What are some common indexing techniques in SQL Server, and how do they impact query performance?

Common Indexing Techniques in SQL Server and Their Impact on Query Performance

Indexing is crucial in optimizing query performance in SQL Server. Here are some common indexing techniques and their impact:

1. Clustered Index:

A clustered index physically orders the data rows in the table based on the indexed column(s). It impacts query performance by speeding up data retrieval for range queries and sorting operations.

2. Non-Clustered Index:

A non-clustered index stores a separate copy of the indexed columns along with pointers to the actual data rows. It improves query performance for SELECT, WHERE, and JOIN operations by quickly locating the required data.

3. Unique Index:

A unique index enforces uniqueness of values in the indexed column(s). It improves query performance by enabling faster data retrieval for unique value searches and ensuring data integrity.

4. Composite Index:

A composite index includes multiple columns in the index key. It impacts query performance by optimizing searches and sorting on multiple columns, improving query execution speed for multi-column WHERE clauses.

5. Covering Index:

A covering index includes all the columns required by a query in the index itself. It reduces the need to access the actual data rows, improving query performance by minimizing disk I/O operations.

By utilizing these indexing techniques effectively, SQL Server can significantly enhance query performance, resulting in faster and more efficient data retrieval operations.