SQL Server is a powerful database management system that requires careful consideration and best practices to ensure optimal performance and efficiency. In this article, we will discuss 20 commandments for SQL Server that can help you improve your database performance and avoid common pitfalls.

1. Know your data and business application well

It is crucial to familiarize yourself with the data and business application that you are working with. Understanding the data volume and distribution in your database will help you make informed decisions and optimize your queries accordingly.

2. Test your queries with realistic data

Testing your queries with realistic data is essential to ensure that they perform well in a production environment. Unrealistic data may behave differently, and rigorous testing with data that closely resembles the production environment is necessary.

3. Write identical SQL statements in your applications

Take full advantage of stored procedures and functions whenever possible. These precompiled SQL statements can provide performance gains and consistency across your applications.

4. Use indexes on the tables carefully

Creating the necessary indexes on your tables is important for query optimization. However, be cautious not to create too many indexes, as they can degrade performance. Strike a balance between the number of indexes and their impact on query execution.

5. Make an indexed path available

To take advantage of indexes, write your SQL statements in such a way that an indexed path is available. Using SQL hints is one way to ensure that the index is used and can improve query performance.

6. Understand the Optimizer

Take the time to understand how the SQL Server optimizer works. Learn how it uses indexes, the WHERE clause, ORDER BY clause, HAVING clause, and other query components to optimize query execution.

7. Think globally when acting locally

Any changes you make in the database to tune one SQL statement may affect the performance of other statements used by applications and users. Consider the broader impact of your changes and test thoroughly to ensure overall performance improvement.

8. The WHERE clause is crucial

Be mindful of the WHERE clause in your queries. Certain conditions, such as using comparison operators like >, <, >=, <=, or using the LIKE operator with a pattern, may not use the index access path even if an index is available. Understand the limitations and optimize your WHERE clauses accordingly.

9. Use WHERE instead of HAVING for record filtering

Avoid using the HAVING clause along with GROUP BY on an indexed column. Instead, use the WHERE clause for record filtering, as it can provide better performance.

10. Specify the leading index columns in WHERE clauses

For composite indexes, ensure that the leading column of the index is specified in the WHERE clause. This allows the query to use the index effectively and improve performance.

11. Evaluate index scan vs. full table scan

Consider the percentage of rows being accessed when deciding between an index scan and a full table scan. In general, if selecting more than 15 percent of the rows from a table, a full table scan is usually faster. However, if the percentage is 15 percent or less, an index scan may be more efficient.

12. Use ORDER BY for index scan

If you have an ORDER BY clause on an indexed column, the SQL Server optimizer will use an index scan. This can improve query performance when sorting large result sets.

13. Minimize table passes

Reducing the number of table passes in a SQL query can result in better performance. Queries with fewer table passes mean faster execution, so optimize your queries to minimize unnecessary table scans.

14. Join tables in the proper order

When performing multiple table joins, start with the most restrictive search first. This filters out the maximum number of rows in the early phases of the join, improving performance in subsequent phases. Consider the order of table joins carefully to optimize query execution.

15. Redundancy is good in WHERE condition

Provide as much information as possible in the WHERE clause. This helps the optimizer clearly infer conditions and optimize query execution. Including redundant conditions can improve performance.

16. Keep it simple, stupid

Complex SQL statements can overwhelm the optimizer. Sometimes, writing multiple simpler SQL statements can yield better performance than a single complex statement. Keep your queries simple and straightforward for optimal execution.

17. You can reach the same destination in different ways

Remember that different SQL statements may use different access paths and perform differently. Explore different approaches and optimize your queries to achieve the desired results efficiently.

18. Reduce network traffic and increase throughput

Using T-SQL blocks over multiple SQL statements can improve performance and reduce network traffic. Stored procedures are even better, as they are stored in SQL Server and pre-compiled for faster execution.

19. Better hardware

Investing in better hardware can significantly improve SQL Server performance. Consider using SCACI drives, RAID 10 arrays, multi-processor CPUs, and a 64-bit operating system to maximize performance gains.

20. Avoid Cursors

Using SQL Server cursors can result in performance degradation compared to using select statements. Whenever possible, try to use correlated subqueries or derived tables for row-by-row operations to improve query performance.

These 20 commandments for SQL Server provide a solid foundation for optimizing your database performance. By following these best practices, you can ensure that your SQL Server environment is efficient, reliable, and capable of handling your business needs effectively.