What is the main content and structure of a cluster index in a database?

A clustered index is a data structure that determines the physical order of storing rows in a database table. The main content of a clustered index is the key values (index keys) and references to the corresponding data rows.

Unlike a non-clustered index, where the index and data are stored separately, a clustered index essentially reorganizes the table based on the index key. This means that the data in the table is physically ordered according to the clustered index key value.

The structure of a clustered index is usually implemented as a B-tree, where:

• Internal nodes contain keys for navigation.
• Leaf nodes contain the data rows themselves, ordered by key.

This approach speeds up key-based retrieval operations, especially for range queries, but imposes restrictions on the number of clustered indexes (usually one per table).