How to tune Parallel Queries in ClickHouse for Performance and Reliability?

Introduction

ClickHouse is designed to handle parallel queries efficiently out of the box. However, there are several techniques you can use to further optimize parallel query performance in ClickHouse.

Techniques to tune Parallel Queries

Here are some examples of how to handle parallel queries more efficiently in ClickHouse:

  1. Use appropriate shard keys:Shard keys are used to distribute data across ClickHouse servers, and choosing appropriate shard keys can improve parallel query performance by reducing the amount of data that needs to be scanned. For example, if you have a large table with a time-based partitioning key, you can shard the table based on the partitioning key to distribute the data across servers.
  2. Use appropriate data types:Using appropriate data types can also improve parallel query performance by reducing the amount of data that needs to be scanned. For example, using fixed-size data types, such as Int32 instead of String, can significantly reduce the amount of data that needs to be scanned.
  3. Use materialized views: Materialized views are precomputed views that can be used to speed up queries. By precomputing the results of a query and storing them in a materialized view, subsequent queries can be executed more quickly. ClickHouse supports materialized views with automatic refresh intervals, allowing the materialized views to be kept up-to-date with changes to the underlying data.
  4. Use query profiling:ClickHouse provides a built-in query profiler that can be used to identify areas for optimization in parallel queries. By analyzing the query plan and identifying any bottlenecks or performance issues, you can optimize queries to improve parallel query performance.
  5. Use appropriate compression algorithms:ClickHouse supports several compression algorithms, including LZ4, ZSTD, and Brotli, which can be used to compress data and improve parallel query performance by reducing the amount of data that needs to be transferred between servers.
  6. Use asynchronous replication:ClickHouse supports asynchronous replication, which can be used to distribute data across multiple servers and improve parallel query performance. By replicating data asynchronously, you can reduce the amount of time it takes for queries to be executed, as each server can independently process queries.
  7. Use distributed tables: ClickHouse supports distributed tables, which can be used to distribute data across multiple servers and improve parallel query performance. By using distributed tables, you can process large amounts of data in parallel, reducing the amount of time it takes for queries to be executed.
  8. Use query timeouts: ClickHouse supports query timeouts, which can be used to limit the amount of time a query can run. By setting a query timeout, you can prevent long-running queries from affecting the performance of other queries.
  9. Use appropriate replication settings: ClickHouse supports several replication settings, including quorum-based replication and synchronous replication. By choosing the appropriate replication settings, you can balance the trade-off between consistency and performance.
  10. Use appropriate hardware: ClickHouse is designed to run efficiently on commodity hardware, but using appropriate hardware can still improve parallel query performance. For example, using SSDs instead of HDDs can significantly improve query performance by reducing disk I/O times.
  11. Use appropriate resource management: ClickHouse provides several resource management features, including query prioritization and query limits, which can be used to allocate resources based on query priority and limit resource usage for specific queries.
  12. Use appropriate compression settings: ClickHouse supports several compression settings, including compression level and block size. By choosing appropriate compression settings, you can balance the trade-off between compression ratio and query performance.

Use cases

These techniques can be applied to a wide range of scenarios, including:

  • Analyzing user behavior on a website, where data is partitioned based on time and sharded across multiple servers.
  • Processing large amounts of sensor data in real-time, where appropriate data types and compression algorithms are used to reduce the amount of data that needs to be scanned.
  • Generating real-time reports from a large dataset, where materialized views and query profiling are used to speed up query execution times.
  • Replicating data across multiple servers to handle high traffic loads, where asynchronous replication is used to distribute data and improve parallel query performance.
  • Analyzing log data from multiple servers, where distributed tables and appropriate hardware are used to improve parallel query performance.
  • Processing real-time data from sensors in a manufacturing plant, where appropriate resource management and query timeouts are used to prioritize and limit queries.
  • Generating real-time analytics from a large dataset, where appropriate compression settings and query prioritization are used to optimize query performance.
  • Replicating data across multiple data centers, where appropriate replication settings and compression settings are used to balance consistency and performance.

Conclusion

Overall, ClickHouse provides a powerful and flexible platform for handling parallel queries efficiently, and by using these techniques, you can optimize query performance and reduce query execution times in a wide range of scenarios.

To read more about how to configure ClickHouse for performance, do consider the below articles:

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.