| Enqueue Type | Description |
| enq: AD – allocate AU | Synchronizes accesses to a specific OSM disk AU |
| enq: AD – deallocate AU | Synchronizes accesses to a specific OSM disk AU |
| enq: AF – task serialization | This enqueue is used to serialize access to an advisor task |
| enq: AG – contention | Synchronizes generation use of a particular workspace |
| enq: AO – contention | Synchronizes access to objects and scalar variables |
| enq: AS – contention | Synchronizes new service activation |
| enq: AT – contention | Serializes ‘alter tablespace’ operations |
| enq: AW – AW$ table lock | Global access synchronization to the AW$ table |
| enq: AW – AW generation lock | In-use generation state for a particular workspace |
| enq: AW – user access for AW | Synchronizes user accesses to a particular workspace |
| enq: AW – AW state lock | Row lock synchronization for the AW$ table |
| enq: BR – file shrink | Lock held to prevent file from decreasing in physical size during RMAN backup |
| enq: BR – proxy-copy | Lock held to allow cleanup from backup mode during an RMAN proxy-copy backup |
| enq: CF – contention | Synchronizes accesses to the controlfile |
| enq: CI – contention | Coordinates cross-instance function invocations |
| enq: CL – drop label | Synchronizes accesses to label cache when dropping a label |
| enq: CL – compare labels | Synchronizes accesses to label cache for label comparison |
| enq: CM – gate | Serialize access to instance enqueue |
| enq: CM – instance | Indicate OSM disk group is mounted |
| enq: CT – global space management | Lock held during change tracking space management operations that affect the entire change tracking file |
| 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 |
| enq: CT – state change gate 2 | Lock held while enabling or disabling change tracking in RAC |
| enq: CT – reading | Lock held to ensure that change tracking data remains in existence until a reader is done with it |
| enq: CT – CTWR process start/stop | Lock held to ensure that only one CTWR process is started in a single instance |
| enq: CT – state change gate 1 | Lock held while enabling or disabling change tracking in RAC |
| enq: CT – change stream ownership | Lock held by one instance while change tracking is enabled, to guarantee access to thread-specific resources |
| enq: CT – local space management | Lock held during change tracking space management operations that affect just the data for one thread |
| enq: CU – contention | Recovers cursors in case of death while compiling |
| enq: DB – contention | Synchronizes modification of database wide supplemental logging attributes |
| enq: DD – contention | Synchronizes local accesses to ASM disk groups |
| enq: DF – contention | Enqueue held by foreground or DBWR when a datafile is brought online in RAC |
| enq: DG – contention | Synchronizes accesses to ASM disk groups |
| enq: DL – contention | Lock to prevent index DDL during direct load |
| enq: DM – contention | Enqueue held by foreground or DBWR to synchronize database mount/open with other operations |
| enq: DN – contention | Serializes group number generations |
| enq: DP – contention | Synchronizes access to LDAP parameters |
| enq: DR – contention | Serializes the active distributed recovery operation |
| enq: DS – contention | Prevents a database suspend during LMON reconfiguration |
| enq: DT – contention | Serializes changing the default temporary table space and user creation |
| enq: DV – contention | Synchronizes access to lower-version Diana (PL/SQL intermediate representation) |
| enq: DX – contention | Serializes tightly coupled distributed transaction branches |
| enq: FA – access file | Synchronizes accesses to open ASM files |
| enq: FB – contention | Ensures that only one process can format data blocks in auto segment space managed tablespaces |
| enq: FC – open an ACD thread | LGWR opens an ACD thread |
| enq: FC – recover an ACD thread | SMON recovers an ACD thread |
| enq: FD – Marker generation | Synchronization |
| enq: FD – Flashback coordinator | Synchronization |
| enq: FD – Tablespace flashback on/off | Synchronization |
| enq: FD – Flashback on/off | Synchronization |
| enq: FG – serialize ACD relocate | Only 1 process in the cluster may do ACD relocation in a disk group |
| enq: FG – LGWR redo generation enq race | Resolve race condition to acquire Disk Group Redo Generation Enqueue |
| enq: FG – FG redo generation enq race | Resolve race condition to acquire Disk Group Redo Generation Enqueue |
| enq: FL – Flashback database log | Synchronization |
| enq: FL – Flashback db command | Enqueue used to synchronize Flashback Database and deletion of flashback logs. |
| enq: FM – contention | Synchronizes access to global file mapping state |
| enq: FR – contention | Begin recovery of disk group |
| enq: FS – contention | Enqueue used to synchronize recovery and file operations or synchronize dictionary check |
| enq: FT – allow LGWR writes | Allow LGWR to generate redo in this thread |
| enq: FT – disable LGWR writes | Prevent LGWR from generating redo in this thread |
| enq: FU – contention | This enqueue is used to serialize the capture of the DB Feature, Usage and High Water Mark Statistics |
| enq: HD – contention | Serializes accesses to ASM SGA data structures |
| enq: HP – contention | Synchronizes accesses to queue pages |
| enq: HQ – contention | Synchronizes the creation of new queue IDs |
| enq: HV – contention | Lock used to broker the high water mark during parallel inserts |
| enq: HW – contention | Lock used to broker the high water mark during parallel inserts |
| enq: IA – contention | |
| enq: ID – contention | Lock held to prevent other processes from performing controlfile transaction while NID is running |
| enq: IL – contention | Synchronizes accesses to internal label data structures |
| enq: IM – contention for blr | Serializes block recovery for IMU txn |
| enq: IR – contention | Synchronizes instance recovery |
| enq: IR – contention2 | Synchronizes parallel instance recovery and shutdown immediate |
| enq: IS – contention | Enqueue used to synchronize instance state changes |
| enq: IT – contention | Synchronizes accesses to a temp object’s metadata |
| enq: JD – contention | Synchronizes dates between job queue coordinator and slave processes |
| enq: JI – contention | Lock held during materialized view operations (like refresh, alter) to prevent concurrent operations on the same materialized view |
| enq: JQ – contention | Lock to prevent multiple instances from running a single job |
| enq: JS – contention | Synchronizes accesses to the job cache |
| enq: JS – coord post lock | Lock for coordinator posting |
| enq: JS – global wdw lock | Lock acquired when doing wdw ddl |
| enq: JS – job chain evaluate lock | Lock when job chain evaluated for steps to create |
| enq: JS – q mem clnup lck | Lock obtained when cleaning up q memory |
| enq: JS – slave enq get lock2 | Get run info locks before slv objget |
| enq: JS – slave enq get lock1 | Slave locks exec pre to sess strt |
| enq: JS – running job cnt lock3 | Lock to set running job count epost |
| enq: JS – running job cnt lock2 | Lock to set running job count epre |
| enq: JS – running job cnt lock | Lock to get running job count |
| enq: JS – coord rcv lock | Lock when coord receives msg |
| enq: JS – queue lock | Lock on internal scheduler queue |
| enq: JS – job run lock – synchronize | Lock to prevent job from running elsewhere |
| enq: JS – job recov lock | Lock to recover jobs running on crashed RAC inst |
| enq: KK – context | Lock held by open redo thread, used by other instances to force a log switch |
| enq: KM – contention | Synchronizes various Resource Manager operations |
| enq: KP – contention | Synchronizes kupp process startup |
| enq: KT – contention | Synchronizes accesses to the current Resource Manager plan |
| enq: MD – contention | Lock held during materialized view log DDL statements |
| enq: MH – contention | Lock used for recovery when setting Mail Host for AQ e-mail notifications |
| enq: ML – contention | Lock used for recovery when setting Mail Port for AQ e-mail notifications |
| enq: MN – contention | Synchronizes updates to the LogMiner dictionary and prevents multiple instances from preparing the same LogMiner session |
| enq: MR – contention | Lock used to coordinate media recovery with other uses of datafiles |
| enq: MS – contention | Lock held during materialized view refresh to setup MV log |
| enq: MW – contention | This enqueue is used to serialize the calibration of the manageability schedules with the Maintenance Window |
| enq: OC – contention | Synchronizes write accesses to the outline cache |
| enq: OL – contention | Synchronizes accesses to a particular outline name |
| enq: OQ – xsoqhiAlloc | Synchronizes access to olapi history allocation |
| enq: OQ – xsoqhiClose | Synchronizes access to olapi history closing |
| enq: OQ – xsoqhistrecb | Synchronizes access to olapi history globals |
| enq: OQ – xsoqhiFlush | Synchronizes access to olapi history flushing |
| enq: OQ – xsoq*histrecb | Synchronizes access to olapi history parameter CB |
| enq: PD – contention | Prevents others from updating the same property |
| enq: PE – contention | Synchronizes system parameter updates |
| enq: PF – contention | Synchronizes accesses to the password file |
| enq: PG – contention | Synchronizes global system parameter updates |
| enq: PH – contention | Lock used for recovery when setting Proxy for AQ HTTP notifications |
| enq: PI – contention | Communicates remote Parallel Execution Server Process creation status |
| enq: PL – contention | Coordinates plug-in operation of transportable tablespaces |
| enq: PR – contention | Synchronizes process startup |
| enq: PS – contention | Parallel Execution Server Process reservation and synchronization |
| enq: PT – contention | Synchronizes access to ASM PST metadata |
| enq: PV – syncstart | Synchronizes slave start shutdown |
| enq: PV – syncshut | Synchronizes instance shutdown_slvstart |
| enq: PW – perwarm status in dbw0 | DBWR 0 holds enqueue indicating prewarmed buffers present in cache |
| enq: PW – flush prewarm buffers | Direct Load needs to flush pre-warmed buffers if DBWR 0 holds enqueue |
| enq: RB – contention | Serializes OSM rollback recovery operations |
| enq: RF – synch: per-SGA Broker metadata | Ensures r/w atomicity of DG configuration metadata per unique SGA |
| enq: RF – synchronization: critical ai | Synchronizes critical apply instance among primary instances |
| enq: RF – new AI | Synchronizes selection of the new apply instance |
| enq: RF – synchronization: chief | Anoints 1 instance’s DMON as chief to other instances’ DMONs |
| enq: RF – synchronization: HC master | Anoints 1 instance’s DMON as health check master |
| enq: RF – synchronization: aifo master | Synchronizes apply instance failure detection and fail over operation |
| enq: RF – atomicity | Ensures atomicity of log transport setup |
| enq: RN – contention | Coordinates nab computations of online logs during recovery |
| enq: RO – contention | Coordinates flushing of multiple objects |
| enq: RO – fast object reuse | Coordinates fast object reuse |
| enq: RP – contention | Enqueue held when resilvering is needed or when data block is repaired from mirror |
| enq: RS – file delete | Lock held to prevent file from accessing during space reclamation |
| enq: RS – persist alert level | Lock held to make alert level persistent |
| enq: RS – write alert level | Lock held to write alert level |
| enq: RS – read alert level | Lock held to read alert level |
| enq: RS – prevent aging list update | Lock held to prevent aging list update |
| enq: RS – record reuse | Lock held to prevent file from accessing while reusing circular record |
| enq: RS – prevent file delete | Lock held to prevent deleting file to reclaim space |
| enq: RT – contention | Thread locks held by LGWR, DBW0, and RVWR to indicate mounted or open status |
| enq: SB – contention | Synchronizes Logical Standby metadata operations |
| enq: SF – contention | Lock used for recovery when setting Sender for AQ e-mail notifications |
| enq: SH – contention | Should seldom see this contention as this Enqueue is always acquired in no-wait mode |
| enq: SI – contention | Prevents multiple streams table instantiations |
| enq: SK – contention | Serialize shrink of a segment |
| enq: SQ – contention | Lock to ensure that only one process can replenish the sequence cache |
| enq: SR – contention | Coordinates replication / streams operations |
| enq: SS – contention | Ensures that sort segments created during parallel DML operations aren’t prematurely cleaned up |
| enq: ST – contention | Synchronizes space management activities in dictionary-managed tablespaces |
| enq: SU – contention | Serializes access to SaveUndo Segment |
| enq: SW – contention | Coordinates the ‘alter system suspend’ operation |
| enq: TA – contention | Serializes operations on undo segments and undo tablespaces |
| enq: TB – SQL Tuning Base Cache Update | Synchronizes writes to the SQL Tuning Base Existence Cache |
| enq: TB – SQL Tuning Base Cache Load | Synchronizes writes to the SQL Tuning Base Existence Cache |
| enq: TC – contention | Lock held to guarantee uniqueness of a tablespace checkpoint |
| enq: TC – contention2 | Lock of setup of a unique tablespace checkpoint in null mode |
| enq: TD – KTF dump entries | KTF dumping time/scn mappings in SMON_SCN_TIME table |
| enq: TE – KTF broadcast | KTF broadcasting |
| enq: TF – contention | Serializes dropping of a temporary file |
| enq: TL – contention | Serializes threshold log table read and update |
| enq: TM – contention | Synchronizes accesses to an object |
| enq: TO – contention | Synchronizes DDL and DML operations on a temp object |
| enq: TQ – TM contention | TM access to the queue table |
| enq: TQ – DDL contention | TM access to the queue table |
| enq: TQ – INI contention | TM access to the queue table |
| enq: TS – contention | Serializes accesses to temp segments |
| enq: TT – contention | Serializes DDL operations on tablespaces |
| enq: TW – contention | Lock held by one instance to wait for transactions on all instances to finish |
| enq: TX – contention | Lock held by a transaction to allow other transactions to wait for it |
| enq: TX – row lock contention | Lock held on a particular row by a transaction to prevent other transactions from modifying it |
| enq: TX – allocate ITL entry | Allocating an ITL entry in order to begin a transaction |
| enq: TX – index contention | Lock held on an index during a split to prevent other operations on it |
| enq: UL – contention | Lock used by user applications |
| enq: US – contention | Lock held to perform DDL on the undo segment |
| enq: WA – contention | Lock used for recovery when setting Watermark for memory usage in AQ notifications |
| enq: WF – contention | This enqueue is used to serialize the flushing of snapshots |
| enq: WL – contention | Coordinates access to redo log files and archive logs |
| enq: WP – contention | This enqueue handles concurrency between purging and baselines |
| enq: XH – contention | Lock used for recovery when setting No Proxy Domains for AQ HTTP notifications |
| enq: XR – quiesce database | Lock held during database quiesce |
| enq: XR – database force logging | Lock held during database force logging mode |
| enq: XY – contention | Lock used for internal testing |
