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ZPOPMIN

Introduction​

In Dragonfly, as well as in Redis and Valkey, the ZPOPMIN command is used to remove and return the member with the smallest score in a sorted set. Sorted sets are particularly useful when you need to maintain ranking systems or work with ordered collections. With ZPOPMIN, you can efficiently extract the lowest-ranked item along with its score.

Syntax​

ZPOPMIN key [count]
  • Time complexity: O(log(N)*M) with N being the number of elements in the sorted set, and M being the number of elements popped.
  • ACL categories: @write, @sortedset, @fast

Parameter Explanations​

  • key: The key of the sorted set.
  • count (optional): An integer specifying how many members with the lowest scores to pop. If omitted, the default value is 1.

Return Values​

  • If the count is 1 or not provided, the command returns a single array of two elements (the first is the member and the second is its score).
  • If a count greater than 1 is specified, it returns an array with up to count elements, each being a two-element array of [member, score].

Code Examples​

Basic Example: Pop the member with the smallest score​

In this example, we will insert a few members into a sorted set, and then we will use ZPOPMIN to remove and return the member with the smallest score.

dragonfly> ZADD myzset 1 "one" 2 "two" 3 "three"
(integer) 3
dragonfly> ZPOPMIN myzset
1) "one"
2) "1"

The member "one" with the score "1" is returned and removed from the sorted set.

Pop multiple members with the smallest scores​

You can also specify a count to remove and return multiple members with the smallest scores. This can be useful in scenarios where you need to process more than one item at a time.

dragonfly> ZADD myzset 4 "four" 5 "five" 6 "six"
(integer) 3
dragonfly> ZPOPMIN myzset 2
1) "two"
2) "2"
3) "three"
4) "3"

The members "two" and "three" are both returned, as they had the two smallest scores, and they are removed from the set.

Using ZPOPMIN in real-time ranking systems​

Imagine you are implementing a ranking system for a game, and you frequently need to pop the lowest-ranked player for removal or recalculations.

dragonfly> ZADD leaderboard 1000 "playerA" 1200 "playerB" 1500 "playerC"
(integer) 3
dragonfly> ZPOPMIN leaderboard
1) "playerA"
2) "1000"

In this case, "playerA" with the score of 1000 is returned and removed, as they were the lowest-ranked player.

Popping from an empty sorted set​

If the sorted set is empty, ZPOPMIN returns nil:

dragonfly> ZPOPMIN emptyset
(nil)

Best Practices​

  • Use ZPOPMIN to maintain performance in real-time systems where you need to process the lowest-scored items first, such as in event queues or job prioritization systems.
  • If you frequently need to remove more than one item, specify the count parameter to batch multiple removals in a single atomic operation.

Common Mistakes​

  • Assuming ZPOPMIN operates on unordered sets; it works only with sorted sets.
  • Forgetting that when using ZPOPMIN, the members will be permanently removed from the sorted set.

FAQs​

What happens if the key does not exist?​

If the key does not exist, ZPOPMIN returns nil, as there are no members to pop.

Can I use ZPOPMIN to atomically pop and process multiple items?​

Yes, you can pass a count argument to pop multiple members atomically in a single call. Keep in mind that members are returned in ascending order of their scores.

Will ZPOPMIN return ties if multiple members have the same score?​

Yes, if multiple members have the same score, ZPOPMIN will return them in lexicographical order. For example, if two or more members have an identical score, the one that comes first alphabetically will be returned first.