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* Break out 'Limitations' into separate page * Add REST API docs * Restructure commands, functions, and operators refs * Add placeholder for getting started guide * Group 'Syntax', 'Metafields', and 'MV fields' under 'Language' * Add placeholder for Kibana page * Add link from landing page * Apply uniform formatting to ACOS, CASE, and DATE_PARSE function refs * Reword default LIMIT * Add support for COUNT(*) * Move 'Commands' and 'Functions and operators' to individual pages --------- Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>
46 lines
1.5 KiB
Text
46 lines
1.5 KiB
Text
[discrete]
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[[esql-agg-count-distinct]]
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=== `COUNT_DISTINCT`
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The approximate number of distinct values.
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[source.merge.styled,esql]
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----
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include::{esql-specs}/stats_count_distinct.csv-spec[tag=count-distinct]
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----
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[%header.monospaced.styled,format=dsv,separator=|]
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|===
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include::{esql-specs}/stats_count_distinct.csv-spec[tag=count-distinct-result]
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|===
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Can take any field type as input and the result is always a `long` not matter
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the input type.
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[discrete]
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==== Counts are approximate
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Computing exact counts requires loading values into a set and returning its
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size. This doesn't scale when working on high-cardinality sets and/or large
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values as the required memory usage and the need to communicate those
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per-shard sets between nodes would utilize too many resources of the cluster.
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This `COUNT_DISTINCT` function is based on the
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https://static.googleusercontent.com/media/research.google.com/fr//pubs/archive/40671.pdf[HyperLogLog++]
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algorithm, which counts based on the hashes of the values with some interesting
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properties:
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include::../../aggregations/metrics/cardinality-aggregation.asciidoc[tag=explanation]
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[discrete]
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==== Precision is configurable
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The `COUNT_DISTINCT` function takes an optional second parameter to configure the
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precision discussed previously.
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[source.merge.styled,esql]
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----
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include::{esql-specs}/stats_count_distinct.csv-spec[tag=count-distinct-precision]
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----
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[%header.monospaced.styled,format=dsv,separator=|]
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|===
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include::{esql-specs}/stats_count_distinct.csv-spec[tag=count-distinct-precision-result]
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|===
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