ClickHouse uniqCombined Function

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[, ...])

Official Documentation

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.

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