读UNDO引发的db file sequential read

SQL> select * from (select SESSION_ID,
       NAME,
       P1,
       P2,
       P3,
       WAIT_TIME,
       CURRENT_OBJ#,
       CURRENT_FILE#,
       CURRENT_BLOCK#
  from v$active_session_history ash, v$event_name enm
 where ash.event# = enm.event#
and ash.session_id=49)
where rownum<10;  2    3    4    5    6    7    8    9   10   11   12   13  

SESSION_ID NAME 				  P1	     P2 	P3  WAIT_TIME CURRENT_OBJ# CURRENT_FILE# CURRENT_BLOCK#
---------- ------------------------------ ---------- ---------- ---------- ---------- ------------ ------------- --------------
	49 resmgr:cpu quantum			   2	      0 	 0	    0		 0	       3	 666568
	49 db file sequential read		   3	 661691 	 1	    0		 0	       3	 661691
	49 db file sequential read		   3	 661568 	 1	    0		 0	       3	 661568
	49 db file sequential read		   3	 665469 	 1	    0		 0	       3	 665469
	49 resmgr:cpu quantum			   2	      0 	 0	    0		 0	       3	 933207
	49 db file sequential read		   3	 933125 	 1	    0		 0	       3	 933125
	49 db file sequential read		   3	 656639 	 1	    0		 0	       3	 656639
	49 db file sequential read		   3	 656473 	 1	    0		 0	       3	 656473
	49 db file sequential read		   3	 663120 	 1	    0		 0	       3	 663120

9 rows selected.

SQL> select owner, segment_name, segment_type
  from dba_extents
 where file_id = 3
   and 661691 between block_id
   and block_id + blocks - 1 ;  2    3    4    5  

OWNER			       SEGMENT_NAME									 SEGMENT_TYPE
------------------------------ --------------------------------------------------------------------------------- ------------------
SYS			       _SYSSMU2_967517682$								 TYPE2 UNDO