Aggregation functions
HLL_COUNT_BUILD
Reference material for HLL_COUNT_BUILD
Counts the approximate number of unique not NULL values, aggregating the values to HLL++ sketches represented as
the BYTEA data type.
Multiple sketches can be merged to a single sketch using the aggregate function HLL_COUNT_MERGE.
To estimate the final distinct count value, the scalar function HLL_COUNT_ESTIMATE can be
used.
HLL_COUNT_BUILD
uses the HLL++ algorithm and allows you to control the set sketch size precision, similar
to HLL_COUNT_DISTINCT.
HLL_COUNT_BUILD
requires less memory than exact count distinct aggregation, but also introduces statistical uncertainty.
The default precision is 12, with a maximum of 20 set optionally.
Higher precision comes at a memory and performance cost.
Syntax
Parameters
Parameter | Description | Supported input types |
---|---|---|
<expression> | Any column name or function that return a column name. | Any type |
<precision> | Optional literal integer value to set precision. If not included, the default precision is 12. Precision range: 12-20. | INTEGER , BIGINT |
Return Type
BYTEA
Example
accurate_count (BIGINT) |
---|
3333334 |
accurate_count (BIGINT) |
---|
5000001 |
hll_estimate (BIGINT) | sketch (BYTEA) |
---|---|
3291008 | \x2f41676772656761746546… |
4948957 | \x2f41676772656761746546… |
hll_estimate (BIGINT) |
---|
6606880 |