All about my opinions on IT and much, much more …
I’m currently in Menlo Park, California at the old Sun lab from Oracle and do some Exadata testing for work. Everything TOP SECRET of course so I cannot tell anything about it. But what I can do is to share some nice pics from the facilities and the boxes! 🙂
What happens if you forget to move our redo logs to a fast disk? Well you will see something like this:
Over 1 and a half year ago I posted the enqueue waits for Oracle 10g. Well, a lot has changed in 11g including the enqueue waits. This is the new list of enqueue waits in 11gR2:
| Group | 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 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 – PST split check | Synchronizes check for Storage (PST) split in disk groups |
| ASM Enqueue | enq: AM – background COD reservation | Reserve a background COD entry |
| 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 |
| Service Operations | enq: AS – service activation | Synchronizes new service activation |
| Alter Tablespace | enq: AT – contention | Serializes ‘alter tablespace’ operations |
| Audit index file | enq: AU – audit index file | lock held to operate on the audit index file |
| 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 |
| 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 – 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 |
| 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 |
| 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 – 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 |
| LogicalStandby | enq: SB – contention | Synchronizes Logical Standby metadata 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) 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 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$, prevents multiple threads analyzing the same table during a load |
| Global Context Action | enq: ZZ – update hash tables | lock held for updating global context hash tables |
For those who made it all to the bottom here a little extra:
While I just found the enqueue waits for 10g on the internet I managed to find out how to retrieve that information. And believe it or not – it’s available just over a simple query:
SELECT eq_name "Group", ev.name "Enqueue Type", eq.req_description "Description" FROM v$enqueue_statistics eq, v$event_name ev WHERE eq.event#=ev.event# ORDER BY ev.name;
In my previous post I talked about bitmap indexes, where they should be used and where they shouldn’t be used or what you have to keep in mind when using them. Today I show you a example where they are very helpful. Remember I said that bitmap indexes where designed for read intensive environments and that they are useful on columns with low cardinality. Lets assume you have a table for all your employees in your company. The company has three different locations, lets say 1=New York; 2=London; 3=Paris:
SQL> CREATE TABLE employees ( empId NUMBER, deptId NUMBER, name VARCHAR2(50), gender VARCHAR2(6) ); Table created.
Now I insert lets say 100,000 employees distributed over the three locations. The distribution on the locations is based on a random number out of DBMS_RANDOM. Same for the gender:
SQL> BEGIN 2 FOR n IN 1..100000 LOOP 3 INSERT INTO employees VALUES (n, ROUND(DBMS_RANDOM.VALUE(0.5,3.5),0), 'Employee ' || n, DECODE(ROUND(DBMS_RANDOM.VALUE(1,2),0),1,'MALE','FEMALE')); 4 END LOOP; 5 COMMIT; 6 END; 7 / PL/SQL procedure successfully completed.
A quick look at the distributions:
SQL> SELECT gender, COUNT(*) FROM employees GROUP BY gender; GENDER COUNT(*) ------ ---------- FEMALE 49985 MALE 50015 SQL> SELECT deptId, COUNT(*) from employees GROUP BY deptId; DEPTID COUNT(*) ---------- ---------- 1 33532 2 33456 3 33012
So as you can see the distribution is good and the cardinality is “bad”. As the next step I create a normal index on it and look how it behaves.
SQL> CREATE INDEX EMPLOYEES_GENDER_I001 ON employees (gender); Index created. SQL> CREATE INDEX EMPLOYEES_DEPT_I001 ON employees (deptId); Index created. SQL> exec DBMS_STATS.GATHER_TABLE_STATS(USER, 'EMPLOYEES', CASCADE=>TRUE); PL/SQL procedure successfully completed.
