The uniqCombined function in ClickHouse is an aggregation function used to calculate the approximate number of distinct values in a dataset. It provides a good balance between accuracy and performance, making it suitable for large-scale data analysis where exact counts are not necessary.
Syntax
uniqCombined(x[, ...])
Example usage
SELECT uniqCombined(user_id)
FROM user_actions
WHERE action_date = '2023-05-01'
This query estimates the number of unique users who performed actions on May 1, 2023.
Common issues
- Results may differ slightly from exact counts, especially for small datasets.
- Not suitable for scenarios requiring precise unique counts.
Best practices
- Use uniqCombined for large datasets where approximate counts are acceptable.
- For smaller datasets or when exact counts are needed, consider using the uniq function instead.
- Combine with other aggregations for comprehensive analytics.
Frequently Asked Questions
Q: How accurate is uniqCombined compared to exact counting methods?
A: uniqCombined typically has a relative error not exceeding 1.6%. It's more accurate than uniqHLL but less precise than uniq for exact counts.
Q: Can uniqCombined be used with multiple arguments?
A: Yes, uniqCombined can take multiple arguments. It will calculate the number of distinct combinations of these arguments.
Q: How does uniqCombined perform on large datasets?
A: uniqCombined is optimized for large datasets, offering better performance than exact counting methods while maintaining good accuracy.
Q: Is it possible to use uniqCombined in a HAVING clause?
A: Yes, you can use uniqCombined in a HAVING clause to filter groups based on the approximate count of unique values.
Q: Can the result of uniqCombined be used in further calculations?
A: Yes, the result is a number that can be used in further calculations or comparisons within your query.
