You recently gathered statistics for a table by using the following commands: 
You noticed that the performance of queries has degraded after gathering statistics. You want to
use the old statistics. The optimizer statistics retention period is default.
What must you do to use the old statistics?
A.
Use the flashback to bring back the statistics to the desired time.
B.
Restore statistics from statistics history up to the desired time.
C.
Delete all the statistics collected after the desired time.
D.
Set OPTIMIZER_USE_PENDING_STATISTICS to TRUE.
Explanation:
Whenever statistics in dictionary are modified, old versions of statistics are saved
automatically for future restoration. Statistics can be restored using RESTORE procedures of
DBMS_STATS package. These procedures use a time stamp as an argument and restore
statistics as of that time stamp. This is useful in case newly collected statistics leads to some suboptimal execution plans and the administrator wants to revert to the previous set of statistics.
Reference: Oracle Database Performance Tuning Guide, Restoring Previous Versions of Statistics