Count distinct unique items in Redis with HyperLogLog (PFADD/PFCOUNT)
domain: redis.io · 4 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
PFADD key element [element ...] records each observed item and creates the key if it doesn't exist.
PFCOUNT key returns the estimated number of unique items (cardinality).
Union several HyperLogLogs in one call: PFCOUNT k1 k2 k3 returns the cardinality of their union.
Pattern: call PFADD on each search query or visit, then call PFCOUNT any time to get the unique-count metric (unique queries, unique daily users, etc.).
Known gotchas
The count is approximated with ~0.81% standard error — not exact.
Each HyperLogLog uses ~12KB of memory regardless of the number of items — great for very large sets, wasteful for tiny ones.
PFCOUNT across multiple keys performs an on-the-fly merge that cannot be cached and has a much larger cost than the single-key O(1) path — don't call it in hot loops.
PFADD returns 1 if any internal register changed, else 0; PFCOUNT of a nonexistent key returns 0.
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