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approx_set

Returns the HyperLogLog sketch of the input data set of x. This data sketch underlies approx_distinct() and can be stored and used later by calling cardinality(). Supported Signatures 
function approx_set(bigint) returns hyperloglog
function approx_set(double) returns hyperloglog
function approx_set(varchar) returns hyperloglog
🔗 Official Documentation

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empty_approx_set

Returns an empty HyperLogLog. Supported Signatures 
function empty_approx_set() returns hyperloglog
🔗 Official Documentation

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merge

Returns the HyperLogLog of the aggregate union of the individual hll HyperLogLog structures. Supported Signatures 
function merge(hyperloglog) returns hyperloglog
function merge(qdigest) returns qdigest
function merge(tdigest) returns tdigest
🔗 Official Documentation