When working with SQL Server, it’s important to understand the difference between Index Seek and Index Scan. These two concepts play a crucial role in optimizing query performance and improving overall database efficiency.

Index Scan

An Index Scan retrieves all the rows from a table. It scans through every row in the table, regardless of whether it qualifies for the query’s predicate. The cost of an Index Scan is proportional to the total number of rows in the table. This makes it an efficient strategy for small tables or when most of the rows qualify for the query.

If there is an index on a table and the query is retrieving a large amount of data (more than 50% or 90% of the data), the optimizer may choose to perform an Index Scan. In this case, the query will scan all the data pages to retrieve the necessary data rows. If there is no index, a Table Scan may be used instead.

Index Seek

In contrast, an Index Seek retrieves selective rows from a table. It only touches rows that qualify for the query’s predicate and the pages that contain these qualifying rows. The cost of an Index Seek is proportional to the number of qualifying rows and pages, rather than the total number of rows in the table.

Index Seeks are generally more efficient than Index Scans when it comes to retrieving specific data. They are particularly useful when dealing with larger tables or queries that require selective data retrieval.

Conclusion

Understanding the difference between Index Seek and Index Scan is essential for optimizing query performance in SQL Server. By utilizing the appropriate strategy based on the size of the table and the selectivity of the query, you can significantly improve the efficiency of your database operations.

For more information on related topics, you may find the following articles useful:

• SQL SERVER – Index Seek Vs. Index Scan (Table Scan)
• SQL SERVER – Primary Key and NonClustered Index in Simple Words
• SQL SERVER – Query to Find Duplicate Indexes – Script to Find Redundant Indexes
• SQL SERVER – Difference Between Index Rebuild and Index Reorganize Explained with T-SQL Script
• SQL SERVER – Fundamentals of Columnstore Index