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Important update:  The rules for identification of candidates for index coalescing/rebuilding depend on your specific index state.  See MOSC notes 989186.1, 122008.1, 989093.1 for Oracle?s suggestions on when to coalesce/rebuild indexes. Also see my notes on index coalesce/rebuilding and note this demonstration of an index that benefits from scheduled oracle index coalesce/rebuilding.


Oracle Index rebuilding

There is a lot of great information about Oracle index rebuilding, and current research reveals how Oracle rebuilds indexes.  Index rebuilding is risk free when done properly, and on OTN, Chen Shapira reports that an index rebuild reduced index range scan I/O:

"We noticed no change of the execution plan after rebuild. The plan was exactly the same, but an index scan that required lots of block reads and was very costly, now takes significantly less. Which is exactly what we needed!"

Check out our notes on the subject: Oracle Index rebuilding details and scripts

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:

Oracle author claims to have perfected script to detect bad indexes

In this article, author Boris Milrud says that Oracle indexes are not self-balancing and proposed a method for detecting when an index needs rebuilding.  Milrud notes that Oracle b-tree indexes become out of balance via high DML and he claims that they have lower performance:

"Oracle indexes are not self-balancing. They become fragmented after a large number of INSERTs and DELETEs, which may lead to significant performance degradation. In this 10-Minute Solution I show you how to detect these "out-of-shape" indexes and cure them.

The author claims that these statistics are relevant to the index rebuilding decision:

"The following INDEX_STATS columns are especially useful:

  • height refers to the maximum number of levels encountered within the index. An index could have 90 percent of the nodes at three levels, but excessive splitting and spawning in one area of the index with heavy DML operations could make nodes in that area to have more than three levels.
     
  • lf_rows refers to the total number of leafs nodes in the index.
     
  • del_lf_rows refers to the number of leaf rows that have been marked deleted as a result of table DELETEs."
Milrud also claims that an index rebuild made a batch job run 1000x faster:
 
"In other words, massive DELETEs seemed to damage certain index areas, but leave other areas intact.

Anyway, it was clear to me that it was high time to fix that index! After I added code to rebuild the index at the end of the procedure and recompiled it, the next run took only 14 seconds."


 

 
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