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DBA_HIST_UNDOSTAT 显示Oracle UNDO表空间的历史的统计数据直方图，实例中的这些数据包括UNDO消耗、事务一致和查询执行的时长，该视图包含V$UNDOSTAT的快照。
| Column | Datatype | NULL | Description |
|---|---|---|---|
BEGIN_TIME | DATE | NOT NULL | Identifies the beginning of the time interval |
END_TIME | DATE | NOT NULL | Identifies the end of the time interval |
DBID | NUMBER | NOT NULL | Database ID for the snapshot |
INSTANCE_NUMBER | NUMBER | NOT NULL | Instance number for the snapshot |
SNAP_ID | NUMBER | NOT NULL | Unique snapshot ID |
UNDOTSN | NUMBER | NOT NULL | Represents the last active undo tablespace in the duration of time. The tablespace ID of the active undo tablespace is returned in this column. If more than one undo tablespace was active in that period, the active undo tablespace that was active at the end of the period is reported. |
UNDOBLKS | NUMBER | Represents the total number of undo blocks consumed. You can use this column to obtain the consumption rate of undo blocks, and thereby estimate the size of the undo tablespace needed to handle the workload on your system. | |
TXNCOUNT | NUMBER | Identifies the total number of transactions executed within the period | |
MAXQUERYLEN | NUMBER | Identifies the length of the longest query (in number of seconds) executed in the instance during the period. You can use this statistic to estimate the proper setting of the UNDO_RETENTION initialization parameter. The length of a query is measured from the cursor open time to the last fetch/execute time of the cursor. Only the length of those cursors that have been fetched/executed during the period are reflected in the view. | |
MAXQUERYSQLID | VARCHAR2(13) | SQL identifier of the longest running SQL statement in the period | |
MAXCONCURRENCY | NUMBER | Identifies the highest number of transactions executed concurrently within the period | |
UNXPSTEALCNT | NUMBER | Number of attempts to obtain undo space by stealing unexpired extents from other transactions | |
UNXPBLKRELCNT | NUMBER | Number of unexpired blocks removed from certain undo segments so they can be used by other transactions | |
UNXPBLKREUCNT | NUMBER | Number of unexpired undo blocks reused by transactions | |
EXPSTEALCNT | NUMBER | Number of attempts to steal expired undo blocks from other undo segments | |
EXPBLKRELCNT | NUMBER | Number of expired undo blocks stolen from other undo segments | |
EXPBLKREUCNT | NUMBER | Number of expired undo blocks reused within the same undo segments | |
SSOLDERRCNT | NUMBER | Identifies the number of times the error ORA-01555 occurred. You can use this statistic to decide whether or not the UNDO_RETENTION initialization parameter is set properly given the size of the undo tablespace. Increasing the value of UNDO_RETENTION can reduce the occurrence of this error. | |
NOSPACEERRCNT | NUMBER | Identifies the number of times space was requested in the undo tablespace and there was no free space available. That is, all of the space in the undo tablespace was in use by active transactions. The corrective action is to add more space to the undo tablespace. | |
ACTIVEBLKS | NUMBER | Total number of blocks in the active extents of the undo tablespace for the instance at the sampled time in the period | |
UNEXPIREDBLKS | NUMBER | Total number of blocks in the unexpired extents of the undo tablespace for the instance at the sampled time in the period | |
EXPIREDBLKS | NUMBER | Total number of blocks in the expired extents of the undo tablespace for the instance at the sampled time in the period | |
TUNED_UNDORETENTION | NUMBER | System tuned value indicating the period for which undo is being retained |
默认情况下V$UNDOSTAT中的信息每10分钟增加一条记录，V$UNDOSTAT一般仅保留576条最近的UNDO记录(相当于4天的UNDO信息，10g 之前保存1008条，相当于7天的信息量)，超过的该时间的信息则被保存在DBA_HIST_UNDOSTAT视图中。该统计仅在UNDO自动管理的模式下有效。
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