A Clustered Index is a type of database index where the data rows in a table are physically organized in the same order as the index. This structure makes data retrieval faster for queries that involve the indexed columns, as the index provides a direct pathway to the data. Unlike non-clustered indexes, where the index maintains a separate structure pointing to the data, a clustered index defines the physical order of the data in a table.

Characteristics of Clustered Indexes:

One Per Table: A table can have only one clustered index because the data rows can only be ordered in one way.
Primary Key by Default: In many database systems, the primary key of a table automatically becomes the clustered index unless specified otherwise.
Efficient Range Queries: Particularly useful for queries that retrieve a range of data, such as between two dates.

How It Works:
- When a clustered index is created on a column, the database rearranges the table's data rows in ascending or descending order based on that column's values.

- This index serves as a roadmap for retrieving data, significantly improving search and retrieval performance for queries targeting the indexed column.

Benefits:
1. Fast Data Retrieval: Optimizes query performance for range-based or sorted queries.
2. Reduced I/O: Minimizes the number of read operations required to fetch data.
3. Better Sorting: Automatically organizes data in a predictable order, which is advantageous for reports and analytics.

Drawbacks:
1. Costly Updates: Inserting, updating, or deleting data in a clustered index may require reorganizing the table, leading to higher processing costs.
2. Larger Size: Clustered indexes may increase the storage requirements of a database.
3. Limited Flexibility: Only one clustered index is allowed per table.

Use Cases:
Transactional Systems: Tables where data retrieval speed is critical, such as in banking or e-commerce applications.

Sorting Needs: Tables requiring frequent sorting or range-based queries, such as log files or historical data.

Clustered indexes are an essential tool for optimizing database performance, particularly for large tables where query efficiency can make a significant difference in application responsiveness.