HyperLogLog can estimate how many distinct values have appeared without retaining every value. In Redis, its dense sketch uses 12,288 bytes, and Redis documents a standard error of 0.81%. Those figures describe a compact probabilistic counter—not an exact set, a hard error limit, or a benchmark proving a particular 100-billion-item count.
What “counting” means here
Suppose you want to answer, “How many unique visitors did the site have today?” An exact approach stores visitor identifiers in a set and counts the set. That can also answer whether a particular identifier was seen, but retaining distinct identifiers takes memory that grows with the set.
HyperLogLog (HLL) takes a different tradeoff: it summarizes observations and estimates the number of distinct inputs. The summary does not preserve the original identifiers, so it cannot tell you whether one specific visitor has appeared. Redis describes its result as an approximation, not an exact count. Redis HyperLogLog documentation
How a small sketch can say something about a huge set
Hash values, then look for rare patterns
Conceptually, HLL hashes each input into a well-distributed string of bits. Some bits select a register in the sketch; the remaining bits are examined for a pattern such as a run of leading zeros. A long run is uncommon: if one appears, it is evidence that many hashes have been observed.
#1 Best Overall
- Read Before You Buy — No Video Output: These adapters support charging and USB 2.0 data transfer, but cannot transmit video signals. Except for standard USB webcams (which use USB data only), they are not compatible with HDMI/DisplayPort cables, video-capable USB-C hubs, or docking stations with video output.
- Convert USB-A Ports to USB-C: Designed to connect USB-C earphones, cables, flash drives, card readers, and other USB-C accessories to standard USB-A ports. Plug-and-play with no drivers or software required.
- Aluminum Alloy Housing: Built with a sturdy aluminum alloy shell that aids in heat dissipation and protects against daily wear and scratches. Designed to maintain a stable and secure connection.
- Compact & Travel-Friendly: The ultra-compact design allows the adapter to stay plugged into your device without blocking adjacent ports or adding bulk, reducing wear and tear on your original USB ports.
- 12-Month Warranty: Backed by a 12-month manufacturer warranty for peace of mind. Designed to meet strict quality control standards for reliable everyday performance.
Combine many noisy observations
One register’s observation is too noisy to give a dependable count by itself. HLL aggregates information from many registers, using a harmonic-mean estimator and corrections for small and large ranges. The result is a compact estimate rather than a record of which values produced it. This is an intuitive description, not a full derivation of the algorithm. An overview of HyperLogLog’s counting intuition
What Redis’s “12 KB” figure covers
Redis documents a dense HyperLogLog representation of 12,288 bytes: 16,384 six-bit counters plus a 16-byte header. A sketch may use a sparse representation instead, which can take less space. Redis describes the sketch as using up to 12 KB, plus a few bytes for the key; the figure is specific to Redis’s implementation, not a universal size for every HLL library. Redis PFCOUNT documentation
Rank #2
- 5-in-1 USB-C Hub: Experience comprehensive connectivity featuring a Power Delivery input, two USB-A 2.0 ports, a USB-A 3.0 port, and an HDMI port. (Note: The USB-C power delivery input port is only for connecting an external wall charger to power your laptop and cannot power peripheral devices.)
- 90W Pass-Through Charging: Achieve optimal charging with 90W pass-through power to your laptop, supported by a total input of 100W, with the hub reserving 10W for operational efficiency. (Note: Wall charger not included.)
- Quick Data Transfers: Accelerate your productivity with rapid data transfers using a high-speed 5Gbps USB 3.0 port and two 480Mbps USB 2.0 ports.
- 4K HDMI Display: Enhance your visual experience with a hub capable of delivering 4K resolution at 30Hz in both mirror and extend modes. Please note that this hub is compatible with MacBook (macOS 12 and newer), Windows 10 and 11, ChromeOS, and laptops equipped with DP Alt Mode and Power Delivery. Note: This device is not compatible with Linux.
- What You Get: Anker USB-C Hub (5-in-1, 4K HDMI), welcome guide, 18-month warranty, and our friendly customer service.
The title’s 100-billion scale is illustrative, not a reported benchmark in the cited sources. The important idea is that sketch memory is bounded by the implementation’s configuration rather than growing in proportion to every observation processed.
How accurate is the estimate?
Redis documents a 0.81% standard error for its HyperLogLog estimate. Standard error is not a guarantee that every result will be within 0.81% of the true count, nor should it be treated as a hard maximum deviation. It is Redis’s stated error characteristic for its implementation. Redis HyperLogLog documentation
Recommended Free Tools
Rank #3
- Sleek 7-in-1 USB-C Hub: Features an HDMI port, two USB-A 3.0 ports, and a USB-C data port, each providing 5Gbps transfer speeds. It also includes a USB-C PD input port for charging up to 100W and dual SD and TF card slots, all in a compact design.
