Oracle has 2 kinds of Index: B-tree index and Bitmap Index.

B-tree index is commonly used index, which is created using CREATE INDEX command.

B-tree index structure:

Root: Pointer to branch node
Branch: Pointer to Leaf node. The distinct value of index columns are sorted and divided into different groups. Each branch is mapped to a group of data.
Leaf: Each Leaf contains the rowid of the records it refers to. Leaf node are linked together in both directions, allowing ascending or descending search. Also leaf nodes can be sorted by ascending or descending.

Bitmap index is some special index, which is created using CREATE BITMAP INDEX command.

Bitmap Index structure:

Root: Pointer to branch node

Branch: Pointer to Leaf node. Branch contains the start rowid and end rowid of the leaf blocks it refers to. (It is different from B-tree here. Here is start rowid and end rowid.)

Leaf: Leaf is a bitmap. Each bit in the bitmap corresponds to a row in the table. You can think the bitmap like a table with RowID as Row Header and Values as column header. When the row has the value of the column header. It is marked to 1, else 0.

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B-tree and Bitmap Index

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