HLL_COUNT_BUILD

Counts the approximate number of unique or not NULL values, aggregating the values to HLL++ sketches represented as the BYTEA data type. Later these sketches can be merged to a single sketch using the aggregate function HLL_COUNT_MERGE_PARTIAL, or the estimated cardinality extracted (to get the final estimated distinct count value) using the HLL_COUNT_EXTRACT scalar function. HLL_COUNT_BUILD uses the HLL++ algorithm and allows you to control the set sketch size precision, similar to HLL_COUNT_DISTINCT.

HLL_COUNT_BUILD requires less memory than exact aggregation functions, but also introduces statistical uncertainty. The default precision is 12, with a maximum of 20 set optionally.

Higher precision comes at a memory and performance cost.

Syntax

HLL_COUNT_BUILD ( <expression> [, <precision> ] )

Parameters

Parameter	Description	Supported input types
`<expression>`	Any column name or function that return a column name.	Any type
`<precision>`	Optional literal integer value to set precision. If not included, the default precision is 12. Precision range: 12-20.	`INTEGER`, `BIGINT`

Return Type

BYTEA

If the input is NULL, this function returns NULL.

Example

SELECT
    HLL_COUNT_BUILD(name, 12) as hll_sketch
FROM
    levels;

Returns:

An HLL++ sketch represented as data type BYTEA.

hll_sketch
\x3041676772656761746546756e6374696f6e28312c20756e6971436f6d62696e65643634283132292c20537472696e6729000100