I am not getting into the moot of when to rebuild index or not, since these things already discussed by many experts at their sites. I am just providing the internal machanism, pros & cons of rebuilding index.
Oracle has 4 main features with regard to its internal maintenance of
indexes that makes index rebuilds such a rare requirement.
1) 50-50 block split and self-balancing mechanism
2) 90-10 block split mechanism for monotonically increasing values
3) Reusability of deleted row space within an index node
4) Reusability of emptied nodes for subsequent index splits.
These 4 features combined almost eliminates the need for index rebuilds.
In fact most indexes that have index entries randomly distributed throughout
the index tree structure remain both balanced (as all Oracle indexes must
be) and fragmentation free *because* leaf row entries are reused.
Inserts/Updates and Deletes result in "holes" being created but importantly
"refilled" typically at even rates. The issue of wasted deleted space is
therefore typically a non-issue.
Rebuilding such indexes can actually be detrimental to overall performance
for a number of reasons. Firstly, it requires a significant amount of
resources and can conflict with the general running of the database. But
perhaps more importantly, it can actually be self-defeating in what rebuilds
are supposed to achieve. That's because after an index rebuild, the index is
more tightly packed with less overall free space (else why rebuild). This
means however that index splits are more likely to now occur which directly
impacts performance due to the additional I/O and CPU this entails. And
after the block split, we now have two blocks each with 50% free space.
After a period of time, the index potentially has "issues" due to
insufficient used space and the vicious rebuild cycle continues. The better
course of action is to do nothing and let the index evolve to it's natural
When should one perform a rebuild?
Firstly, if the index value were to have monotonically increasing values
then any deleted space could be a problem as this space may not be reused
(making feature 3 above redundant). However, if sufficient entries are
deleted resulting in index nodes being fully emptied (say via a bulk delete)
then feature 4 would kick in and the deleted space could be reused. The
question now becomes one of *when* would the equivalent amount of index
entries be reinserted from the time of the deletions, as index scans (in all
it's manifestations) would be impacted during this interim period. So
monotonically increasing values *and* sparse deletions would present one
case for an index rebuild. These types of indexes can be identified as
having predominately 90-10 splits rather than the usual 50-50 split.
Another case would be an index that has deletions without subsequent inserts
or inserts within an acceptable period of time. Such a case would result in
wasted space that can't be effectively reused as there's not the sufficient
insert activity to reclaim the space. However, in this scenario, it's really
the *table* itself rather than the indexes directly that should be rebuilt.
Because such "shrinkage" results in both the table and associated indexes
being fragmented with HWMs that need resetting (to prevent performance
issues with Full Table Scans and all types of Index Scans). Yes the index
needs rebuilding but only as a result of the dependent table being rebuilt
The information has taken from multiple Metalink Notes.