// This is generated by ESQL's AbstractFunctionTestCase. Do no edit it. See ../README.md for how to regenerate it.

[discrete]
[[esql-agg-count-distinct-approximate]]
==== Counts are approximate

Computing exact counts requires loading values into a set and returning its
size. This doesn't scale when working on high-cardinality sets and/or large
values as the required memory usage and the need to communicate those
per-shard sets between nodes would utilize too many resources of the cluster.

This `COUNT_DISTINCT` function is based on the
https://static.googleusercontent.com/media/research.google.com/fr//pubs/archive/40671.pdf[HyperLogLog++]
algorithm, which counts based on the hashes of the values with some interesting
properties:

include::../../../aggregations/metrics/cardinality-aggregation.asciidoc[tag=explanation]

The `COUNT_DISTINCT` function takes an optional second parameter to configure
the precision threshold. The precision_threshold options allows to trade memory
for accuracy, and defines a unique count below which counts are expected to be
close to accurate. Above this value, counts might become a bit more fuzzy. The
maximum supported value is 40000, thresholds above this number will have the
same effect as a threshold of 40000. The default value is `3000`.

