openLooKeng implements the
approx_distinct function using the HyperLogLog data structure.
openLooKeng implements HyperLogLog data sketches as a set of 32-bit buckets which store a maximum hash. They can be stored sparsely (as a map from bucket ID to bucket), or densely (as a contiguous memory block). The HyperLogLog data structure starts as the sparse representation, switching to dense when it is more efficient. The P4HyperLogLog structure is initialized densely and remains dense for its lifetime.
hyperloglog_type implicitly casts to
p4hyperloglog_type, while one can explicitly cast
cast(hll AS P4HyperLogLog)
Data sketches can be serialized to and deserialized from
varbinary. This allows them to be stored for later use. Combined with the ability to merge multiple sketches, this allows one to calculate
approx_distinct of the elements of a partition of a query, then for the entirety of a query with very little
For example, calculating the
HyperLogLog for daily unique users will allow weekly or monthly unique users to be calculated incrementally by combining the dailies. This is similar to computing weekly revenue by
summing daily revenue. Uses of
GROUPING SETS can be converted to use
CREATE TABLE visit_summaries ( visit_date date, hll varbinary ); INSERT INTO visit_summaries SELECT visit_date, cast(approx_set(user_id) AS varbinary) FROM user_visits GROUP BY visit_date; SELECT cardinality(merge(cast(hll AS HyperLogLog))) AS weekly_unique_users FROM visit_summaries WHERE visit_date >= current_date - interval '7' day;
approx_set(x) -> HyperLogLog
HyperLogLog sketch of the input data set of
x. This data sketch underlies
approx_distinct and can be stored and used later by calling
cardinality(hll) -> bigint
This will perform
approx_distinct on the data summarized by the
hll HyperLogLog data sketch.
empty_approx_set() -> HyperLogLog
Returns an empty
merge(HyperLogLog) -> HyperLogLog
HyperLogLog of the aggregate union of the individual
hll HyperLogLog structures.