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Oracle index maintenance

Oracle Tips by Burleson Consulting

Oracle Indexes Is Maintenance Required?

The question about whether Oracle indexes are self-balancing is largely a matter of semantics.  As rows are added to an empty table, Oracle controls the addition of same level blocks, called splitting, until the higher level index node is unable to hold any more key pointer pairs.  When the index can no longer split because the owner block is full, Oracle will spawn a whole new index level, keeping the index tree in perfect logical and physical balance.

 

However, DELETEs are a different story.  Physically, Oracle indexes are always balanced because empty blocks stay inside the tree structure after a massive DELETE.  Logically, Oracle indexes are not self-balancing because Oracle does not remove the dead blocks as they become empty.  For example, Figure 15.10 shows an Oracle index before a massive delete.

 

Figure 15.10:  A physical index before a massive row delete

 

Now, after a massive delete, the physical representation of the index is exactly the same because the empty data blocks remain as illustrated in Figure 15.11.  However, the logical internal pointer structure is quite unbalanced, because Oracle has routed around the deleted leaf nodes and has placed the empty index blocks back on the freelist, where they can be reused anywhere in the index tree structure

 

Figure 15.11:  The logical pointer structure of an index after a massive row delete

 

This type of sparse index is typical of an index on highly active tables with large scale INSERTs, DELETEs and UPDATEs.  There may be thousands of empty or near empty index blocks, and the sparse data can cause excessive I/O.  There are several types of Oracle execution steps that will run longer on this type of sparse index:

       Index Range Scans: Index range scans that must access many near empty blocks will have excessive I/O compared to a rebuilt index.

       Index Fast Full Scans: Because 70% of an index can be deleted and the index will still have the same number of data blocks, a full index scan might run many times slower before it is rebuilt.

Since the SQL must visit the sparse blocks, the task will take longer to execute.

SEE CODE DEPOT FOR FULL SCRIPTS


This is an excerpt from my latest book "Oracle Tuning: The Definitive Reference". 

You can buy it direct from the publisher for 50%-off and get instant access to the code depot of Oracle tuning scripts:

http://www.rampant-books.com/book_1002_oracle_tuning_definitive_reference_2nd_ed.htm

 


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