V $ waitstat and save undo segment Header/save undo Block

For example

SQL> select * from V $ waitstat;

Class count time
--------------------------------------
Data Block 2 1
Sort Block 0 0
Save undo Block 0 0
Segment header 1 0
Save undo header 0 0
Free List 0 0
Extent map 0 0
1st Level BMB 0 0
2nd Level BMB 0 0
3rd Level BMB 0 0
Bitmap Block 0 0

Class count time
--------------------------------------
Bitmap index Block 0 0
File Header Block 0 0
Unused 0 0
System undo header 0 0
System undo Block 0 0
Undo header 0 0
Undo Block 0 0

18 rows have been selected.

SQL>

In x $ BH. Class
5: Save undo segment Header
3: Save undo Block

It indicates the defferd rollback segment block (refer to Oracle concepts)

SQL> select count (*) from x $ BH where class = 3;

Count (*)
----------
0

SQL> select count (*) from x $ BH where class = 5;

Count (*)
----------
0

SQL> Delete Rainy. t where rownum & lt; 11;

10 rows have been deleted.

SQL> alter tablespace users offline;

The tablespace has been changed.

SQL> select count (*) from x $ BH where class = 5;

Count (*)
----------
1

SQL> select count (*) from x $ BH where class = 3;

Count (*)
----------
1

SQL> alter tablespace users online;

The tablespace has been changed.

SQL> select count (*) from Rainy. t;

Count (*)
----------
90

SQL> select count (*) from x $ BH where class = 5;

Count (*)
----------
0

SQL> select count (*) from x $ BH where class = 3;

Count (*)
----------
0

SQL>