes changing the default temporary table spaceand user creation
Diana Versioning	enq: DV – contention	Synchronizes access to lower-version Diana (PL/SQL intermediate representation)
In memory Dispenser	enq: DW – contention	Serialize in memory dispenser operations
Distributed Transaction	enq: DX – contention	Serializes tightly coupled distributed transaction branches
ASM File Access Lock	enq: FA – access file	Synchronizes accesses to open ASM files
Format Block	enq: FB – contention	Ensures that only one process can format data blcoks in auto segment space managed tablespaces
Disk Group Chunk Mount	enq: FC – open an ACD thread	LGWR opens an ACD thread
Disk Group Chunk Mount	enq: FC – recover an ACD thread	SMON recovers an ACD thread
Flashback Database	enq: FD – Flashback coordinator	Synchronization
Flashback Database	enq: FD – Flashback logical operations	Synchronization
Flashback Database	enq: FD – Flashback on/off	Synchronization
Flashback Database	enq: FD – Marker generation	Synchronization
Flashback Database	enq: FD – Restore point create/drop	Synchronization
Flashback Database	enq: FD – Tablespace flashback on/off	Synchronization
KTFA Recovery	enq: FE – contention	Serializes flashback archive recovery
ACD Relocation Gate Enqueue	enq: FG – FG redo generation enq race	resolve race condition to acquire Disk Group Redo Generation Enqueue
ACD Relocation Gate Enqueue	enq: FG – LGWR redo generation enq race	resolve race condition to acquire Disk Group Redo Generation Enqueue
ACD Relocation Gate Enqueue	enq: FG – serialize ACD relocate	only 1 process in the cluster may do ACD relocation in a disk group
Flashback database log	enq: FL – Flashback database log	Synchronization
Flashback database log	enq: FL – Flashback db command	Enqueue used to synchronize Flashback Database and and deletion of flashback logs.
File Mapping	enq: FM – contention	Synchronizes access to global file mapping state
File Object	enq: FP – global fob contention	Synchronizes various File Object(FOB) operations
Disk Group Recovery	enq: FR – contention	begin recovery of disk group
Disk Group Recovery	enq: FR – recover the thread	wait for lock domain detach
Disk Group Recovery	enq: FR – use the thread	indicate this ACD thread is alive
File Set / Dictionary Check	enq: FS – contention	Enqueue used to synchronize recovery and file operations or synchronize dictionary check
Disk Group Redo Generation	enq: FT – allow LGWR writes	allow LGWR to generate redo in this thread
Disk Group Redo Generation	enq: FT – disable LGWR writes	prevent LGWR from generating redo in this thread
DBFUS	enq: FU – contention	This enqueue is used to serialize the capture of the DB Feature           Usage and High Water Mark Statistics
ACD Xtnt Info CIC	enq: FX – issue ACD Xtnt Relocation CIC	ARB relocates ACD extent
ASM Disk Header	enq: HD – contention	Serializes accesses to ASM SGA data structures
Queue Page	enq: HP – contention	Synchronizes accesses to queue pages
Hash Queue	enq: HQ – contention	Synchronizes the creation of new queue IDs
Direct Loader High Water Mark	enq: HV – contention	Lock used to broker the high water mark during parallel inserts
Segment High Water Mark	enq: HW – contention	Lock used to broker the high water mark during parallel inserts
Internal	enq: IA – contention
NID	enq: ID – contention	Lock held to prevent other processes from performing controlfile transaction while NID is running
Label Security	enq: IL – contention	Synchronizes accesses to internal label data structures
Kti blr lock	enq: IM – contention for blr	Serializes block recovery for IMU txn
Instance Recovery	enq: IR – contention	Synchronizes instance recovery
Instance Recovery	enq: IR – contention2	Synchronizes parallel instance recovery and shutdown immediate
Instance State	enq: IS – contention	Enqueue used to synchronize instance state changes
In-Mem Temp Table Meta Creation	enq: IT – contention	Synchronizes accesses to a temp object’s metadata
Job Queue Date	enq: JD – contention	Synchronizes dates between job queue coordinator and slave processes
