ClickHouse groupArray Function

The groupArray function in ClickHouse is an aggregation function that collects all the values of a column into an array. It's commonly used when you need to gather all values from a group into a single array field, preserving duplicates and maintaining the order of appearance.

Syntax

groupArray(x)

Official ClickHouse Documentation on groupArray

Example Usage

SELECT 
    user_id,
    groupArray(product_id) AS purchased_products
FROM purchases
GROUP BY user_id

This query will return an array of all product IDs purchased by each user.

Common Issues

  1. Memory usage: groupArray can consume a lot of memory if the grouped column contains many rows or large values.
  2. Order of elements: The order of elements in the resulting array is not guaranteed unless used with ORDER BY.

Best Practices

  1. Use groupArrayInsertAt if you need to control the position of elements in the resulting array.
  2. Consider using groupArrayMoving if you need a moving window of values.
  3. For large datasets, consider using groupArrayState and groupArrayMerge for two-level aggregation to optimize memory usage.

Frequently Asked Questions

Q: How does groupArray handle NULL values?
A: groupArray includes NULL values in the resulting array. If you want to exclude NULLs, you can use groupArrayIf with a condition to filter them out.

Q: Is there a limit to the size of the array groupArray can create?
A: There's no hard limit, but the size is constrained by available memory. For very large arrays, consider using groupArrayState for memory-efficient aggregation.

Q: Can I sort the elements in the resulting array?
A: groupArray itself doesn't sort the elements. You can use ORDER BY in your query to influence the order, or use array functions like arraySort on the result.

Q: How does groupArray perform compared to other aggregation functions?
A: groupArray is generally fast but can be memory-intensive. For simple counts or sums, other aggregation functions might be more efficient.

Q: Can I use groupArray with multiple columns?
A: Yes, you can use groupArray on multiple columns separately in the same query to create multiple array columns.

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