How are indexes built?
An index contains the values of the indexed table columns in a sorted order. A normal B* tree index is a tree made up of blocks with up to five levels. The highest level consists of only one block which is called root block and which is the basis for every index access. The lowest level consists of a potentially very large number of blocks that are called leaf blocks and that contain the actual data from left to right in a sorted order (values of the indexed columns and ROWID for the access to the relevant table entry). The entries are called leaf rows. The leaf rows also include earlier index entries that were not physically removed from the index, but were only marked as deleted. This special type of leaf row is called a deleted leaf row.