Now lets have a closer look at some selects and their explain plans. First I just count the number of female employees and the number of employees in the New York (deptId=1) office. As I have indexes on both columns Oracle should retrieve the data directly from them – no need to go to the table:
SQL> set autotrace traceonly explain;
SQL> SELECT COUNT(*) FROM employees WHERE gender = 'FEMALE';
Execution Plan
----------------------------------------------------------
Plan hash value: 938208788
-----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 7 | 88 (18)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 7 | | |
|* 2 | INDEX FAST FULL SCAN| EMPLOYEES_GENDER_I001 | 50108 | 342K| 88 (18)| 00:00:01 |
-----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("GENDER"='FEMALE')
SQL> SELECT COUNT(*) FROM employees WHERE deptId = 1;
Execution Plan
----------------------------------------------------------
Plan hash value: 3728864332
-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 74 (9)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | INDEX RANGE SCAN| EMPLOYEES_DEPT_I001 | 34186 | 100K| 74 (9)| 00:00:01 |
-----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("DEPTID"=1)
I got an INDEX FAST FULL SCAN and an INDEX RANGE SCAN – fair enough. Costs are 88 and 74. Now what if I have a more complex statement? What if I want to have all male employees who are either in the New York or the London office:
SQL> SELECT COUNT(*) FROM employees WHERE gender = 'MALE' AND deptId IN (1,2);
Execution Plan
----------------------------------------------------------
Plan hash value: 1712853197
--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 9 | 177 (16)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 9 | | |
|* 2 | TABLE ACCESS FULL| EMPLOYEES | 34047 | 299K| 177 (16)| 00:00:01 |
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("GENDER"='MALE' AND ("DEPTID"=1 OR "DEPTID"=2))
This leads already to a TABLE ACCESS FULL with costs of 177. Now the same story with bitmap indexes on the columns:
SQL> DROP INDEX EMPLOYEES_GENDER_I001; Index dropped. SQL> DROP INDEX EMPLOYEES_DEPT_I001; Index dropped. SQL> CREATE BITMAP INDEX EMPLOYEES_GENDER_I001 ON employees (gender); Index created. SQL> CREATE BITMAP INDEX EMPLOYEES_DEPT_I001 ON employees (deptId); Index created. SQL> exec DBMS_STATS.GATHER_TABLE_STATS(USER, 'EMPLOYEES', CASCADE=>TRUE); PL/SQL procedure successfully completed.
First lets do the counts again:
SQL> SELECT COUNT(*) FROM employees WHERE gender = 'FEMALE';
Execution Plan
----------------------------------------------------------
Plan hash value: 3004312514
-----------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 7 | 3 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 7 | | |
| 2 | BITMAP CONVERSION COUNT | | 51145 | 349K| 3 (0)| 00:00:01 |
|* 3 | BITMAP INDEX SINGLE VALUE| EMPLOYEES_GENDER_I001 | | | | |
-----------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("GENDER"='FEMALE')
SQL> SELECT COUNT(*) FROM employees WHERE deptId = 1;
Execution Plan
----------------------------------------------------------
Plan hash value: 2661849472
---------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 3 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
| 2 | BITMAP CONVERSION COUNT | | 34039 | 99K| 3 (0)| 00:00:01 |
|* 3 | BITMAP INDEX SINGLE VALUE| EMPLOYEES_DEPT_I001 | | | | |
---------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("DEPTID"=1)
As you can see the optimizer chose another plan now for both tables. Instead of an INDEX FAST FULL SCAN and an INDEX RANGE SCAN I got now both times a BITMAP INDEX SINGLE VALUE and a BITMAP CONVERSION COUNT. Costs dropped from 88 and 74 to 3! Now the complexer statement:
SQL> SELECT COUNT(*) FROM employees WHERE gender = 'MALE' AND deptId IN (1,2);
Execution Plan
----------------------------------------------------------
Plan hash value: 597175463
-------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 9 | 9 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 9 | | |
| 2 | BITMAP CONVERSION COUNT | | 33998 | 298K| 9 (0)| 00:00:01 |
| 3 | BITMAP AND | | | | | |
|* 4 | BITMAP INDEX SINGLE VALUE | EMPLOYEES_GENDER_I001 | | | | |
| 5 | BITMAP OR | | | | | |
|* 6 | BITMAP INDEX SINGLE VALUE| EMPLOYEES_DEPT_I001 | | | | |
|* 7 | BITMAP INDEX SINGLE VALUE| EMPLOYEES_DEPT_I001 | | | | |
-------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("GENDER"='MALE')
6 - access("DEPTID"=1)
7 - access("DEPTID"=2)
I got again another plan and here you can see the power of bitmap indexes now. Oracle takes the department ids and does now a BINARY OR on the bitmap. Out of the result it does a BINARY AND with the gender bitmap and look at the result: The costs dropped from 177 on a FULL TABLE SCAN to 9 on some bitmap operations! So as you can see on this short demonstration bitmap indexes are quite powerful!
