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ZREVRANGE

Introduction​

In Dragonfly, as well as in Redis and Valkey, the ZREVRANGE command is used to return a range of members in a sorted set, ordered from the highest to the lowest score. It is the reverse of the ZRANGE command, which retrieves members in ascending order of their scores. This command is particularly useful for ranking scenarios like leaderboards, where you want to list the top performers.

Syntax​

ZREVRANGE key start stop [WITHSCORES]
  • Time complexity: O(log(N)+M) with N being the number of elements in the sorted set and M the number of elements returned.
  • ACL categories: @read, @sortedset, @slow

Parameter Explanations​

  • key: The key of the sorted set.
  • start: The starting rank (0-based index) where 0 represents the highest score in the set (inclusive).
  • stop: The ending rank (inclusive).
  • WITHSCORES (optional): If provided, the command returns both the member and its associated score.

Return Values​

  • Without WITHSCORES, the command returns a list of members in the specified range, ordered from highest to lowest score.
  • With WITHSCORES, the command returns a list where each member is followed by its score in the sorted set.

Code Examples​

Basic Example​

Retrieve the top 3 members of a leaderboard:

dragonfly$> ZADD leaderboard 100 "PlayerA" 90 "PlayerB" 85 "PlayerC" 75 "PlayerD"
(integer) 4

dragonfly$> ZREVRANGE leaderboard 0 2
1) "PlayerA"
2) "PlayerB"
3) "PlayerC"

Using the WITHSCORES Option​

Fetch the top 3 members along with their scores:

dragonfly$> ZADD leaderboard 100 "PlayerA" 90 "PlayerB" 85 "PlayerC" 75 "PlayerD"
(integer) 4

dragonfly$> ZREVRANGE leaderboard 0 2 WITHSCORES
1) "PlayerA"
2) "100"
3) "PlayerB"
4) "90"
5) "PlayerC"
6) "85"

Specifying Negative Indexes​

You can use negative indexes to fetch members starting from the end of the sorted set. For example, to retrieve the last 2 members:

dragonfly$> ZADD leaderboard 100 "PlayerA" 90 "PlayerB" 85 "PlayerC" 75 "PlayerD"
(integer) 4

dragonfly$> ZREVRANGE leaderboard -2 -1
1) "PlayerC"
2) "PlayerD"

Best Practices​

  • Use ZREVRANGE with WITHSCORES when you need both the members and their associated scores, especially in ranking applications.
  • For leaderboards that might grow large over time, consider paginating results by adjusting the start and stop parameters dynamically.
  • Whenever only member values are needed (and not scores), exclude the WITHSCORES option to optimize performance and reduce response size.

Common Mistakes​

  • Confusing the start and stop parameters to be based on the score range rather than the index in the sorted set. They are 0-based positional indexes, not score values.
  • Assuming that the command affects the sorting order or scores in the set. ZREVRANGE only retrieves the data and doesn't modify any existing scores or members.

FAQs​

What happens if the key does not exist?​

If the key doesn't exist, ZREVRANGE returns an empty list.

Can I use ZREVRANGE with large datasets?​

Yes. However, it's advised to use ranges (start and stop) to paginate through large sorted sets. Fetching a large number of members at once could result in performance degradation.

What happens if start or stop exceeds the set's bounds?​

If start or stop exceeds the sorted set's length, ZREVRANGE will return the elements within the valid range. If there are no elements in the specified range, an empty list is returned.