What is the difference between clustered and non-clustered indexes?
What is the difference between clustered and non-clustered indexes?
Ask by Steilla Mitchel
Updated 19 Oct 2023
Clustered and non-clustered indexes are both database index structures used in SQL Server and other relational database systems. They play a significant role in improving query performance by facilitating rapid data retrieval. However, they have key differences:
Clustered Index:
Physical Order: A clustered index determines the physical order of data rows in a table. In other words, the actual data rows are stored on disk in the order specified by the clustered index.
One Per Table: A table can have only one clustered index. This index defines the logical order of data and the physical storage layout.
Primary Key: If a table has a primary key, the primary key is, by default, implemented as a clustered index, but this can be changed during table creation.
Performance: Clustered indexes are typically faster for retrieval of single rows or for range queries because the data is stored in the order of the index.
Data Storage Impact: Because the data rows are stored in the order of the clustered index, making updates to the indexed columns can be more resource-intensive. Inserts and updates may result in data being moved within the storage.
Unique Key Requirement: Clustered indexes don't have to be unique, but if they are not, SQL Server adds a hidden 4-byte uniqueifier to ensure uniqueness at the physical level.
Non-Clustered Index:
Logical Order: A non-clustered index does not affect the physical order of data rows. It defines a logical order for data retrieval, pointing to the location of data in the clustered index or the table heap (the unsorted storage).
Multiple Per Table: A table can have multiple non-clustered indexes, each tailored to specific query patterns or columns. These are separate from the clustered index.
Performance: Non-clustered indexes are beneficial for speeding up query operations, especially when searching for specific values in one or more columns, but they may require an additional lookup to retrieve the actual data rows.
Data Storage Impact: Non-clustered indexes store a copy of the indexed column(s) and a pointer to the location of the actual data row.
Unique Constraint: Non-clustered indexes can be unique or non-unique. When unique, they enforce data integrity by ensuring that the indexed column(s) contain unique values.
In summary, the primary difference between clustered and non-clustered indexes is in how they determine the physical storage layout of the data. Clustered indexes dictate the physical order of data rows, while non-clustered indexes do not. Non-clustered indexes can be used to optimize query performance for specific search patterns, but they involve an additional lookup to retrieve the actual data. Understanding when and how to use each type of index is important for optimizing database performance.