ClickHouse's Christmas Gifts: Performance Boosts!

ClickHouse: Speed and Efficiency Unleashed

This ClickHouse blog post presents a compelling case for the performance improvements delivered as 'Christmas gifts' in version 25.12. The optimization of distinctJSONPaths and DISTINCT queries on LowCardinality columns, along with the filter pushdown for SEMI JOIN operations, demonstrate significant gains in query execution speed and resource utilization. The data-driven performance comparisons, particularly the BlueSky dataset example for distinctJSONPaths, and the UK property prices dataset examples, effectively showcase the impact of these optimizations. The article's clear presentation of before-and-after performance metrics, including elapsed time, processed rows, and data read, makes the benefits readily apparent. The post is well-written, with contributions from the engineering team highlighted, enhancing its credibility.

However, the article could be improved by providing more context on the underlying technical mechanisms of the optimizations. While the summaries are helpful, a deeper dive into the specific algorithms and data structures used would be valuable for technically inclined readers. For instance, explaining the metadata used for distinctJSONPaths in more detail would enrich the reader's understanding. Additionally, while the article mentions the limitation of the DISTINCT optimization on the postcode1 column, further analysis of the data distribution characteristics that cause this behavior would be beneficial. Furthermore, exploring the trade-offs of these optimizations, such as potential increased memory consumption or impact on write performance, would provide a more complete picture. Finally, the article primarily focuses on performance; exploring the impact on cost savings, especially in cloud environments, would add further value.

Key Points

• Significant performance improvements for distinctJSONPaths function (50x speedup). The optimization relies on metadata at the data-part level, avoiding full JSON data scanning.

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📖 Source: Christmas gifts from the ClickHouse engineering team