enq: TA – contention 等待事件

enq: TA – contention 等待事件

This enqueue is used when undo tablespace operations are being performed. Some examples of such
operations are:
When dropping an undo tablespace we acquire the enqueue in exclsuive mode to stop other sessions
using the undo tablespace at that point. If we cannot acquire the enqueue because the undo tablespace
is in use then we return an error.
When onlining an undo segment.
Undo segment recovery.
Instance shutdown.

The id1 value records the operation in use and maps to the following values under 11.2 (beta):
KTU_INSTANCE_SEG1 (1) – instance lock on undo segment lock 1
KTU_INSTANCE_IUP (2) – instance lock on instance existence
KTU_INSTANCE_SEG2 (3) – instance lock on undo segment lock 2
KTU_INSTANCE_EXC (4) – mutual-exclusive lock on starting up
KTU_XA_LOCKS (5) – transaction enqueue
KTU_INSTANCE_UT (6) – instance lock on undo tablespace
KTU_SWITCH_UT (7) – switch (& recovery of) undo tbs operation
KTU_INSTANCE_INIT (8) – undo segment initialisation (during start of cache recovery)
Id2 is typically used to identify the undo segment but for some operations it is just hard-coded as 0.