Index Re-building is a debatable concept amongst DBA’s. Experts like Ask Tom think rebuilding indexes doesn’t create any considerable performance improvement.
Testimony to this is a small exercise I did in my production environment. We have a table with over 10 million rows. A full table scan on this takes approximately 15 minutues. the table has 6 b*tress indexes. I rebuild all the indexes online, the result was same. Full table scan took a few seconds less. The result was neglible. For performance tuning you have to carefully consider all symptoms and not blinly rebuild all indexes. The Cost Based Optimizer(CBO) in Oracle takes care of optimizing your queries. Below are the general myths associated with index rebuilding.
1. The vast majority of indexes do not require rebuilding
2. Oracle B-tree indexes can become “unbalanced” and need to
be rebuilt is a myth
3. deleted space in an index is “deadwood” and over time
requires it to be rebuilt is a myth
4. If an index has a height of “X” levels, it’s inefficient
and must be rebuilt is a myth
5. If an index has a poor CF, rebuilding it is a myth
6. To improve performance indexes must be rebuilt on a regular basis is a myth