Materialized View	enq: JI – contention	Lock held during materialized view operations (like refresh, alter) to prevent concurrent operations on the same materialized view
Job Queue	enq: JQ – contention	Lock to prevent multiple instances from running a single job
Job Scheduler	enq: JS – aq sync	Scheduler evt code and AQ sync
Job Scheduler	enq: JS – contention	Synchronizes accesses to the job cache
Job Scheduler	enq: JS – evt notify	Lock got during event notification
Job Scheduler	enq: JS – evtsub add	Lock got when adding subscriber to event q
Job Scheduler	enq: JS – evtsub drop	Lock got when dropping subscriber to event q
Job Scheduler	enq: JS – job recov lock	Lock to recover jobs running on crashed RAC inst
Job Scheduler	enq: JS – job run lock – synchronize	Lock to prevent job from running elsewhere
Job Scheduler	enq: JS – q mem clnup lck	Lock obtained when cleaning up q memory
Job Scheduler	enq: JS – queue lock	Lock on internal scheduler queue
Job Scheduler	enq: JS – sch locl enqs	Scheduler non-global enqueues
Job Scheduler	enq: JS – wdw op	Lock got when doing window open/close
SQL STATEMENT QUEUE	enq: JX – SQL statement queue	statement
SQL STATEMENT QUEUE	enq: JX – cleanup of  queue	release SQL statement resources
Scheduler Master DBRM	enq: KD – determine DBRM master	Determine DBRM master
Scheduler	enq: KM – contention	Synchronizes various Resource Manager operations
Multiple Object Checkpoint	enq: KO – fast object checkpoint	Coordinates fast object checkpoint
Kupp Process Startup	enq: KP – contention	Synchronizes kupp process startup
ASM Attributes Enque	enq: KQ – access ASM attribute	Synchronization of ASM cached attributes
Scheduler Plan	enq: KT – contention	Synchronizes accesses to the current Resource Manager plan
Materialized View Log DDL	enq: MD – contention	Lock held during materialized view log DDL statements
AQ Notification Mail Host	enq: MH – contention	Lock used for recovery when setting Mail Host for AQ e-mail notifications
Master Key	enq: MK – contention	changing values in enc$
AQ Notification Mail Port	enq: ML – contention	Lock used for recovery when setting Mail Port for AQ e-mail notifications
LogMiner	enq: MN – contention	Synchronizes updates to the LogMiner dictionary and prevents multiple instances from preparing the same LogMiner session
MMON restricted session	enq: MO – contention	Serialize MMON operations for restricted sessions
Media Recovery	enq: MR – contention	Lock used to coordinate media recovery with other uses of datafiles
Media Recovery	enq: MR – standby role transition	Lock used to disallow concurrent standby role transition attempt
Materialized View Refresh Log	enq: MS – contention	Lock held during materialized view refresh to setup MV log
Online Datafile Move	enq: MV – datafile move	Held during online datafile move operation or cleanup
MWIN Schedule	enq: MW – contention	This enqueue is used to serialize the calibration of the           manageability schedules with the Maintenance Window
ksz synch	enq: MX – sync storage server info	Lock held to generate a response to the storage server information request when an instance is starting up
Outline Cache	enq: OC – contention	Synchronizes write accesses to the outline cache
Online DDLs	enq: OD – Serializing DDLs	Lock to prevent concurrent online DDLs
Outline Name	enq: OL – contention	Synchronizes accesses to a particular outline name
OLAPI Histories	enq: OQ – xsoq*histrecb	Synchronizes access to olapi history parameter CB
OLAPI Histories	enq: OQ – xsoqhiAlloc	Synchronizes access to olapi history allocation
OLAPI Histories	enq: OQ – xsoqhiClose	Synchronizes access to olapi history closing
OLAPI Histories	enq: OQ – xsoqhiFlush	Synchronizes access to olapi history flushing
OLAPI Histories	enq: OQ – xsoqhistrecb	Synchronizes access to olapi history globals
TEXT: Generic Lock	enq: OT – TEXT: Generic Lock	CTX Generic Locks
Encryption Wallet	enq: OW – initialization	initializing the wallet context
Encryption Wallet	enq: OW – termination	terminate the wallet context
