Count unique items with Redis HyperLogLog (PFADD/PFCOUNT/PFMERGE)

domain: redis.io · 6 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗

Documented steps

  1. Add elements to a HyperLogLog: `PFADD site:visits user-1 user-2 user-3`. Re-adding the same element is a no-op.
  2. Get the estimated cardinality: `PFCOUNT site:visits`.
  3. Merge multiple HLLs: `PFMERGE dest hll1 hll2` - union of sets, correct for aggregating daily buckets into a weekly/global estimate.
  4. HLL uses a fixed ~12 KB per key regardless of cardinality.
  5. python (redis-py): `r.pfadd('hll','a','b'); r.pfcount('hll')`.
  6. Use for unique visitors, distinct IPs, unique searches - anywhere an exact set is too memory-heavy.

Known gotchas

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