DBA_HIST_UNDOSTAT

DBA_HIST_UNDOSTAT displays the history of histograms of statistical data to show how well the system is working. The available statistics include undo space consumption, transaction concurrency, and length of queries executed in the instance. This view contains snapshots of V$UNDOSTAT.

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