What are the differences between clustered and non-clustered indexes?

Clustered and non-clustered indexes serve distinct roles in database performance and data organization. A clustered index determines the physical order of data rows in a table, meaning the rows are stored on disk in the same sequence as the index. Each table can have only one clustered index because the data cannot be physically sorted in multiple ways simultaneously. For example, if a table has a clustered index on a user_id column, the rows are stored in ascending or descending order of user_id. In contrast, a non-clustered index is a separate structure that points to the data without rearranging it. A table can have multiple non-clustered indexes, each providing a different “path” to access the data. For instance, a non-clustered index on an email column would store email values alongside pointers (like row addresses) to the corresponding rows in the main table.

The performance characteristics of these indexes differ significantly. Clustered indexes excel at retrieving large ranges of ordered data quickly because the rows are physically contiguous. For example, querying all orders from January 2024 using a clustered index on order_date would require minimal disk I/O. However, inserting or updating data in a clustered index can be slower if the new data disrupts the physical order. Non-clustered indexes, on the other hand, are faster for write operations (inserts, updates) since they don’t reorganize the table. But querying via a non-clustered index often requires a “lookup” step to fetch the actual row data, which adds overhead. For example, searching for a user by email using a non-clustered index would first find the email in the index, then retrieve the full row from the main table using the pointer.

Use cases for each index depend on query patterns. Clustered indexes are ideal for columns frequently used in range queries (e.g., BETWEEN, ORDER BY) or as the primary key. For example, a transactions table might use a clustered index on transaction_id to optimize lookups by ID. Non-clustered indexes are better for columns used in search conditions (e.g., WHERE clauses) or joins where the column isn’t part of the clustered index. For instance, a customers table with a clustered index on customer_id might have a non-clustered index on country to speed up filtering by country. Properly balancing clustered and non-clustered indexes reduces redundant storage and ensures efficient query execution.