mirror of
https://github.com/elastic/elasticsearch.git
synced 2025-06-29 18:03:32 -04:00
25 lines
1.3 KiB
Text
Generated
25 lines
1.3 KiB
Text
Generated
// 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`.
|
|
|