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Don Burleson Blog 









Predict table and index full-scan activity

Oracle Database Tips by Donald Burleson

In  Oracle 10g we have the dba_hist_seg_stat table with a wealth of information about all active segments within the database, including their invocations of full-scans (full-table scans, index fast-full scans).

Full scans are used when the CBO determines that it's is better to access all of the table of index blocks, rather than use an index, and tracking full-scans is an important tool for measuring the efficiency of indexes.

The dba_hist_seg_stat table has information of table usage, especially the buffer gets (logical reads), disk reads (physical reads) and the number of full-scans experienced by any database segment:

As I note in my book Oracle Tuning: The Definitive Reference, we can quickly plot this information for trending and predictions:

   to_char(begin_interval_time,'yy/mm/dd/hh24') c1,
   table_scans_total c2
   dba_hist_seg_stat s,
   dba_hist_seg_stat_obj o,
   dba_hist_snapshot sn
   o.owner = 'TCI_SCHEMA_071805'
   s.obj# = o.obj#
   sn.snap_id = s.snap_id
   object_name = 'GEN_PERSON'
order by

Now, a quick paste into the MS Excel spreadsheet and the chart wizard gives us a great trend line for full-scan for our Oracle table.  Remember, not all full-scans are bad, but a full-scan is an indicator if a missing or sub-optimal index.

As we can see, the dba_hist_seg_stat view is a wonderful way to capture object-level space and usage information for predictive modeling purposes.

This is part of my latest book "Oracle Tuning: The Definitive Reference". 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts:



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