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ZLEXCOUNT

Introduction

In Dragonfly, as well as in Redis and Valkey, the ZLEXCOUNT command is used to count the number of elements in a sorted set that fall between a given lexicographical range (i.e., the dictionary order of the string values within the set). It is useful in a wide range of scenarios, particularly when dealing with ordered or alphabetical sorting, range-based queries, or when managing complex datasets that require retrieval of elements within specific textual limits.

Syntax

ZLEXCOUNT key min max
  • Time complexity: O(log(N)) with N being the number of elements in the sorted set.
  • ACL categories: @read, @sortedset, @fast

Parameter Explanations

  • key: The key for the sorted set where the lexicographical counting occurs.
  • min: The minimum lexicographical string value for the range query (inclusive or exclusive).
  • max: The maximum lexicographical string value for the range query (inclusive or exclusive).
  • For the min and max parameters, use [ for inclusive and ( for exclusive boundaries.
  • For the min and max parameters, - and + represent the lowest and highest possible strings in the sorted set, respectively.

Return Values

  • An integer indicating the number of members in the sorted set within the provided lexicographical range.

Code Examples

Basic Example

Count the number of elements in a lexicographical range in a sorted set:

dragonfly$> ZADD myset 0 a 0 b 0 c 0 d 0 e
(integer) 5

dragonfly$> ZLEXCOUNT myset [b [d
(integer) 3

In this example, the elements b, c, and d fall within the lexicographical range [b, d], returning a count of 3.

Range with Exclusive Boundaries

Specify an exclusive upper range limit in a lexicographical query:

dragonfly$> ZADD myset 0 a 0 b 0 c 0 d 0 e
(integer) 5

dragonfly$> ZLEXCOUNT myset [b (d
(integer) 2

Here, only b and c fall within the range [b, d), as d is excluded due to the exclusive boundary.

Counting All Elements with a Lexicographical Range Across All Members

To count all elements in the sorted set, use - and + as argument values denoting the lowest and highest possible strings:

dragonfly$> ZADD myset 0 a 0 b 0 c 0 d 0 e
(integer) 5

dragonfly$> ZLEXCOUNT myset - +
(integer) 5

In this case, all members from a to e are counted.

Best Practices

  • When using large sorted sets, lexicographical queries can be quite efficient for text-based range retrievals.
  • Use inclusive and exclusive boundaries carefully ([ and () to define the exact range of elements you need.
  • For lexicographical queries covering large segments with efficient performance, consider using ZLEXCOUNT instead of scanning large parts of the sorted set.

Common Mistakes

  • Assuming ZLEXCOUNT operates on numeric values—this command only works on lexicographical (text) comparisons.
  • Misunderstanding the boundary markers [, (—inclusive and exclusive boundaries can lead to incorrect or unintended results if not used properly.

FAQs

What happens if the key does not exist?

If the key does not exist, ZLEXCOUNT will return 0.

Can I use ZLEXCOUNT with integer values?

While ZLEXCOUNT can technically handle integer-like strings, the values are still compared lexicographically, not numerically. This means "100" will come before "20" when sorted lexicographically.

What does - and + mean in ZLEXCOUNT ranges?

When using ZLEXCOUNT, - represents the lowest possible string in lexicographical order, and + represents the highest. This allows you to cover the entire range of possible elements in the sorted set.