The most prominent myth is that an index can become degenerated
after a while and must be re-built regularly. First of all, the database keeps the tree
balance—always. It is not possible that a single
fragment of the index grows deeper and deeper until the tree traversal
becomes slow. What can happen is that the index become bigger than needed.
If there are many UPDATE or DELETE statements involved space utilization can
become suboptimal. However, even if the index is bigger than required, it
is very unlikely that the depth of the index grows because of that. As
explained in “The Search Tree (B-Tree) Makes the Index Fast”, the number of entries in the
index must typically grow by a factor of hundred to increase the index
depth by one level.
Rebuilding an index might reduce the number of leaf nodes by about
20% - 30%. The most you can possibly expect from this reduction is 20%-30%
for very expensive operations like a FULL INDEX
SCAN. The typical INDEX UNIQUE SCAN gain of an
index rebuild is 0%-2% because the depth of the index is not reduced by
the rebuild.
