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Version: Candidate-3.4

APPROX_COUNT_DISTINCT_HLL_SKETCH

Returns the approximate value of aggregate function similar to the result of COUNT(DISTINCT col). Like APPROX_COUNT_DISTINCT(expr).

It is faster than the COUNT and DISTINCT combination and uses a fixed-size memory, so less memory is used for columns of high cardinality.

It is slower than APPROX_COUNT_DISTINCT(expr) but with higher precision. Which takes advantages of Apache Datasketches.

Syntax​

APPROX_COUNT_DISTINCT_HLL_SKETCH(expr)

Examples​

MySQL > select APPROX_COUNT_DISTINCT_HLL_SKETCH(query_id) from log_statis group by datetime;
+-----------------------------------+
| approx_count_distinct_hll_sketch(`query_id`) |
+-----------------------------------+
| 17721                             |
+-----------------------------------+

keyword​

APPROX_COUNT_DISTINCT_HLL_SKETCH,APPROX_COUNT_DISTINCT