ClickHouse EXPLAIN: Display & Analyze Execution Plans

Table of Contents

Introduction

To display and read the execution plans for a SQL statement in ClickHouse, you can follow these steps using real-life data sets:

  1. Connect to ClickHouse: Open a ClickHouse client or connect to the ClickHouse server using a client tool like clickhouse-client or a SQL IDE that supports ClickHouse.
  2. Choose a SQL Statement: Select a SQL statement for which you want to see the execution plan. Let’s consider an example where you have a table called orders with columns like order_id, customer_id, order_date, total_price, and shipping_address.

For this example, we’ll use a simple SELECT statement to retrieve data from the orders table:

SELECT customer_id, COUNT(*) AS order_count
FROM orders
GROUP BY customer_id
ORDER BY order_count DESC;

This query will calculate the number of orders per customer and display the results in descending order of the order count.

  1. Display Execution Plan: Use the EXPLAIN command to display the execution plan for the chosen SQL statement. Execute the following command:
EXPLAIN SELECT customer_id, COUNT(*) AS order_count
FROM orders
GROUP BY customer_id
ORDER BY order_count DESC;

ClickHouse will provide the execution plan for the query, which includes details about how the query will be executed and the steps involved in processing the data. The execution plan may include information such as the order of operations, the use of indexes, data transformations, and any optimizations applied by ClickHouse.

  1. Analyze the Execution Plan: Review and analyze the execution plan to understand how ClickHouse will execute the query. Consider the following aspects:
  • Order of operations: Check the sequence of operations performed by ClickHouse to process the data.
  • Data sources: Identify the tables or data sources involved in the query.
  • Joins and aggregations: Look for any joins or aggregations specified in the query and how they are processed.
  • Index usage: Determine if ClickHouse utilizes any indexes to optimize query execution.
  • Data transformations: Note any data transformations or operations applied during query processing.

By examining the execution plan, you can gain insights into the query optimization strategy used by ClickHouse and identify areas for potential performance improvements.It’s important to remember that ClickHouse’s execution plans are specific to its columnar storage and distributed architecture, which may differ from traditional relational databases. Therefore, it’s recommended to consult with ChistaDATA ClickHouse Performance Engineering Experts to better understand and optimize the execution plans for your specific use cases and data sets.

Conclusion

By following these steps and analyzing the execution plans, you can gain a deeper understanding of how ClickHouse processes your SQL statements and make informed decisions to optimize your queries for improved performance.

To read more about EXPLAIN in ClickHouse, do consider reading the following articles:

  1. ClickHouse Performance: Decoding Query Execution Plan with EXPLAIN
  2. ClickHouse EXPLAIN: Determine JOIN Order in Query Execution Plans
  3. Comprehensive Guide to ClickHouse EXPLAIN
About Shiv Iyer 215 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.