latch: undo global data Oracle等待:
This latch is taken out when we need to search or manipulate the array of rollback (undo) segments used by
the instance (this is pointed to by the fixed SGA variable “ktugd” of type “struct ktugt”).
We need to search / manipulate the array to locate the rollback segment for such operations as SET
TRANSACTION USE ROLLBACK SEGMENT, shutdown, any ALTER ROLLBACK SEGMENT
operation, binding a rollback segment to a transaction etc.
1. General latch contention issues.
2. This latch is acquired in many places throughout the code and hence a good first step is to determine
exactly where in the code you are seeing most of the contention. Repeated selecting from
v$latch_misses for this latch and noting the increments may reveal where the contention lies and give a
clue as to how best to avoid it.
latch: undo global data在11g上是非共享的,有多个child的latch