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Oracle Metric system undo header

Oracle Consulting Tips by Burleson


The system undo header Oracle metric shows waits on the system rollback segment header.

the dba_hist_active_sess_history view can be queried to identify particular sessions and objects that caused the high contention. The datafile and objects ids can be found in the v$session_wait dynamic view. The wait_stat_int_10g.sql query can be used to retrieve historical block contention statistics:


select e.class                                 "E.CLASS"
     , e.wait_count   - nvl(b.wait_count,0)     "Waits"
     , e.time        - nvl(b.time,0)           "Total Wait Time (cs)"
     , (e.time       - nvl(b.time,0)) /
       (e.wait_count - nvl(b.wait_count,0))    "Avg Time (cs)"
  from dba_hist_waitstat b
     , dba_hist_waitstat e
 where b.snap_id         = &pBgnSnap
   and e.snap_id         = &pEndSnap
   and b.dbid            = &pDbId
   and e.dbid            = &pDbId
   and b.dbid            = e.dbid
   and b.instance_number = &pInstNum
   and e.instance_number = &pInstNum
   and b.instance_number = e.instance_number
   and b.class           = e.class
   and b.wait_count      < e.wait_count
 order by 3 desc, 2 desc


The sample query output looks like:

SQL> @Wait_stat_int_10g.sql
E.CLASS                 Waits Total Wait Time (cs) Avg Time (cs)
------------------ ---------- -------------------- -------------
undo header                97                  121    1,24742268
file header block           2                  114            57

The output of the wait_stat_int_10g.sql script shows which particular buffer wait events play a significant role.

 The Ion tool has a corresponding report named Buffer Waits Summary that generates time-series charts for the block contention statistics.

This, and many other Oracle performance metrics are discussed in my book "Oracle Tuning" by Rampant TechPress.  You can buy it directly from the publisher and save 30% at this link:



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