In SQL Server, indexes are crucial for improving query performance by providing a structured way to access data. There are two primary types: clustered and non-clustered.

Clustered Index

  • Defines the physical order of the data: A clustered index determines how the rows are physically arranged on disk.
  • Can only have one per table: A table can only have one clustered index.
  • Impacts data retrieval: Queries that use the clustered index columns are generally faster as they directly access the data.
  • Often based on primary key: The primary key is often defined as a clustered index, ensuring data integrity and efficient retrieval.

Non-Clustered Index

  • Points to the physical location of data: A non-clustered index contains a list of pointers to the actual data rows.
  • Can have multiple per table: A table can have multiple non-clustered indexes.
  • Improves query performance: Non-clustered indexes can significantly improve query performance, especially for queries that frequently filter on or join data based on the indexed columns.

Key Differences

Feature Clustered Index Non-Clustered Index
Physical order Defines the physical order of data Points to the physical location
Number per table Only one per table Multiple per table
Impact on data retrieval Directly accesses data Indirectly accesses data
Typical use Primary key Frequently filtered columns

When to Use Which

  • Clustered index: Use for columns that are frequently used in primary key operations or for data retrieval based on the clustered index columns.
  • Non-clustered index: Use for columns that are frequently used in filtering or joining operations.

Example: If you have a table Orders with columns OrderID, CustomerID, OrderDate, and TotalAmount, you might:

  • Create a clustered index on OrderID to ensure data integrity and efficient retrieval of orders by ID.
  • Create non-clustered indexes on CustomerID and OrderDate to improve performance for queries that filter based on these columns.

By understanding the differences between clustered and non-clustered indexes, you can optimize your SQL Server database design for efficient data retrieval and query performance.