---
title: "How clustered and non-clustered indexes impact query performance."  
description: "How clustered and non-clustered indexes impact query performance."  
author: "Revati S Misra"  
published: 2023-10-26  
updated: 2023-10-27  
canonical: https://www.mindstick.com/forum/160275/how-clustered-and-non-clustered-indexes-impact-query-performance  
category: "mssql server"  
tags: ["sql server", "sql", "query optimization"]  
reading_time: 3 minutes  

---

# How clustered and non-clustered indexes impact query performance.

How [clustered and non-clustered](https://answers.mindstick.com/qa/92501/what-is-the-difference-between-clustered-and-non-clustered-index) [indexes](https://www.mindstick.com/articles/12525/indexes-in-sql-server) [impact](https://yourviews.mindstick.com/audio/1149/the-evolution-and-impact-of-movies-from-invention-to-modern-era) [query performance](https://www.mindstick.com/forum/160277/how-execution-plan-can-help-identify-query-performance-issues).

## Replies

### Reply by Aryan Kumar

[Clustered](https://www.mindstick.com/interview/1892/what-is-a-clustered-index) and non-clustered indexes are two types of indexes used in a relational database management system (RDBMS) like SQL Server. They impact [query](https://www.mindstick.com/blog/202/sub-query-in-sqlserver) performance in different ways, and their use depends on the nature of the query and the specific requirements of your database. Here's how clustered and non-clustered indexes affect query performance:

**Clustered Index**:

**Physical Data Order**: A clustered index determines the physical order of data rows in a table. In other words, the rows are physically sorted in the same order as the clustered index key.

**Unique**: A table can have only one clustered index, and by default, it enforces uniqueness on the indexed column(s).

**Primary Key**: In many cases, the primary key of a table is implemented as a clustered index. This enforces the uniqueness of the primary key and physically sorts the data.

**Query Performance**:

- Excellent for range queries: Clustered indexes are well-suited for queries that involve range searches or equality comparisons on the indexed column(s).
- Fast retrieval of entire rows: Because the data rows are physically stored in the order of the clustered index, retrieving an entire row based on the index key is efficient.

**Insert, Update, and Delete Performance**:

- Inserting new rows can be slower, as data may need to be reorganized to maintain the physical order of the index.
- Updating the clustered index key can be slow for similar reasons.
- Deleting rows can be faster, as the data is already physically ordered.

**Non-Clustered Index**:

**Logical Order**: A non-clustered index does not dictate the physical order of data; instead, it creates a separate structure that stores a copy of the indexed column(s) along with a reference to the data rows.

**Multiple Indexes**: A table can have multiple non-clustered indexes, allowing for efficient access to data based on various criteria.

**Query Performance**:

- Efficient for specific lookups: Non-clustered indexes are excellent for queries that need to look up individual rows based on the indexed column(s).
- Useful for covering queries: Non-clustered indexes can "cover" a query by including all the columns needed in the query, reducing the need to access the actual data rows.

**Insert, Update, and Delete Performance**:

- Inserting new rows is generally faster, as there is no need to reorder the data.
- Updating the indexed column(s) is typically faster.
- Deleting rows is similar in performance to clustered indexes.

In summary, the choice between clustered and non-clustered indexes depends on your specific use case:

**Clustered Index**: Use when you need to physically order the data rows or enforce the uniqueness of a primary key. It's best for range queries and for retrieving entire rows efficiently.

**Non-Clustered Index**: Use when you need to optimize specific lookups or cover specific queries with a subset of columns. You can have multiple non-clustered indexes on a table to support various query patterns.

In practice, it's common to use a combination of clustered and non-clustered indexes to optimize different aspects of query performance in a database.


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Original Source: https://www.mindstick.com/forum/160275/how-clustered-and-non-clustered-indexes-impact-query-performance

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