Clustered vs NonClustered Index

Now that you understand both clustered and nonclustered indexes individually, it is time to put them side by side. Knowing the differences between these two index types is essential for making good database design decisions.

In this tutorial, you will learn:

• A direct comparison of clustered vs nonclustered indexes
• How each type stores data internally
• Performance differences for common operations
• How they work together on the same table
• Decision guidelines for choosing between them

Here is the complete comparison between clustered and nonclustered indexes:

The most important difference is WHERE the data lives:

Different query patterns perform differently depending on the index type:

1. Point Lookup (Single Row)

SELECT * FROM orders WHERE order_id = 42;

2. Range Query

SELECT * FROM orders WHERE order_date BETWEEN '2017-01-01' AND '2017-06-30';

3. Covering Query (All Needed Columns in Index)

SELECT customer_id, order_status FROM orders WHERE customer_id = 'ABC';

4. Full Table Scan

SELECT * FROM orders;

In practice, a well-designed table has one clustered index and several nonclustered indexes that work together:

Important Implication: Clustered Key Width

Since every nonclustered index stores the clustered key in its leaf pages, a wide clustered key (like a GUID) makes ALL nonclustered indexes larger:

Choose Clustered Index For:

Choose NonClustered Index For:

Decision Flowchart

Here are the key takeaways from comparing clustered and nonclustered indexes:

Golden Rules

1. Every table should have a clustered index (avoid heaps)
2. Choose the clustered key carefully — it affects everything
3. Keep the clustered key narrow (INT or BIGINT preferred)
4. Use nonclustered indexes for secondary access patterns
5. Use INCLUDE columns to create covering indexes and avoid key lookups

In the next tutorial, you will learn the exact SQL syntax for creating indexes.