Property Lock	enq: PD – contention	Prevents others from updating the same property
Parameter	enq: PE – contention	Synchronizes system parameter updates
Password File	enq: PF – contention	Synchronizes accesses to the password file
Global Parameter	enq: PG – contention	Synchronizes global system parameter updates
AQ Notification Proxy	enq: PH – contention	Lock used for recovery when setting Proxy for AQ HTTP notifications
Remote PX Process Spawn Status	enq: PI – contention	Communicates remote Parallel Execution Server Process creation status
Transportable Tablespace	enq: PL – contention	Coordinates plug-in operation of transportable tablespaces
Process Startup	enq: PR – contention	Synchronizes process startup
PX Process Reservation	enq: PS – contention	Parallel Execution Server Process reservation and synchronization
ASM Partnership and Status Table	enq: PT – contention	Synchronizes access to ASM PST metadata
KSV slave startup	enq: PV – syncshut	Synchronizes instance shutdown_slvstart
KSV slave startup	enq: PV – syncstart	Synchronizes slave start_shutdown
Buffer Cache PreWarm	enq: PW – flush prewarm buffers	Direct Load needs to flush prewarmed buffers if DBWR 0 holds enqueue
Buffer Cache PreWarm	enq: PW – perwarm status in dbw0	DBWR 0 holds enqueue indicating prewarmed buffers present in cache
ASM Rollback Recovery	enq: RB – contention	Serializes ASM rollback recovery operations
Result Cache: Enqueue	enq: RC – Result Cache: Contention	Coordinates access to a result-set
RAC Load	enq: RD – RAC load	update RAC load info
Block Repair/Resilvering	enq: RE – block repair contention	Synchronize block repair/resilvering operations
Data Guard Broker	enq: RF – DG Broker Current File ID	Identifies which configuration metadata file is current
Data Guard Broker	enq: RF – FSFO Observer Heartbeat	Captures recent Fast-Start Failover Observer heartbeat information
Data Guard Broker	enq: RF – FSFO Primary Shutdown suspended	Records when FSFO Primary Shutdown is suspended
Data Guard Broker	enq: RF – RF – Database Automatic Disable	Means for detecting when database is being automatically disabled
Data Guard Broker	enq: RF – atomicity	Ensures atomicity of log transport setup
Data Guard Broker	enq: RF – new AI	Synchronizes selection of the new apply instance
Data Guard Broker	enq: RF – synch: DG Broker metadata	Ensures r/w atomicity of DG configuration metadata
Data Guard Broker	enq: RF – synchronization: aifo master	Synchronizes apply instance failure detection and failover operation
Data Guard Broker	enq: RF – synchronization: critical ai	Synchronizes critical apply instance among primary instances
wallet_set_mkey	enq: RK – set key	wallet master key rekey
RAC Encryption Wallet Lock	enq: RL – RAC wallet lock	RAC wallet lock
Redo Log Nab Computation	enq: RN – contention	Coordinates nab computations of online logs during recovery
Multiple Object Reuse	enq: RO – contention	Coordinates flushing of multiple objects
Multiple Object Reuse	enq: RO – fast object reuse	Coordinates fast object reuse
Resilver / Repair	enq: RP – contention	Enqueue held when resilvering is needed or when datablock is repaired from mirror
Workload Capture and Replay	enq: RR – contention	Concurrent invocation of DBMS_WORKLOAD_* package API
Reclaimable Space	enq: RS – file delete	Lock held to prevent file from accessing during space reclaimation
Reclaimable Space	enq: RS – persist alert level	Lock held to make alert level persistent
Reclaimable Space	enq: RS – prevent aging list update	Lock held to prevent aging list update
Reclaimable Space	enq: RS – prevent file delete	Lock held to prevent deleting file to reclaim space
Reclaimable Space	enq: RS – read alert level	Lock held to read alert level
Reclaimable Space	enq: RS – record reuse	Lock held to prevent file from accessing while reusing circular record
Reclaimable Space	enq: RS – write alert level	Lock held to write alert level
Redo Thread	enq: RT – contention	Thread locks held by LGWR, DBW0, and RVWR to indicate mounted or open status
