The topKWeighted function in ClickHouse is an aggregation function that returns an array of the most frequent values based on their weights. It's particularly useful when you need to find the top-k elements while considering their importance or frequency represented by weights.
Syntax
topKWeighted(k)(x, weight)
For the official documentation, visit the ClickHouse topKWeighted function page.
Example Usage
SELECT topKWeighted(3)(product_name, sales_amount) AS top_products
FROM sales_data
GROUP BY category;
This query returns the top 3 products in each category, weighted by their sales amount.
Common Issues
- Ensure that the weight parameter is always non-negative, as negative weights can lead to unexpected results.
- Be aware that the function may return fewer than k elements if there are not enough unique values in the input.
Best Practices
- Use topKWeighted when you need to consider both frequency and importance of elements.
- For large datasets, consider using approximate versions like topKWeighted64 for better performance.
- Always validate the weight column to ensure it contains meaningful and non-negative values.
Frequently Asked Questions
Q: How does topKWeighted differ from regular topK?
A: While topK considers only the frequency of occurrences, topKWeighted takes into account both the frequency and a weight associated with each occurrence, allowing for more nuanced ranking.
Q: Can topKWeighted handle decimal weights?
A: Yes, topKWeighted can handle decimal weights. It's important to ensure that the weight column is of a numeric type that can represent your desired precision.
Q: Is there a limit to the value of k in topKWeighted?
A: While there's no hard limit, it's recommended to keep k reasonably small (typically under 100) for performance reasons, especially on large datasets.
Q: How does topKWeighted handle ties?
A: In case of ties, topKWeighted may return any of the tied elements. The order is not guaranteed to be stable across different runs or data distributions.
Q: Can topKWeighted be used with window functions?
A: No, topKWeighted is an aggregate function and cannot be directly used as a window function. However, you can use it within a subquery and then incorporate that into a window function scenario if needed.
