`approx_count_distinct`

Description

Returns an approximate count of unique, non-null values for a given expression.

A document is counted if it contains a unique, non-null value.
The result is approximate, optimized for performance rather than precision.

Note

approx_count_distinct is an aggregation function and must be used with a grouping keyword such as groupby.

Syntax

approx_count_distinct(expression: any): number

Arguments

Name	Type	Required	Description
expression	any	true	Expression whose unique, non-null values are counted approximately

Example

Use case: Estimate the number of active users per application

Suppose you want to know how many unique users have been active in a given timeframe. Since you only care whether the number is above a threshold, an approximate count is sufficient.

Example data

{ "applicationname": "checkout-service", "username": "alice" },
{ "applicationname": "checkout-service", "username": "bob" },
{ "applicationname": "checkout-service", "username": "charlie" },
{ "applicationname": "checkout-service", "username": "alice" }

Example query

groupby $l.applicationname aggregate approx_count_distinct($d.username) as active_users

Example output

applicationname	active_users
checkout-service	3

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