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Oracle Metric undo segment extension

Oracle Consulting Tips by Burleson


The undo segment extension Oracle metric occurs when the undo segment extension is being extended or shrunk. The session must wait until the operation on the undo segment has finished.

   sess.username username,
   swh.event     event,
   v$session               sess,
   v$session_wait_history  swh
   sess.sid = 74
   sess.sid = swh.sid
order by
SEQ# SID USERNAME EVENT                       P1         P2
---- --- -------- --------------------------- ---------- ----------
   1  74 PCS      buffer busy waits                    3      21277
   2  74 PCS      latch: cache buffers chains 1556332118        172
   3  74 PCS      latch: cache buffers chains 1556332118        172
   4  74 PCS      buffer busy waits                    4        155


By querying the $session_wait_history view, it becomes clear that users were faced with some additional waits including buffer busy waits for UNDO segment determined by P1=3 and P2=21277.  Also, it appears that the session had many waits for cache buffer chain latch. This contention has two likely causes: very long buffer chains or very heavy access to the same data blocks.

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