- Flawless 4K@60Hz Video with HDMI: Delivers exceptional clarity and smoothness with its 4K@60Hz HDMI port, making it ideal for high-definition presentations and entertainment. (Note: Only the HDMI port supports video projection; the USB-C port is for data transfer only.)
- Double Up on Efficiency: The two USB-A 3.0 ports and a USB-C port support a fast 5Gbps data rate, significantly boosting your transfer speeds and improving productivity.
- Fast and Reliable 85W Charging: Offers high-capacity, speedy charging for laptops up to 85W, so you spend less time tethered to an outlet and more time being productive.
- What You Get: Anker USB-C Hub (7-in-1), welcome guide, 18-month warranty, and our friendly customer service.
Redis commands for adding, counting, and combining
| Goal | Command | What it does |
|---|---|---|
| Add observations | PFADD key element... |
Adds values to the sketch. Repeated values do not increase the estimated distinct count as though they were new distinct items. |
| Estimate a count | PFCOUNT key |
Returns the approximate cardinality represented by one sketch. |
| Combine sketches | PFMERGE destination source... |
Merges source sketches into a destination sketch, useful for rolling up partitions or periods. |
Redis also accepts multiple keys with PFCOUNT to estimate their union without first issuing a separate merge. That multi-key count does more work than counting one key, so the two forms have different performance characteristics. Sketch merging handles overlap approximately; it does not reconstruct an exact union of original identifiers. Redis PFCOUNT command documentation
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose an exact set or HyperLogLog based on the consequence
| Decision | Exact hash set | HyperLogLog |
|---|---|---|
| Cardinality | Exact | Approximate |
| Check whether one item was seen | Yes | No; the sketch cannot answer membership questions |
| Memory as distinct values grow | Grows with retained values | Bounded by implementation and configuration; Redis uses up to 12 KB per sketch |
| Typical fit | Billing, payment deduplication, or eligibility decisions requiring exactness | Aggregate counts such as unique visitors or distinct search queries |
| Combine partitions | Requires retaining items or another exact set-union strategy | Sketches can be merged, or counted together in Redis |
If an incorrect or approximate count could charge someone, allow a duplicate payment, or wrongly grant or deny a coupon, use an exact structure or another exact workflow. HLL is for aggregate questions where a compact estimate is useful and individual membership is not required.
Quick Recap
Best Value
- 5-in-1 Connectivity: Equipped with a 4K HDMI port, a 5 Gbps USB-C data port, two 5 Gbps USB-A ports, and a USB C 100W PD-IN port. Note: The USB C 100W PD-IN port supports only charging and does not support data transfer devices such as headphones or speakers.
- Powerful Pass-Through Charging: Supports up to 85W pass-through charging so you can power up your laptop while you use the hub. Note: Pass-through charging requires a charger (not included). Note: To achieve full power for iPad, we recommend using a 45W wall charger.
- Transfer Files in Seconds: Move files to and from your laptop at speeds of up to 5 Gbps via the USB-C and USB-A data ports. Note: The USB C 5Gbps Data port does not support video output.
- HD Display: Connect to the HDMI port to stream or mirror content to an external monitor in resolutions of up to 4K@30Hz. Note: The USB-C ports do not support video output.
- What You Get: Anker 332 USB-C Hub (5-in-1), welcome guide, our worry-free 18-month warranty, and friendly customer service.
Rank #4
- Dual Converters, Infinite Potential：Includes 2× USB C male to USB A female adapters and 2× USB A male to USB C female adapters. Perfect for a wide range of uses—tablets with Bluetooth keyboards, expand USB ports on macbook, and more. Two different converters for all your daily needs
- Next-Level 10Gbps & 3A Charging: No more slow 480Mbps, this usb to usb c adapter has a transfer speed of up to 10Gbps, allowing you to do more transferring in less time. This usb adapter fits both USB A and USB C charger, supporting up to 3A fast charging
- Upgraded Exquisite Craftsmanship: With an aluminum alloy housing and metal connector, the usbc to usb adapter is extremely durable and sturdy. Rigorously tested to withstand more than 10,000 times of plugging and unplugging, ensuring long-lasting performance
- Broad Compatible: The usb c to usb adapter widely supports all USB C/ USB A devices like laptops, tablets, cellphones, car chargers, and phone chargers. Such as compatible with MacBook Pro/Air 2023/2022, Thunderbolt 4/3 Devices,Apple MagSafe Watch 9/8/7/SE/Ultra, iPad Pro 2022/2021, Samsung Galaxy S23/S20/S10, and iPhone 17/16/15 Pro. Plug and play
- Please Note: To reach 10Gbps speed, keep the cable under 3.3 ft. For USB A Male to USB C adapters, try flipping the USB C connector. USB C Male to USB A adapters support bidirectional 10Gbps transfer within 3.3 ft
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
