Top 20 SQL Query Optimization Techniques for Improved Performance

Nader Fares
3 min readMar 15


SQL query optimization is essential for improving the performance and efficiency of your database operations. Here, we’ll explore 20 techniques to optimize SQL queries and discuss the role of the query optimizer in determining the most efficient execution plan for your queries.

1. Create an index on very large tables (>1,000,000 rows) Creating an index on large tables helps speed up searches and can improve overall query performance.

2. Use EXIST() instead of COUNT() to find an element in the table The EXISTS() function is more efficient than COUNT() when checking for the presence of an element.

3. SELECT fields instead of using SELECT * Selecting specific fields instead of using SELECT * reduces the amount of data that needs to be transferred and processed.

4. Avoid subqueries in WHERE clause Subqueries can slow down query performance. Whenever possible, use joins instead of subqueries in the WHERE clause.

5. Avoid SELECT DISTINCT where possible The SELECT DISTINCT clause can increase processing time. If possible, use GROUP BY or other alternatives to achieve the same result.

6. Use WHERE clause instead of HAVING Using the WHERE clause filters data before aggregation, reducing the number of rows that need to be processed.

7. Create joins with INNER JOIN (not WHERE) Using INNER JOIN instead of WHERE can help the query optimizer determine the best execution plan.

8. Use LIMIT to sample query results The LIMIT clause can be used to retrieve a subset of the data, speeding up query execution and reducing the amount of data returned.

9. Use UNION ALL instead of UNION wherever possible UNION ALL is faster than UNION because it doesn’t require sorting and removing duplicates.

10. Use UNION where instead of WHERE … or … query Using UNION can be more efficient than using multiple WHERE clauses with the OR operator.

11. Run your query during off-peak hours Running queries during off-peak hours can reduce competition for system resources and improve performance.

12. Avoid using OR in join queries The OR operator can slow down join queries. Try to use other join methods, such as INNER JOIN or OUTER JOIN.

13. Choose GROUP BY over window functions GROUP BY is generally more efficient than window functions, especially when dealing with large datasets.

14. Use derived and temporary tables Derived and temporary tables can help break down complex queries and improve performance.

15. Drop the index before loading bulk data Dropping indexes before bulk data loads can speed up the process and reduce resource usage.

16. Use materialized views instead of views Materialized views store the results of a query, making them faster to access than standard views.

17. Avoid != or <> (not equal) operator The not equal operator can be less efficient than other comparison operators. Try to rewrite your query to avoid using it.

18. Minimize the number of subqueries Reducing the number of subqueries can help improve query performance.

19. Use INNER join as little as possible when you can get the same output using LEFT/RIGHT join Using LEFT/RIGHT join instead of INNER join can help reduce the amount of data that needs to be processed in certain cases.

20. For retrieving the same dataset frequently try to use temporary sources.

SQL query optimization is crucial for maintaining database performance and efficiency. By implementing these top 20 techniques, you can significantly improve your SQL queries and ensure that your database operations run smoothly. Understanding the role of the query optimizer and how it determines the most efficient execution plan for your queries is essential for making informed decisions about optimizing your SQL code. Keep these techniques in mind and apply them whenever possible to enhance your database’s performance and minimize resource usage. Happy querying!