HLL_UNION_AGG

Description

The HLL_UNION_AGG function is an aggregate function mainly used to merge multiple HyperLogLog data structures and estimate the approximate cardinality after merging.

Syntax

hll_union_agg(<hll>)

Parameters

Parameter	Description
<hll>	The expression to be calculated, type HLL supported.

Return Value

Returns a BIGINT cardinality value. If there is no valid data in the group, returns 0.

Example

-- setup
create table test_uv(
    id int,
    uv_set string
) distributed by hash(id) buckets 1
properties ("replication_num"="1");
insert into test_uv values
    (1, ('a')),
    (1, ('b')),
    (2, ('c')),
    (2, ('d')),
    (3, null);

select HLL_UNION_AGG(HLL_HASH(uv_set)) from test_uv;

+---------------------------------+
| HLL_UNION_AGG(HLL_HASH(uv_set)) |
+---------------------------------+
|                               4 |
+---------------------------------+

select HLL_UNION_AGG(HLL_HASH(uv_set)) from test_uv where uv_set is null;

+---------------------------------+
| HLL_UNION_AGG(HLL_HASH(uv_set)) |
+---------------------------------+
|                               0 |
+---------------------------------+