In the last few months I got more and more involved into the Data Warehouse side. This is actually not too bad – learning some new stuff brings some change into the daily routine. So on this post I tell something about BITMAP indexes as I just had this subject in work. Normally I don’t write about indexes as Richard Foote is way better in that but however:
Bitmap indexes got introduced into Oracle with 7.3 but are currently only available in the Enterprise and Personal Edition. So why did I mention the Data Warehouse above? Well, because BITMAP indexes are designed for them and ad hoc query environments. Usually you use them on columns with low cardinality. However they are NOT designed for OLTP environments where a lot of data manipulation happens in many concurrent sessions. Reason for that is because you will face some locking issues but more on that later. So how does a BITMAP index look like? The index stores a bitmap for a single value on all the rows. Lets take the emp table for example (I took here the example from Tom Kyte‘s book Expert Oracle Database Architecture site 448). I create a bitmap index on the job column. You can have a lot of employees but the jobs will keep within a limit so you will have a low cardinality there. Your index would look like this:
| Value/Row | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ANALYST | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 | 0 | 1 | 0 |
| CLERK | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 0 | 1 |
| MANAGER | 0 | 0 | 0 | 1 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| PRESIDENT | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 |
| SALESMAN | 0 | 1 | 1 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
So instead of a normal B*Tree index you don’t get a new index entry with every value pointing to the rowid but you have a bitmap for each single value and Oracle puts either a 0 or an 1 in there for a specific row. One thing to mention here: BITMAP indexes also store NULL values as 0. So why is there an issue with locking on concurrent sessions? The reason for that is that one single bitmap entry points to many rows. In the above example thing about the CLERK entry. One company will have many clerks but you have only one index entry with the underlying bitmap. Oracle cannot lock individual bits in a bitmap index entry, it has to lock the specific entry with the entire bitmap. Any other session which also tries to update the same index entry at the same time will be locked until the first session does a commit or rollback! So lets have a look into this:
First I create a new table. The table has just one column “departementID”. Lets stay at the example with the employees. We got many employees which belong to 3 different departments. Normally you would have columns like name and so forth but for this I concentrate on the departmentId:
SQL> CREATE TABLE BITMAPTEST (departmentId NUMBER) TABLESPACE DATA1; Table created.
Now I add a bitmap index on the departmentId. All I have to do is to put the word BITMAP between CREATE and INDEX:
SQL> CREATE BITMAP INDEX BITMAPTEST_BI001 ON BITMAPTEST (departmentId); Index created.
So, now I have two sessions:
Session 1 has the SID 1539
Session 2 has the SID 1470
Now session 1 inserts a row with the first departmentId:
SESS1 – 1539:
SQL> INSERT INTO BITMAPTEST VALUES (1); 1 row created.
No commit so far. Now a second session inserts a new row with the same department – another employee who started at the same department:
SESS2 – 1470:
SQL> INSERT INTO BITMAPTEST VALUES (1);
The insert is locked with a “enq: TX – row lock contention until the first session commits or does a rollback:
SQL> SELECT sid FROM v$session_wait WHERE event = 'enq: TX - row lock contention'; SID ---------- 1470
After I do a commit the lock is released:
SESS1 – 1539:
SQL> COMMIT;
Commit complete.
The second session can now create the row:
SESS2 – 1470:
1 row created.
As I said before the lock is only on the bitmap of the specific index entry. So if session 1 (session 2 did not commit or rollback yet) tries now to insert another row on another department everything is fine:
SESS1 – 1539:
SQL> INSERT INTO BITMAPTEST VALUES (2); 1 row created.
What does that mean on an OLTP environment? As an OLTP environment is very write/modification intensive system concurrent sessions will lock each other out when they try do modify a bitmap index entry. Here a small test:
I take again the bitmaptest table with the departmentId. Now I insert 1,000 rows with 3 different department IDs like this:
DECLARE departmentID NUMBER:=1; maxLoop NUMBER := 1000; BEGIN FOR n1 IN 1..maxLoop LOOP IF departmentID = 4 THEN departmentID := 1; END IF; INSERT INTO bitmapTest VALUES (departmentID); departmentID := departmentID+1; END LOOP; COMMIT; END; /
Note: The commit happens after the loop. So all other sessions will have to wait on this one to finish before they can start.
