PFMERGE

Introduction and Use Case(s)

PFMERGE is a Redis command used to merge multiple HyperLogLog data structures into a single HyperLogLog. This is particularly useful when you want to combine the approximations of distinct elements from different sets. Typical use cases include merging user activity logs from multiple shards, aggregating statistics, or combining results from distributed computations.

Syntax

  1. PFMERGE destkey sourcekey [sourcekey ...]

Parameter Explanations

  • destkey: The key where the merged HyperLogLog will be stored.
  • sourcekey: One or more keys of the HyperLogLogs that you want to merge.

Return Values

The command returns OK if the merge operation is successful. If any of the source keys do not contain a valid HyperLogLog, an error is returned.

Code Examples

  1. dragonfly> PFADD hll1 "a" "b" "c"
  2. (integer) 1
  3. dragonfly> PFADD hll2 "b" "c" "d"
  4. (integer) 1
  5. dragonfly> PFADD hll3 "d" "e" "f"
  6. (integer) 1
  7. dragonfly> PFMERGE hllmerged hll1 hll2 hll3
  8. OK
  9. dragonfly> PFCOUNT hllmerged
  10. (integer) 6

Best Practices

When using PFMERGE, ensure that all the sourcekeys contain valid HyperLogLog structures. This prevents errors and ensures accurate merging of the datasets.

Common Mistakes

  • Invalid HyperLogLog Keys: Attempting to merge keys that do not contain HyperLogLog structures will result in an error.
  • Overwriting Existing Data: Be cautious when specifying destkey. If it already exists, its data will be overwritten with the merged result.

FAQs

Can I merge HyperLogLogs with non-HyperLogLog keys?

No, all source keys must be HyperLogLogs. Attempting to merge a HyperLogLog with a non-HyperLogLog key will result in an error.

What happens if destkey already exists?

If destkey already exists, the existing data will be overwritten by the merged result of the specified sourcekeys.