V$DATAGUARD_STATS displays how much redo data generated by the primary database is not yet available on the standby database, showing how much redo data could be lost if the primary database were to crash at the time you queried this view. You can query this view on any instance of a standby database in a Data Guard configuration. If you query this view on a primary database, then the column values are cleared.
Name of the parameter:
APPLY FINISH TIME - Estimated time before log apply services will finish applying the redo data available on the standby database. If there are gaps in the log files, then this parameter shows the time it will take to resolve the gap with the lowest SCN.
APPLY LAG - Amount of time that the application of redo data on the standby database lags behind the primary database.
TRANSPORT LAG - How much redo data (in time units) generated by the primary database is not available or applicable on the standby database at the time of computation.
Note: This parameter includes the redo data that has not been transmitted to the standby database and redo data that is available on the standby database but has not been applied yet, such as the redo data for resolving a gap.
ESTIMATED STARTUP TIME - Estimated time the physical standby database was started. This is not displayed for logical standby databases.
STANDBY HAS BEEN OPEN - Indicates if the physical standby database was opened in read-only mode at any time since the last time the database was started. The parameter value indicates if the instance must be stopped and restarted after a failover. If the physical standby database has been opened in read-only mode since the last time it was started and it is the target of a failover, then you must shut down the database and restart it in read/write mode. If the primary database must be shut down and restarted, then use the sum of APPLY FINISH TIME and ESTIMATED STARTUP TIME to obtain an estimated failover time (the estimate assumes SHUTDOWN ABORT).
Value of the parameter. For example, the APPLY FINISH TIME parameter value +00 00:00:01.7 indicates the standby database needs 1.7 seconds to finish applying the remaining redo data.
Time unit for the parameter, displayed as day (2) to second (n) interval, where n is 0 or 1.
Local time on the standby database when the value of the parameter was computed or sampled.
The APPLY LAG and TRANSPORT LAG metrics are computed based on information received from the primary database. An unchanging value in this column for the APPLY LAG and TRANSPORT LAG metrics indicates that these metrics are not being updated (that is, they have become stale), possibly due to a communications fault between the primary and standby.