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ZRANGEBYLEX

Introduction​

In Dragonfly, as well as in Redis and Valkey, the ZRANGEBYLEX command is used to return a range of members in a sorted set, filtered by the member's lexicographical order. It is especially useful for situations where you need to retrieve elements based purely on their defined string order, rather than by their scores. This command offers a convenient way to perform efficient range queries on the lexicographical ordering of the elements in a sorted set.

Syntax​

ZRANGEBYLEX key min max [LIMIT offset count]
  • Time complexity: O(log(N)+M) with N being the number of elements in the sorted set and M the number of elements being returned. If M is constant (e.g. always asking for the first 10 elements with LIMIT), you can consider it O(log(N)).
  • ACL categories: @read, @sortedset, @slow

Parameter Explanations​

  • key: The key of the sorted set where the range query is performed.
  • min: The minimum lexicographical value (inclusive or exclusive). Use ( to exclude the value and [ to include it.
  • max: The maximum lexicographical value (inclusive or exclusive). Again, use [ to include the value and ( to exclude it.
  • LIMIT offset count (optional): Allows you to limit the number of elements returned starting from a specific position (offset), and up to the maximum number of elements (count) to return.

Return Values​

A list of members in the sorted set that fall within the specified lexicographical range.

Code Examples​

Basic Example: Retrieve Members in a Lexicographical Range​

Let's create a sorted set of words and retrieve words that fall between apple and banana inclusively:

dragonfly> ZADD fruits 0 apple 0 apricot 0 avocado 0 banana 0 berry
(integer) 5
dragonfly> ZRANGEBYLEX fruits [apple [banana
1) "apple"
2) "apricot"
3) "avocado"
4) "banana"

Exclusive Range​

You can exclude the start or end of the range by using parentheses ( instead of square brackets [:

dragonfly> ZRANGEBYLEX fruits (apple [banana
1) "apricot"
2) "avocado"
3) "banana"

Using LIMIT to Restrict Results​

You can limit the number of results by specifying an offset and count:

dragonfly> ZRANGEBYLEX fruits [a [z LIMIT 1 2
1) "apricot"
2) "avocado"

In this example, the query starts from the second lexicographically-smallest member (offset = 1) and returns up to 2 members (count = 2).

Querying a Range with No Matching Elements​

If no elements fall within the specified range, the command will return an empty list:

dragonfly> ZRANGEBYLEX fruits [zzz [zzzz
(empty array)

Best Practices​

  • Take advantage of [ or ( when defining lexicographical bounds to specify whether to include or exclude the boundary values.
  • Use the LIMIT clause to avoid processing too many elements when you only need a subset of the range.
  • Use ZRANGEBYLEX on datasets where elements are naturally ordered lexicographically, such as dictionary words or sequential, lexicographically sortable IDs.

Common Mistakes​

  • Forgetting to use the correct boundary markers ([ for inclusive, ( for exclusive) in min and max parameters.
  • Failing to specify the lexicographical ordering when intending to use string-based ordering and mistakenly using ZSCORE operations instead.
  • Misunderstanding that ZRANGEBYLEX operates only on elements themselves, and not on their scores.

FAQs​

Can ZRANGEBYLEX work with numeric scores?​

No, ZRANGEBYLEX operates strictly on the lexicographical ordering of the members in a sorted set, regardless of their scores. To perform range queries on numeric scores, use ZRANGEBYSCORE.

What happens if the key does not exist?​

If the key does not exist, ZRANGEBYLEX returns an empty array because there are no members in the sorted set.

Can I use ZRANGEBYLEX with a set that has numeric-like string values?​

Yes, you can use numeric-like strings such as "123", "456", etc. ZRANGEBYLEX will still interpret these strings lexicographically, meaning "123" may come before "12" because it's based on string comparison, not numeric order.