Redo Thread	enq: RT – thread internal enable/disable	Thread locks held by CKPT to synchronize thread enable and disable
Rolling Migration	enq: RU – contention	Serializes rolling migration operations
Rolling Migration	enq: RU – waiting	Results of rolling migration CIC
Materialized View Flags	enq: RW – MV metadata contention	Lock held by CREATE/ALTER/DROP materialized viewwhile updating materialized view flags in detail tables
ASM Extent Relocation Lock	enq: RX – relocate extent	Synchronizes relocating ASM extents
ASM Extent Relocation Lock	enq: RX – unlock extent	Synchronizes unlocking ASM extents
LogicalStandby	enq: SB – logical standby metadata	Synchronizes Logical Standby metadata operations
LogicalStandby	enq: SB – table instantiation	Synchronizes table instantiation and EDS operations
Session Migration	enq: SE – contention	Synchronizes transparent session migration operations
AQ Notification Sender	enq: SF – contention	Lock used for recovery when setting Sender for AQ e-mail notifications
Active Session History Flushing	enq: SH – contention	Should seldom see this contention as this Enqueue is always  acquired in no-wait mode
Streams Table Instantiation	enq: SI – contention	Prevents multiple streams tabel instantiations
KTSJ Slave Task Cancel	enq: SJ – Slave Task Cancel	Serializes cancelling task executed by slave process
Shrink Segment	enq: SK – contention	Serialize shrink of a segment
Serialize Lock request	enq: SL – escalate lock	sending lock escalate to LCK0
Serialize Lock request	enq: SL – get lock	sending lock req to LCK0
Serialize Lock request	enq: SL – get lock for undo	sending lock req for undo to LCK0
Shared Object	enq: SO – contention	Synchronizes access to Shared Object (PL/SQL Shared Object Manager)
Spare Enqueue	enq: SP – contention 1	(1) due to one-off patch
Spare Enqueue	enq: SP – contention 2	(2) due to one-off patch
Spare Enqueue	enq: SP – contention 3	(3) due to one-off patch
Spare Enqueue	enq: SP – contention 4	(4) due to one-off patch
Sequence Cache	enq: SQ – contention	Lock to ensure that only one process can replenish the sequence cache
Synchronized Replication	enq: SR – contention	Coordinates replication / streams operations
Sort Segment	enq: SS – contention	Ensures that sort segments created during parallel DML operations aren’t prematurely cleaned up
Space Transaction	enq: ST – contention	Synchronizes space management activities in dictionary-managed tablespaces
SaveUndo Segment	enq: SU – contention	Serializes access to SaveUndo Segment
Suspend Writes	enq: SW – contention	Coordinates the ‘alter system suspend’ operation
Instance Undo	enq: TA – contention	Serializes operations on undo segments and undo tablespaces
SQL Tuning Base Existence Cache	enq: TB – SQL Tuning Base Cache Load	Synchronizes writes to the SQL Tuning Base Existence Cache
SQL Tuning Base Existence Cache	enq: TB – SQL Tuning Base Cache Update	Synchronizes writes to the SQL Tuning Base Existence Cache
Tablespace Checkpoint	enq: TC – contention	Lock held to guarantee uniqueness of a tablespace checkpoint
Tablespace Checkpoint	enq: TC – contention2	Lock of setup of a unqiue tablespace checkpoint in null mode
KTF map table enqueue	enq: TD – KTF dump entries	KTF dumping time/scn mappings in SMON_SCN_TIME table
KTF broadcast	enq: TE – KTF broadcast	KTF broadcasting
Temporary File	enq: TF – contention	Serializes dropping of a temporary file
Threshold Chain	enq: TH – metric threshold evaluation	Serializes threshold in-memory chain access
Auto Task Serialization	enq: TK – Auto Task Serialization	Lock held by MMON to prevent other MMON spawning of Autotask Slave
Auto Task Serialization	enq: TK – Auto Task Slave Lockout	Serializes spawned Autotask Slaves
Log Lock	enq: TL – contention	Serializes threshold log table read and update
DML	enq: TM – contention	Synchronizes accesses to an object
Temp Object	enq: TO – contention	Synchronizes DDL and DML operations on a temp object
Runtime Fixed Table Purge	enq: TP – contention	Lock held during purge and dynamic reconfiguration of fixed tables.
