Enqueue

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.

Following are the Classes of enqueue event
BL, Buffer Cache Management
CF, Controlfile Transaction
CI, Cross-instance Call Invocation
CU, Bind Enqueue
DF, Datafile
DL, Direct Loader Index Creation
DM, Database Mount
DR, Distributed Recovery
DX, Distributed TX
FS, File Set
IN, Instance Number
IR, Instance Recovery
IS, Instance State
IV, Library Cache Invalidation
JQ, Job Queue
KK, Redo Log "Kick"
L[A-P], Library Cache Lock
MR, Media Recovery
N[A-Z], Library Cache Pin
PF, Password File
PI, Parallel Slaves
PR, Process Startup
PS, Parallel Slave Synchronization
Q[A-Z], Row Cache
RT, Redo Thread
SC, System Commit Number
SM, SMON
SQ, Sequence Number Enqueue
SR, Synchronized Replication
SS, Sort Segment
ST, Space Management Transaction
SV, Sequence Number Value
TA, Transaction Recovery
TM, DML Enqueue
TS, Temporary Segment (also TableSpace)
TT, Temporary Table
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