Using alert type with REASON_ID=123 as an example, an alert can be initiated when the free
space in the database recovery area falls below a specified percentage.
V$UNDOSTAT
Having too much undo space and having not enough undo space are both problems. Although
an alert can be set up to notify the DBA when the undo space is not sufficient to provide enough
transaction history to satisfy Flashback queries or enough ???before??? image data to prevent
???Snapshot Too Old??? errors, a DBA can be proactive by monitoring the dynamic performance
view V$UNDOSTAT during heavy database usage periods.
V$UNDOSTAT displays historical information about the consumption of undo space for tenminute
intervals. By analyzing the results from this table, a DBA can make informed decisions
when adjusting the size of the undo tablespace or changing the value of the UNDO_RETENTION
initialization parameter.
V$OBJECT_USAGE
If an index is not being used, it not only takes up space that could be used by other objects, but
the overhead of maintaining the index whenever an insert, update, or delete occurs is wasted.
By using the alter index . . . monitoring usage command, the view V$OBJECT_USAGE will be
updated when the index has been accessed indirectly because of a select statement.
V$SORT_SEGMENT
The view V$SORT_SEGMENT can be used to view the allocation and deallocation of space in a
temporary tablespace??™s sort segment. The column CURRENT_USERS indicates how many distinct
users are actively using a given segment.
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