Categories:

Aggregate Functions (Cardinality Estimation) , Window Functions

# HLL¶

Uses HyperLogLog to return an approximation of the distinct cardinality of the input (i.e. HLL(col1, col2, ... ) returns an approximation of COUNT(DISTINCT col1, col2, ... )).

Aliases:

## Syntax¶

Aggregate function

HLL( [ DISTINCT ] <expr1> [ , ... ] )

HLL(*)


Window function

HLL( [ DISTINCT ] <expr1> [ , ... ] ) OVER ( [ PARTITION BY <expr2> ] )

HLL(*) OVER ( [ PARTITION BY <expr2> ] )


## Arguments¶

expr1

This is the expression for which you want to know the number of distinct values.

expr2

This is the optional expression used to group rows into partitions.

## Returns¶

The data type of the returned value is INTEGER.

## Usage Notes¶

• DISTINCT can be included as an argument, but has no effect.

• For information about NULL values and aggregate functions, see Aggregate Functions and NULL Values.

• When used as a window function:

• This function does not support:

• ORDER BY sub-clause in the OVER() clause.

• Window frames.

## Examples¶

This example shows how to use HLL and its alias APPROX_COUNT_DISTINCT. This example calls both COUNT(DISTINCT i) and APPROX_COUNT_DISTINCT(i) to emphasize that the results do not always match exactly.

SELECT COUNT(i), COUNT(DISTINCT i), APPROX_COUNT_DISTINCT(i), HLL(i)
FROM sequence_demo;


Output:

The results might vary because APPROX_COUNT_DISTINCT() returns an approximation, not an exact value.

SELECT COUNT(i), COUNT(DISTINCT i), APPROX_COUNT_DISTINCT(i), HLL(i)
FROM sequence_demo;
+----------+-------------------+--------------------------+--------+
| COUNT(I) | COUNT(DISTINCT I) | APPROX_COUNT_DISTINCT(I) | HLL(I) |
|----------+-------------------+--------------------------+--------|
|     1024 |              1024 |                     1030 |   1030 |
+----------+-------------------+--------------------------+--------+