ClickHouse topK Function

The topK function in ClickHouse is an aggregation function used to find the most frequent values in a dataset. It returns an array of the approximate top K most frequent elements, making it useful for analyzing trends, identifying popular items, or finding common patterns in large datasets.

Syntax

topK(N)(column)

Official ClickHouse topK documentation

Example Usage

SELECT topK(3)(product_name) AS top_3_products
FROM sales_data

This query returns an array of the 3 most frequently occurring product names in the sales_data table.

Common Issues

  1. Results may not be exact for very large datasets due to the approximate nature of the algorithm.
  2. The order of elements in the result array is not guaranteed to be sorted by frequency.

Best Practices

  1. Use topK when exact results are not required, as it's more efficient than sorting the entire dataset.
  2. Combine with other aggregations for more comprehensive analysis, e.g., topK with count to get both frequent items and their counts.
  3. Consider using topKWeighted if you need to account for weights in your frequency calculations.

Frequently Asked Questions

Q: How accurate is the topK function?
A: The topK function provides approximate results, which are generally accurate enough for most use cases. The accuracy improves with larger datasets and smaller K values.

Q: Can I use topK with multiple columns?
A: Yes, you can use topK with multiple columns by applying it separately to each column or by combining columns into a tuple.

Q: Is there a limit to the value of K in topK?
A: While there's no strict limit, using very large K values can impact performance and memory usage. It's recommended to use reasonable K values based on your specific needs.

Q: How does topK differ from ORDER BY and LIMIT?
A: topK is more efficient for large datasets as it uses an approximate algorithm, while ORDER BY with LIMIT performs an exact sort which can be slower for large datasets.

Q: Can topK be used in combination with other aggregate functions?
A: Yes, topK can be used alongside other aggregate functions in the same query to provide comprehensive analysis of your data.

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