PFCOUNT

Returns the approximated cardinality of the set(s) observed by the HyperLogLog key(s).

Browse all commands · Upstream source documentation

Reference field	Value
Canonical name	PFCOUNT
Catalog group	HyperLogLog commands
Upstream since	2.8.9 — upstream Redis/module version, not an ffkv release
Pinned catalog	Redis 8.10.0

Syntax

PFCOUNT key [key ...]

ffkv behavior and limits

Cardinality is approximate. ffkv uses Redis-compatible HYLL registers and unions; this does not establish compatibility for every arbitrary or corrupted upstream dump.

Source: README.md:288; src/engine/hyperloglog.rs; tests/bdd/features/hyperloglog.feature.

All requests share the 8 MiB framing/1,024-argument ingress limit; normal RESP replies have 64 MiB wire/logical budgets with bounded nesting/value counts. Management and Lua origins impose smaller budgets. These are not a process RSS cap. Catalog inclusion is not exhaustive option or wire-format parity. See limits and compatibility.

Upstream metadata

The following fields come from the pinned Redis metadata, not measured ffkv performance or an assertion that ffkv enforces every upstream flag. Running COMMAND INFO/COMMAND DOCS describe the current engine.

Upstream Redis complexity: O(1) with a very small average constant time when called with a single key. O(N) with N being the number of keys, and much bigger constant times, when called with multiple keys.

Upstream arity: -2 (includes the command name; a negative number is a minimum).

Command flags: READONLY, MAY_REPLICATE.

ACL categories: HYPERLOGLOG.

Argument structure

Argument	Type	Constraints / metadata
key	key	multiple; key_spec_index: 0

Key specifications

Upstream key discovery descriptors (indexes are relative to the full command vector):

[
  {
    "notes": "RW because it may change the internal representation of the key, and propagate to replicas",
    "flags": [
      "RW",
      "ACCESS"
    ],
    "begin_search": {
      "index": {
        "pos": 1
      }
    },
    "find_keys": {
      "range": {
        "lastkey": -1,
        "step": 1,
        "limit": 0
      }
    }
  }
]

Additional pinned upstream metadata

{
  "group": "hyperloglog",
  "since": "2.8.9"
}

Metadata source · Local snapshot: src/command_metadata.json, key pfcount.

BDD coverage and source examples

These are source observations, not a test run performed by this documentation build. A tag identifies scenario intent; an extracted vector identifies a command invocation, which may intentionally produce an error. Scenarios may require setup, multiple connections, persistence, cluster nodes or native-module fixtures.

• WRITE pauses hold may-replicate operations without holding reads (Scenario Outline) — tests/bdd/features/client_control_repair.feature:98; command tag and invocation observed.
• Duplicate elements do not update cardinality and cache invalidates — tests/bdd/features/hyperloglog.feature:5; command tag and invocation observed.
• Union deduplicates overlapping input and supports destination aliases — tests/bdd/features/hyperloglog.feature:89; command tag and invocation observed.
• Sparse decoding and dense conversion expose real representation — tests/bdd/features/hyperloglog.feature:161; invocation observed.
• Register diagnostics return decoded registers without changing cardinality — tests/bdd/features/hyperloglog.feature:232; invocation observed.
• Seeded register and estimator diagnostics preserve database keys — tests/bdd/features/hyperloglog.feature:257; invocation observed.
• PFADD rejects invalid HYLL without mutating existing values — tests/bdd/features/hyperloglog.feature:294; invocation observed.
• PFCOUNT rejects invalid HYLL without mutating existing values — tests/bdd/features/hyperloglog.feature:323; command tag and invocation observed.
• PFMERGE rejects invalid HYLL without mutating existing values — tests/bdd/features/hyperloglog.feature:352; invocation observed.
• PFDEBUG rejects invalid HYLL without mutating existing values — tests/bdd/features/hyperloglog.feature:381; invocation observed.
• Malformed sparse runs are rejected before cache or register access — tests/bdd/features/hyperloglog.feature:410; command tag and invocation observed.
• Sketch merges and cache refresh retain expiry and survive recovery — tests/bdd/features/hyperloglog.feature:436; command tag and invocation observed.

Extracted command vectors

The following JSON arrays are the test harness’s argument vectors, not shell commands. Numeric arguments are converted by the harness; objects of the form {"base64":"…"} represent decoded binary bytes, not a literal JSON command argument. Preserve scenario setup and connection ordering. See the CLI guide for shell quoting and binary stdin. No reply is invented or inferred.

Duplicate elements do not update cardinality and cache invalidates — tests/bdd/features/hyperloglog.feature:15 (I execute:)

[
  "PFCOUNT",
  "h"
]

Next source assertion: the reply equals:

0

Union deduplicates overlapping input and supports destination aliases — tests/bdd/features/hyperloglog.feature:111 (I execute:)

[
  "PFCOUNT",
  "a"
]

Next source assertion: the reply equals:

4

PFADD rejects invalid HYLL without mutating existing values — tests/bdd/features/hyperloglog.feature:313 (I execute:)

[
  "PFCOUNT",
  "dest"
]

Next source assertion: the reply equals:

2