HLL_CREATE_SKETCH function

The HLL_CREATE_SKETCH function returns an HLLSKETCH data type that encapsulates the input expression values. The HLL_CREATE_SKETCH function works with any data type and ignores NULL values. When there are no rows in a table or all rows are NULL, the resulting sketch has no index-value pairs such as {"version":1,"logm":15,"sparse":{"indices":[],"values":[]}}.

Syntax

HLL_CREATE_SKETCH (aggregate_expression)

Argument

aggregate_expression

Any valid expression that provides the value to an aggregate, such as a column name. NULL values are ignored. This function supports any data type as input except HLLSKETCH, GEOMETRY, GEOGRAPHY, and VARBYTE.

Return type

The HLL_CREATE_SKETCH function returns an HLLSKETCH value.

Examples

The following example returns the HLLSKETCH type for column an_int in table a_table. A JSON object is used to represent a sparse HyperLogLog sketch when importing, exporting, or printing sketches. A string representation (in Base64 format) is used to represent a dense HyperLogLog sketch.

CREATE TABLE a_table(an_int INT);
INSERT INTO a_table VALUES (1), (2), (3), (4);

SELECT hll_create_sketch(an_int) AS sketch FROM a_table;
sketch
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{"version":1,"logm":15,"sparse":{"indices":[20812342,20850007,22362299,47158030],"values":[1,2,1,1]}}
(1 row)