ClickHouse JOIN: Understanding Advanced Hash and Merge Joins


In the realm of data analytics, performance is often the key differentiator between success and stagnation. ClickHouse, an open-source columnar database management system, has gained popularity for its exceptional query performance, especially when it comes to handling complex joins efficiently. Understanding the intricacies of ClickHouse’s advanced join types is essential for optimizing queries and unlocking the full potential of your data.

This technical document dives deep into six advanced join types offered by ClickHouse:

  • Hash Join
  • Parallel Hash Join
  • Grace Hash Join
  • Full Sorting Merge Join
  • Partial Merge Join
  • Direct Join

We’ll explore the technical underpinnings of each join type and provide real-life use cases to illustrate their practical relevance. By the end of this document, you’ll have a comprehensive understanding of how to harness these join types to turbocharge your data analytics tasks.

Hash Join


Hash Join is a fundamental join type in which the database hashes join keys to create buckets and efficiently matches keys within these buckets.

Practical Example

Use Case: Imagine a scenario where an e-commerce company needs to analyze sales data. They can perform a Hash Join between sales and products tables on the product_id field:

FROM sales
INNER JOIN products ON sales.product_id = products.product_id

Parallel Hash Join


Parallel Hash Join enhances the Hash Join algorithm by parallelizing the hashing process, distributing it across multiple threads or nodes.

Practical Example

Use Case: In a massive log analysis system, parallelization becomes crucial. ClickHouse can employ Parallel Hash Join to optimize the matching of log data across distributed nodes:

FROM distributed_logs
INNER JOIN distributed_metrics ON distributed_logs.session_id = distributed_metrics.session_id

Grace Hash Join


Grace Hash Join is a sophisticated extension of Hash Join that gracefully handles out-of-memory scenarios for large joins.

Practical Example

Use Case: Consider a scenario where a financial institution needs to join transaction records with customer data. Grace Hash Join ensures that even exceptionally large joins do not cause memory issues:

FROM transactions
INNER JOIN customers ON transactions.customer_id = customers.customer_id

Full Sorting Merge Join


Full Sorting Merge Join involves sorting both input tables before joining, which can be highly efficient for specific use cases.

Practical Example

Use Case: In a time-series database, where chronological order is crucial, Full Sorting Merge Join can significantly enhance performance. Suppose you need to combine historical weather data with sensor readings:

FROM weather_history
INNER JOIN sensor_readings ON weather_history.timestamp = sensor_readings.timestamp
ORDER BY weather_history.timestamp

Partial Merge Join


Partial Merge Join is a hybrid approach that combines sorting and merging techniques, offering optimal performance in various scenarios.

Practical Example

Use Case: In an e-commerce platform, you may need to join customer data with transaction logs for targeted marketing. Partial Merge Join can be a performance savior:

FROM customer_data
INNER JOIN transaction_logs ON customer_data.customer_id = transaction_logs.customer_id
ORDER BY customer_data.customer_id

Direct Join


Direct Join leverages ClickHouse’s columnar storage format for blazing-fast, direct matching of data.

Practical Example

Use Case: In a clickstream analysis system, where raw data is stored column-wise, Direct Join is a game-changer. Suppose you want to correlate user behavior with demographic data:

FROM clickstream_data
DIRECT JOIN demographic_data ON clickstream_data.user_id = demographic_data.user_id


Data analytics is at the heart of informed decision-making in today’s data-driven world. ClickHouse’s advanced join types are the secret sauce that enables organizations to extract actionable insights from vast datasets with lightning speed. Whether you’re dealing with e-commerce transactions, log analysis, financial records, or clickstream data, ClickHouse’s join algorithms offer tailored solutions to optimize your queries.

In this document, we’ve delved into the intricacies of six advanced join types, showcasing their technical nuances and demonstrating their real-world applicability. By choosing the right join strategy for your use case, you can harness the true power of ClickHouse to drive insights and make data-driven decisions that propel your organization forward.

Now armed with this knowledge, you’re well-equipped to tackle even the most challenging data analysis tasks with confidence, leveraging ClickHouse’s remarkable capabilities to deliver results that matter.

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About Shiv Iyer 222 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.