segment which may be located at the high water mark for the segment the hints in the global table may be ordered for example based on the extent commit time when a process uses a hint from the local table shown in fig3 a it may obtain all the other hints from the same bucket the process may then place the other hints from the bucket into the global table of undo hints shown in fig3 b if it is determined that those hints expire sooner than the hints currently in the table an hint may be considered u201c valid u201d if the extent with the commit time contained in the hint has expired even if a hint is valid it is not guaranteed that space will be available because another process may have already used the hint when a process uses a hint from the local table shown in fig3 a it also reduces the undo retention time to ensure that the hint is valid in this case all the other hints in the bucket will be valid as well thus at the time hints are placed into the global table of fig3 b the hints will be valid however in systems with auto tuning of undo retention the undo retention may be automatically adjusted based on the system activity such tuning may be known as u201c active tuning u201d and may occur independently of the tables of undo hints to attempt to prevent the system from encountering space pressures if the system does encounter space pressure u201c reactive tuning u201d may be performed to adjust the undo retention time reactive tuning may include consulting the tables of undo hints to determine an appropriate amount to decrease the undo retention because some time passes between.