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Important 2007 update:  The rules for identification of candidates for index rebuilding are changing.  Please see my updated notes on index rebuilding.


Col. John Garmany, noted Oracle author and indexing expert, gave a great IOUG Live! 2005 presentation on the internals of Oracle indexing.  In a repeat of the top-rated OracleWorld 2004 presentation, Garmany noted several important areas of Oracle indexing:

  • Detecting and removing un-used indexes
  • Managing large multi-column indexes
  • Index rebuilding
  • New Oracle 10g AWR index tables

The full presentation can be downloaded here.  Col. Garmany makes several important notes:

Why Rebuilt Indexes are Problems

A tightly packed index is more efficient for the database as long as no changes are made in the underlying table. Once you start making changes, the index blocks start to split and reintroduce the fluff. Not only is fluff reintroduced, but there is redo created as blocks are added to the index. The higher the rate of change, the faster your nicely paced index will return to a steady state of fluff.

When to Rebuild Indexes

The only time you need to rebuild indexes is when the cost to rebuild is less that the performance gained. The first one is obvious. Pack your tables and indexes (rebuild into the soon to be read-only tablespace) tightly and they will stay that way. The second is much more difficult. First, it is a continuing process since the index will move toward fluff with use. Second, there is the cost of rebuilding the index AND the cost of the additional redo as the index changes. There is only one method to determine is rebuilding an indexes benefits your database, testing.

Oracle 10g Index Tables

The problem has always been that it is very difficult to know what indexes are the most popular. In Oracle10g we can easily see what indexes are used, when they are used and the context where they are used. Here is a simple AWR query to plot index usage:

col c1 heading ‘Object|Name’ format a30
col c2 heading ‘Operation’ format a15
col c3 heading ‘Option’ format a15
col c4 heading ‘Index|Usage|Count’ format 999,999
break on c1 skip 2
break on c2 skip 2

select
   p.object_name c1,
   p.operation c2,
   p.options c3,
   count(1) c4
from
   dba_hist_sql_plan p,
   dba_hist_sqlstat s
where
   p.object_owner <> 'SYS'
and
   p.operation like ‘%INDEX%’
and
   p.sql_id = s.sql_id
group by
   p.object_name,
   p.operation,
   p.options
order by
   1,2,3;

Overall, an excellent and informative presentation.

See my related notes on index rebuilding:

 

Update: (November 1, 2007)

When can we "prove" a benefit from an index rebuild?  Here, Robin Schumacher proves that an index that is rebuilt in a larger tablespace will contain more index entries be block, and have a flatter structure:

"As you can see, the amount of logical reads has been reduced in half simply by using the new 16K tablespace and accompanying 16K data cache."

In an OracleWorld 2003 presentation titled “Oracle Database 10g: The Self-Managing Database” by Sushil Kumar of Oracle Corporation, Kumar states that the new Automatic Maintenance Tasks (AMT) Oracle10g feature will "automatically detect and re-build sub-optimal indexes.“

This Kim Floss article shows the Oracle 10g segment advisor recommending a rebuild of an index: 

“The page lists all the segments (table, index, and so on) that constitute the object under review. The default view ("View Segments Recommended to Shrink") lists any segments that have free space you can reclaim.”


Oracle index rebuild advisor (Source: Oracle Corporation)

See my related notes on index rebuilding:

 

 


 

 
 
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