DATASKETCHES_HLL_UNION_AGG

Description

datasketches_hll_union_agg is an aggregate function used to union multiple Apache DataSketches HLL (hll_sketch) serialized values and return the estimated cardinality (approximate distinct count / NDV) after union.

This function expects the input to be serialized bytes of a DataSketches HLL sketch (for example, generated by hll_sketch.serialize_compact() in the DataSketches library). It does not accept arbitrary strings.

Aliases:

• ds_hll_estimate
• datasketches_hll_estimate

Syntax

datasketches_hll_union_agg(<sketch>)

Parameters

Parameter	Description
<sketch>	The serialized bytes of an Apache DataSketches HLL sketch. Supported types: STRING / VARCHAR / VARBINARY. NULL values are ignored. Empty strings are treated as invalid input and will throw an error.

Return Value

Returns a DOUBLE (Float64) cardinality estimate value.
If there is no valid data in the group (or the input is empty), returns 0.
If the input bytes cannot be deserialized as a valid DataSketches HLL sketch (including empty string), an error is thrown (typically with error code CORRUPTION).

Example

-- setup
CREATE TABLE test_datasketches_hll_union_agg_tbl (
    id INT,
    sk STRING
)
DISTRIBUTED BY HASH(id) BUCKETS 1
PROPERTIES ("replication_num" = "1");

-- The sketch bytes are inserted via Base64 decoding.
INSERT INTO test_datasketches_hll_union_agg_tbl VALUES
    (1, from_base64('AgEHCAMIBwjL18IEK/L7BoYv+Q11gWYHgbxdBntl5gj8LUIK')),
    (2, from_base64('AwEHCAUIAAkKAAAAIjvrBcS1nwfGGWoEyHokBO8t9wc1qTEENkcJB7hWqQxZf9QNnuSbGA==')),
    (3, NULL);

-- The function returns DOUBLE, so use ROUND/CAST if you want an integer display.
SELECT CAST(ROUND(datasketches_hll_union_agg(sk)) AS BIGINT)
FROM test_datasketches_hll_union_agg_tbl;

+-------------------------------------------------------+
| CAST(ROUND(datasketches_hll_union_agg(sk)) AS BIGINT) |
+-------------------------------------------------------+
|                                                    17 |
+-------------------------------------------------------+

-- aliases
SELECT
    CAST(ROUND(datasketches_hll_union_agg(sk)) AS BIGINT) AS v1,
    CAST(ROUND(ds_hll_estimate(sk)) AS BIGINT)            AS v2,
    CAST(ROUND(datasketches_hll_estimate(sk)) AS BIGINT)  AS v3
FROM test_datasketches_hll_union_agg_tbl;

+------+------+------+
| v1   | v2   | v3   |
+------+------+------+
|   17 |   17 |   17 |
+------+------+------+

-- empty input returns 0
SELECT datasketches_hll_union_agg(sk)
FROM test_datasketches_hll_union_agg_tbl
WHERE sk IS NULL;

+--------------------------------+
| datasketches_hll_union_agg(sk) |
+--------------------------------+
|                              0 |
+--------------------------------+

-- invalid sketch bytes will throw
SELECT datasketches_hll_union_agg(from_base64('AA=='));

ERROR 1105 (HY000): errCode = 2, detailMessage = (127.0.0.1)[CORRUPTION]HLL sketch data corrupted when add: Attempt to deserialize unknown object type

-- empty string is invalid and will throw
SELECT datasketches_hll_union_agg('');

ERROR 1105 (HY000): errCode = 2, detailMessage = (127.0.0.1)[CORRUPTION]HLL sketch data corrupted when add: empty input.