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HyperLogLog is a data structure that estimates the cardinality of a set. As a probabilistic data structure, HyperLogLog trades perfect accuracy for efficient space utilization.

The Redis HyperLogLog implementation uses up to 12 KB and provides a standard error of 0.81%.

Examples

Add some items to the HyperLogLog:

> PFADD members 123
(integer) 1
> PFADD members 500
(integer) 1
> PFADD members 12
(integer) 1

Estimate the number of members in the set:
```
> PFCOUNT members
(integer) 3
```

Basic commands

PFADD adds an item to a HyperLogLog.
PFCOUNT returns an estimate of the number of items in the set.
PFMERGE combines two or more HyperLogLogs into one.

See the complete list of HyperLogLog commands.

Performance

Writing (PFADD) to and reading from (PFCOUNT) the HyperLogLog is done in constant time and space. Merging HLLs is O(n), where n is the number of sketches.

Limits

The HyperLogLog can estimate the cardinality of sets with up to 18,446,744,073,709,551,616 (2^64) members.

Learn more

Redis new data structure: the HyperLogLog has a lot of details about the data structure and its implementation in Redis.
Redis HyperLogLog Explained shows you how to use Redis HyperLogLog data structures to build a traffic heat map.

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