ClickHouse SQL Engineering: Rules for Writing Optimal SQL for Query Performance


Performance is an important aspect of any database system, as it directly impacts the ability of a business to access and process its data in a timely and efficient manner.

Some of the most critical areas where performance is important for business are:

  1. Data Warehousing and Business Intelligence: Performance is critical when working with large data sets and generating reports. A slow-performing data warehouse can impede the ability of businesses to make data-driven decisions.
  2. E-commerce and Online Transactions: Performance is critical when processing online transactions, such as credit card payments, as slow response times can lead to customer dissatisfaction and lost sales.
  3. Real-time Analytics: Performance is critical when working with real-time data streams, such as social media feeds, stock prices, and sensor data. Businesses rely on these data streams to make real-time decisions.
  4. Online Gaming: Performance is critical for online gaming and gaming platforms, as slow response times can negatively impact the gaming experience and lead to player dissatisfaction.
  5. IoT Applications: Performance is critical when working with IoT devices, as slow response times can negatively impact the functionality of the devices.
  6. Cloud-based Applications: Performance is critical when working with cloud-based applications, as slow response times can negatively impact the user experience and lead to customer dissatisfaction.
  7. Financial services: Performance is critical for financial services, as slow response times can negatively impact the ability of businesses to make financial decisions and can lead to financial losses.

In general, performance is critical for any business that relies on real-time data processing, high-volume transactions, or large data sets to make decisions. Choosing the appropriate database system that can handle the volume of data, the number of transactions and the response time required is vital for the success of the business.

Rules for writing Optimal SQL in ClickHouse

There are several best practices to keep in mind when writing SQL queries to optimize performance:

  1. Use indexes: Indexes are used to speed up the search process. Make sure to create indexes on columns that are frequently used in WHERE clauses and JOIN conditions.
  2. Avoid using wildcards: Using wildcards (*) in SELECT statements can slow down the query. Instead, specify the exact columns you need to retrieve.
  3. Limit the number of rows returned: Use the LIMIT clause to limit the number of rows returned by a query. This can significantly improve performance when working with large tables.
  4. Avoid using subqueries: Subqueries can slow down a query, try to avoid them when possible and use JOINs instead.
  5. Use EXPLAIN to analyze your queries: The EXPLAIN keyword can be used to analyze the execution plan of a query and identify any issues that may be causing poor performance.
  6. Use prepared statements: Prepared statements are precompiled SQL statements that can be executed multiple times with different parameter values. This can improve performance by reducing the time spent parsing and compiling SQL statements.
  7. Use JOINs instead of UNIONs: JOINs are more efficient than UNIONs when retrieving data from multiple tables.
  8. Avoid using OR in WHERE clause: OR conditions in WHERE clause can slow down the query. Try to use AND conditions instead.
  9. Use appropriate data types: Choose the appropriate data types for the columns in your table. For example, use INT for integer values and VARCHAR for string values.
  10. Use Partitioning: Partitioning data into smaller, more manageable pieces can improve performance when working with large tables.
  11. Avoid using functions on indexed columns: Try to avoid using functions on indexed columns in WHERE clause, it can cause the query to be slow.
  12. Use caching: Caching the results of frequently executed queries can significantly improve performance


By following these best practices, you can write SQL queries that are more efficient and better optimized for performance.

To know more about Clickhouse SQL Engineering, read the below articles: 

About Shiv Iyer 227 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.