A scalar function. Extracts a cardinality estimate of a single Apache DataSketches HLL sketches that was previously built using the aggregate function APACHE_DATASKETCHES_HLL_BUILD. The APACHE_DATASKETCHES_HLL_ESTIMATE function is used to estimate the cardinality (number of unique elements) of a dataset represented by an HLL (HyperLogLog) sketch. This is particularly useful for large datasets where exact counting is computationally expensive.

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Syntax

APACHE_DATASKETCHES_HLL_ESTIMATE
(<expression>)

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Parameters

ParameterDescriptionSupported input types
<expression>An Apache DataSketches HLL sketches in a valid format, e.g. the output of the APACHE_DATASKETCHES_HLL_BUILD function.BYTEA

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Return Type

BIGINT

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Error Handling

If the input expression is not a valid Apache DataSketches HLL sketches, the function will raise an error. Ensure that the input is correctly formatted and generated by the APACHE_DATASKETCHES_HLL_BUILD function.

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Example

Following the example in APACHE_DATASKETCHES_HLL_BUILD: 
SELECT APACHE_DATASKETCHES_HLL_ESTIMATE(a) AS estimate
FROM sketch_of_data_to_count
ORDER BY 1;
estimate (BIGINT)
3333526
5001149
SELECT APACHE_DATASKETCHES_HLL_ESTIMATE(APACHE_DATASKETCHES_HLL_MERGE(a)) AS estimate
FROM sketch_of_data_to_count;
hll_estimate (BIGINT)
6673219