hll_union
function
Applies to: Databricks SQL Databricks Runtime 13.3 LTS and above
This function utilizes the HyperLogLog algorithm to combine two sketches into a single sketch.
Queries can use the resulting buffers to compute approximate unique counts as long integers with the hll_sketch_estimate function.
The implementation uses the Apache Datasketches library. Please see HLL for more information.
Arguments
exprN
: ABINARY
expression holding a sketch generated by hll_sketch_agg.allowDifferentLgConfigK
: A optionalBOOLEAN
expression controlling whether to allow merging two sketches with different lgConfigK values. The default value is false.
Returns
A BINARY
buffer containing the HyperLogLog sketch computed as a result of combining the input expressions.
When the allowDifferentLgConfigK
parameter is true, the result sketch uses the smaller of the two provided lgConfigK
values.
Examples
> SELECT hll_sketch_estimate(
hll_union(
hll_sketch_agg(col1),
hll_sketch_agg(col2)))
FROM VALUES
(1, 4),
(1, 4),
(2, 5),
(2, 5),
(3, 6) AS tab(col1, col2);
6
> SELECT hll_sketch_estimate(
hll_union(
hll_sketch_agg(col1, 4),
hll_sketch_agg(col2, 21)))
FROM VALUES
(1, 4),
(1, 4),
(2, 5),
(2, 5),
(3, 6) AS tab(col1, col2);
error