Queue table enqueue	enq: TQ – DDL contention	TM access to the queue table
Queue table enqueue	enq: TQ – DDL-INI contention	Streams DDL on queue table
Queue table enqueue	enq: TQ – INI contention	TM access to the queue table
Queue table enqueue	enq: TQ – TM contention	TM access to the queue table
Temporary Segment	enq: TS – contention	Serializes accesses to temp segments
Tablespace	enq: TT – contention	Serializes DDL operations on tablespaces
Cross-Instance Transaction	enq: TW – contention	Lock held by one instance to wait for transactions on all instances to finish
Transaction	enq: TX – allocate ITL entry	Allocating an ITL entry in order to begin a transaction
Transaction	enq: TX – contention	Lock held by a transaction to allow other transactions to wait for it
Transaction	enq: TX – index contention	Lock held on an index during a split to prevent other operations on it
Transaction	enq: TX – row lock contention	Lock held on a particular row by a transaction to prevent other transactions from modifying it
User-defined	enq: UL – contention	Lock used by user applications
Undo Segment	enq: US – contention	Lock held to perform DDL on the undo segment
AQ Notification Watermark	enq: WA – contention	Lock used for recovery when setting Watermark for memory usage in AQ notifications
AWR Flush	enq: WF – contention	This enqueue is used to serialize the flushing of snapshots
Write gather local enqueue	enq: WG – delete fso	acquire lobid local enqueue when deleting fso
Write gather local enqueue	enq: WG – lock fso	acquire lobid local enqueue when locking fso
Being Written Redo Log	enq: WL – RAC-wide SGA contention	Serialize access to RAC-wide SGA
Being Written Redo Log	enq: WL – RFS global state contention	Serialize access to RFS global state
Being Written Redo Log	enq: WL – Test access/locking	Testing redo transport access/locking
Being Written Redo Log	enq: WL – contention	Coordinates access to redo log files and archive logs
WLM Plan Operations	enq: WM – WLM Plan activation	Synchronizes new WLM Plan activation
AWR Purge	enq: WP – contention	This enqueue handles concurrency between purging and baselines
LNS archiving log	enq: WR – contention	Coordinates access to logs by Async LNS and ARCH/FG
XDB Configuration	enq: XC – XDB Configuration	Lock obtained when incrementing XDB configuration version number
Auto Online Exadata disks	enq: XD – ASM disk OFFLINE	Serialize OFFLINE Exadata disk operations
Auto Online Exadata disks	enq: XD – ASM disk ONLINE	Serialize ONLINE Exadata disk operations
Auto Online Exadata disks	enq: XD – ASM disk drop/add	Serialize Auto Drop/Add Exadata disk operations
AQ Notification No-Proxy	enq: XH – contention	Lock used for recovery when setting No Proxy Domains for AQ HTTP notifications
ASM Extent Fault Lock	enq: XL – fault extent map	Keep multiple processes from faulting in the same extent chunk
ASM Extent Relocation Enqueue	enq: XQ – purification	wait for relocation before doing block purification
ASM Extent Relocation Enqueue	enq: XQ – recovery	prevent relocation during _recovery_asserts checking
ASM Extent Relocation Enqueue	enq: XQ – relocation	wait for recovery before doing relocation
Quiesce / Force Logging	enq: XR – database force logging	Lock held during database force logging mode
Quiesce / Force Logging	enq: XR – quiesce database	Lock held during database quiesce
Internal Test	enq: XY – contention	Lock used for internal testing
Audit Partition	enq: ZA – add std audit table partition	lock held to add partition to std audit table
FGA Partition	enq: ZF – add fga audit table partition	lock held to add partition to fga audit table
File Group	enq: ZG – contention	Coordinates file group operations
Compression Analyzer	enq: ZH – compression analysis	Synchronizes analysis and insert into compression$, prevents multiple threads analyzing the same table during a load
Global Context Action	enq: ZZ – update hash tables	lock held for updating global context hash tables

 

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