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* Functions starting with A * Functions starting with 'C' * More 'C' functions * Fix tests * Fix missing snippet * DATE_* functions * Apply suggestions from code review Co-authored-by: Bogdan Pintea <pintea@mailbox.org> --------- Co-authored-by: Bogdan Pintea <pintea@mailbox.org>
67 lines
1.8 KiB
Text
67 lines
1.8 KiB
Text
[discrete]
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[[esql-agg-count-distinct]]
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=== `COUNT_DISTINCT`
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*Syntax*
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[source,esql]
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----
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COUNT_DISTINCT(column[, precision])
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----
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*Parameters*
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`column`::
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Column for which to count the number of distinct values.
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`precision`::
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Precision. Refer to <<esql-agg-count-distinct-approximate>>.
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*Description*
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Returns the approximate number of distinct values.
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[discrete]
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[[esql-agg-count-distinct-approximate]]
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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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The `COUNT_DISTINCT` function takes an optional second parameter to configure the
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precision.
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*Supported types*
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Can take any field type as input.
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*Examples*
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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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With the optional second parameter to configure the precision:
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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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