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, ... )).

For more information about HyperLogLog, see Estimating the Number of Distinct Values.

Aliases:: APPROX_COUNT_DISTINCT.
See also:: HLL_ACCUMULATE , HLL_COMBINE , HLL_ESTIMATE

Syntax¶

Aggregate function

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

HLL(*)

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Window function

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

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

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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 this function is called as a window function, it does not support:
- An ORDER BY clause within the OVER clause.
- Explicit 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 of these two functions do not always match exactly.

The exact output from the following query 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;

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