HLL_COUNT_MERGE

Reference material for HLL_COUNT_MERGE

Merges one or more HLL++ sketches that were previously built using the aggregate function HLL_COUNT_BUILD into a new sketch.

Each sketch must be built on the same type and the same precision. Attempts to merge sketches for different types or precisions results in an error. For example, you cannot merge a sketch built from INTEGER data with one built from TEXT data, or a sketch built with precision 13 and a sketch built with precision 14.

Syntax

HLL_COUNT_MERGE(<expression>) [FILTER ([WHERE] <condition>)]

Parameters

Parameter	Description	Supported input types
<expression>	HLL++ sketch in a valid format, e.g. the output of the HLL_COUNT_BUILD function.	BYTEA
<condition>	An optional boolean expression to filter rows used in aggregation	BOOL

Return Type

BYTEA

Example

Following the example in HLL_COUNT_BUILD:

SELECT hll_count_estimate(hll_count_merge(a)) AS hll_estimate, hll_count_merge(a) AS merged_sketch
FROM sketch_of_data_to_count;

hll_estimate BIGINT	merged_sketch BYTEA
6606880	\x2f4167677265676174654675…