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TOPK.RESERVE

Syntax​

TOPK.RESERVE key topk [width depth decay]

Time complexity: O(1)

ACL categories: @topk

Creates a new Top-K data structure at key that tracks the top topk most frequent items.

  • topk: The number of top items to track (between 1 and 100000).
  • width (optional): The number of counters per hash function row (default 8, max 1000000).
  • depth (optional): The number of hash function rows (default 7, max 100).
  • decay (optional): The probability decay factor between 0 and 1 (default 0.9).

The default parameters are recommended for most use cases. In particular, the default decay value of 0.9 uses a globally shared pre-computed lookup table, while a custom decay value forces Dragonfly to allocate a separate lookup table in memory for each key. Using many Top-K structures with different custom decay values will result in higher memory consumption.

If width, depth, and decay are provided, all three must be specified together. If key already exists, an error (item exists) is returned.

Return​

Simple string reply: OK if the Top-K was created successfully.

An error is returned if:

  • The key already exists.
  • The parameters are out of the allowed ranges.

Examples​

dragonfly> TOPK.RESERVE topk 3
OK

dragonfly> TOPK.RESERVE topk_custom 5 20 7 0.9
OK

See also​

TOPK.ADD | TOPK.INFO | TOPK.LIST