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Types of Enqueues in Oracle Database

Oracle Database Contentions

Angudi Tech

6/2/20261 min read

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Enqueues are memory structures that function as locks, ensuring serialized access to database resources. They may be linked to either a session or a transaction. The names of enqueues are shown in the LOCK_TYPE column of the DBA_LOCK and DBA_LOCK_INTERNAL data dictionary views.

Any resource distinctly identifies an object that may be locked by various sessions within a single instance (local resource) or across multiple instances (global resource). Each session attempting to lock the resource will place an enqueue on it.

There are more than 60 different types of such enqueues in Oracle Database,

  • BL, Buffer Cache Management

  • BR, Backup/Restore

  • CF, Controlfile Transaction

  • CI, Cross-instance Call Invocation

  • CU, Bind Enqueue

  • DF, Datafile

  • DL, Direct Loader Index Creation

  • DM, Database Mount

  • DR, Distributed Recovery Process

  • DW, SecureFiles

  • DX, Distributed Transaction

  • FP, File Object

  • FS, File Set

  • HW, High-Water Lock

  • IN, Instance Number

  • IR, Instance Recovery

  • IS, Instance State

  • IV, Library Cache Invalidation

  • JI, Enqueue used during AJV snapshot refresh

  • JQ, Job Queue

  • KK, Redo Log "Kick"

  • KP, contention in Oracle Data Pump startup and shutdown processes

  • KO, Multiple Object Checkpoint

  • L[A-P], Library Cache Lock

  • LS, Log Start or Switch

  • MM, Mount Definition

  • MR, Media Recovery

  • N[A-Z], Library Cache Pin

  • PE, ALTER SYSTEM SET PARAMETER = VALUE

  • PF, Password File

  • PI, Parallel Slaves

  • PR, Process Startup

  • PS, Parallel Slave Synchronization

  • Q[A-Z], Row Cache

  • RO, Object Reuse

  • RT, Redo Thread

  • RW, Row Wait

  • SC, System Change Number

  • SM, SMON

  • SN, Sequence Number

  • SQ, Sequence Number Enqueue

  • SR, Synchronized Replication

  • SS, Sort Segment

  • ST, Space Management Transaction

  • SV, Sequence Number Value

  • TA, Transaction Recovery

  • TC, Thread Checkpoint

  • TE, Extend Table

  • TM, DML Enqueue

  • TO, Temporary Table Object Enqueue

  • TS, Temporary Segment (also TableSpace)

  • TT, Tablespace

  • TX, Transaction

  • UL, User-defined Locks

  • UN, User Name

  • US, Undo Segment, Serialization

  • WL, Being Written Redo Log

  • XA, Instance Attribute Lock

  • XI, Instance Registration Lock

  • ZA, Exclusive Lock When Moving Audit Table