I run this PL/SQL now in 100 concurrent sessions over a shell script:
processes=100; i=0; while [ $i -le $processes ]; do runInserts.sh & i=`expr $i + 1`; done;
Now lets see what happens. To monitor the locks I just do a simple SQL on v$session_wait. I should see there lots of waits with “enq: TX – row lock contention”. So I run the test and what I get is:
SQL> SELECT count(*), event, state, wait_class FROM v$session_wait GROUP BY event, state, wait_class ORDER BY 1 DESC; COUNT(*) EVENT STATE WAIT_CLASS ---------- ---------------------------------------------------------------- ------------------- ----------------- 136 SQL*Net message from client WAITING Idle 90 enq: TX - row lock contention WAITING Application 12 rdbms ipc message WAITING Idle 1 buffer busy waits WAITED SHORT TIME Concurrency 1 smon timer WAITING Idle 1 wait for unread message on broadcast channel WAITING Idle 1 Streams AQ: qmn coordinator idle wait WAITING Idle 1 SQL*Net message to client WAITED SHORT TIME Network 1 pmon timer WAITING Idle 1 Streams AQ: qmn slave idle wait WAITING Idle 1 Streams AQ: waiting for messages in the queue WAITING Idle 11 rows selected.
As you can see I have 90 sessions with row lock contention. When I reexecute this statement several times the amount of sessions reduces just slowly as just one session is running after each other:
SQL> r 1 SELECT count(*), event, state, wait_class 2 FROM v$session_wait 3 GROUP BY event, state, wait_class 4* ORDER BY 1 DESC COUNT(*) EVENT STATE WAIT_CLASS ---------- ---------------------------------------------------------------- ------------------- ----------------- 136 SQL*Net message from client WAITING Idle 86 enq: TX - row lock contention WAITING Application 12 rdbms ipc message WAITING Idle 1 buffer busy waits WAITED SHORT TIME Concurrency 1 smon timer WAITING Idle 1 wait for unread message on broadcast channel WAITING Idle 1 Streams AQ: qmn coordinator idle wait WAITING Idle 1 SQL*Net message to client WAITED SHORT TIME Network 1 pmon timer WAITING Idle 1 Streams AQ: qmn slave idle wait WAITING Idle 1 Streams AQ: waiting for messages in the queue WAITING Idle 11 rows selected. SQL> r 1 SELECT count(*), event, state, wait_class 2 FROM v$session_wait 3 GROUP BY event, state, wait_class 4* ORDER BY 1 DESC COUNT(*) EVENT STATE WAIT_CLASS ---------- ---------------------------------------------------------------- ------------------- ----------------- 136 SQL*Net message from client WAITING Idle 83 enq: TX - row lock contention WAITING Application 12 rdbms ipc message WAITING Idle 1 buffer busy waits WAITED SHORT TIME Concurrency 1 smon timer WAITING Idle 1 wait for unread message on broadcast channel WAITING Idle 1 Streams AQ: qmn coordinator idle wait WAITING Idle 1 SQL*Net message to client WAITED SHORT TIME Network 1 pmon timer WAITING Idle 1 Streams AQ: qmn slave idle wait WAITING Idle 1 Streams AQ: waiting for messages in the queue WAITING Idle 11 rows selected.
So when I pick out one of those sessions I can have a look on what I am waiting on just be doing a select on v$session and all_objects:
SQL> SELECT count(*), s.row_wait_obj# FROM v$session s, v$session_wait w WHERE s.sid=w.sid AND w.event = 'enq: TX - row lock contention' GROUP BY s.row_wait_obj#; COUNT(*) ROW_WAIT_OBJ# ---------- ------------- 96 7154476
96 sessions waiting on the object with the ID 7154476 which is
SQL> SELECT object_name, object_type FROM all_objects WHERE object_id = 7154476; OBJECT_NAME OBJECT_TYPE ------------------------------ ------------------- BITMAPTEST_I001 INDEX
as you can see my BITMAP index! So what does this mean? As I wrote already before: BITMAP index are designed for data warehouse or read intensive environments to speed up queries where you have a low cardinality on it. As Tom Kyte already wrote in his book: Low cardinality always depends on the data. In a table with 10,000 rows a low cardinality might be 10 while in a 10 million table 10,000 might be a low cardinality! It always depends, there is no cut-and-dried answer for this. You simply have to try it out. If you want to use a BITMAP index in an OLTP environment be aware of the locking! There might be valid cases to use them inside there as for example on the “gender” column of a “employees” table to speed up reports. It always depends if the table you load is heavily modified or not and if you load the table concurrent or not!
Sources:
