Clustered Index

Definition of Clustered Index

A clustered index is a type of database index that sorts and stores the data rows in the table based on the key values. This directly affects the table’s physical ordering and storage on the disk. Since a clustered index determines the data’s actual storage order, there can be only one clustered index per table.


Clustered Index in phonetics would be: /ˈklʌstərd ˈɪndɛks/

Key Takeaways

  1. Clustered indexes sort and store the data rows in the table based on their key values, providing a fast and efficient way to access data through the index.
  2. There can be only one clustered index per table, and it affects the physical order of data storage, which is why it is also called the primary key.
  3. Clustered indexes can significantly improve performance for operations that involve range queries, since multiple rows sharing the same key value are stored together, reducing the need to read data from multiple locations.

Importance of Clustered Index

The technology term “Clustered Index” is important because it plays a crucial role in optimizing database performance and improving data retrieval efficiency.

A clustered index determines the physical order of data storage within a table by rearranging the rows of a table based on specified column(s), resulting in quicker access to requested information.

As only one clustered index can be created per table, careful consideration of the most frequently accessed fields can significantly enhance the speed of data querying and processing operations.

In addition, clustered indexes can further improve database performance by minimizing the time and resources associated with data fragmentation, keeping the records organized and resultantly providing better overall system performance.


Clustered indexes serve an essential role in enhancing the efficiency and performance of database operations. The main purpose of a clustered index is to determine the physical order in which data is stored within a table. By organizing the records following a specific attribute, usually a primary key, table reads and writes can be streamlined to minimize I/O operations.

Consequently, this ordering not only expedites searches but also supports ranged queries since the data associated with adjacent values of the indexed column are physically contiguous on disk. Moreover, because each table can only have one clustered index, this underlying structure forms an integral aspect of a table’s design to optimize database operations. Beyond improving runtime performance, clustered indexes serve significant roles in data management.

For instance, when a new row is inserted into a table with a clustered index, the database engine will automatically place the record in the appropriate position based on the indexing order. This automatic sorting and allocation minimize the need for database fragmentation and consistent re-indexing. Additionally, as foreign key relationships often lead to frequent join operations between tables, utilizing clustered indexes optimizes these processes as the related data becomes accessible quicker.

Ultimately, thoughtful implementation of clustered indexes not only contributes to efficient database operation, but it also forms part of best practices in data organization and access.

Examples of Clustered Index

Clustered indexes are commonly utilized in database management systems to optimize data retrieval and ensure efficient data organization. Here are three real-world examples of the technology being implemented:

Banking System:A clustered index is used in a bank’s database management system to store account information in an organized, efficient manner. By employing a clustered index based on account numbers, banks can swiftly retrieve customer balances, transaction history, and other related account details. This not only speeds up data access but also ensures accurate information for smooth banking operations.

E-commerce Platforms:Online shopping platforms often use a clustered index to optimize catalog searches and customer account management. For instance, a clustered index may be created based on the product ID in a product database to facilitate speedy retrieval of product details like price, description, and availability. Similarly, in customer databases, a clustered index could be used to sort user data based on the customer ID for seamless management of user accounts, order history, and shipping addresses.

Hospital Management Systems:To efficiently manage patient records and healthcare services, hospitals rely on database systems that use clustered indexes. For example, a clustered index can be created based on a unique patient identifier to sort patient data such as medical history, appointment schedules, and billing information. This indexing method contributes to faster retrieval of patient data and improved overall healthcare management.

Clustered Index FAQ

1. What is a clustered index?

A clustered index is a type of index in a database where the rows of a table are stored in the same order as the index, providing an efficient way to access the data. This means that the actual data of the table is organized following the clustered index’s order, making the relationship between the index and the data a one-to-one mapping.

2. How does a clustered index work?

A clustered index is created by defining a column or a set of columns in a table to be the key of the index. When a new record is inserted or an existing one is updated, the database engine reorganizes the data according to the clustered index key. The index itself is essentially a schema of how the data should be sorted, allowing the database engine to quickly locate and access the required records.

3. What is the primary difference between a clustered index and a non-clustered index?

The primary difference between a clustered and non-clustered index is how they store and organize the data. In a clustered index, the data itself is ordered and stored based on the index key, while in a non-clustered index, the index and data are stored separately. Non-clustered indexes use a pointer or reference to the actual data, allowing for multiple non-clustered indexes on a single table, whereas there can only be one clustered index per table.

4. Can there be more than one clustered index on a table?

No, there can only be one clustered index per table. This is because the clustered index defines the actual order in which the data is stored in the table. Allowing multiple clustered indexes would result in conflicting orders and confusion in data storage.

5. Does a table always need a clustered index?

A table does not necessarily need a clustered index, but it is highly recommended in most cases. Having a clustered index results in faster query performance and efficient data retrieval, as it allows the database engine to quickly locate the desired rows. If a table does not have a clustered index, it is called a heap, and it stores the data in a non-ordered way, which might lead to slower performance for certain queries.

Related Technology Terms

  • Database Management System (DBMS)
  • Non-Clustered Index
  • Primary Key
  • B-Tree
  • Index Reorganization

Sources for More Information


About The Authors

The DevX Technology Glossary is reviewed by technology experts and writers from our community. Terms and definitions continue to go under updates to stay relevant and up-to-date. These experts help us maintain the almost 10,000+ technology terms on DevX. Our reviewers have a strong technical background in software development, engineering, and startup businesses. They are experts with real-world experience working in the tech industry and academia.

See our full expert review panel.

These experts include:


About Our Editorial Process

At DevX, we’re dedicated to tech entrepreneurship. Our team closely follows industry shifts, new products, AI breakthroughs, technology trends, and funding announcements. Articles undergo thorough editing to ensure accuracy and clarity, reflecting DevX’s style and supporting entrepreneurs in the tech sphere.

See our full editorial policy.

More Technology Terms

Technology Glossary

Table of Contents