Redis ZUNIONSTORE in Python (Detailed Guide w/ Code Examples)

Use Case(s)

The ZUNIONSTORE command in Redis is used to compute the union of multiple sorted sets and store the resulting sorted set in a new key. It's particularly useful in scenarios such as combining scores from different games into a single leaderboard, or aggregating results from various metrics that have a notion of ranking or scoring.

Code Examples

Basic Example of ZUNIONSTORE

In this example, we use ZUNIONSTORE to combine scores from two game leaderboards into a combined leaderboard.

import redis r = redis.Redis(host='localhost', port=6379, db=0) # Assuming 'game1_scores' and 'game2_scores' are existing sorted sets result = r.zunionstore('combined_scores', ['game1_scores', 'game2_scores']) print(f"Number of elements in 'combined_scores': {result}")

This code initializes a connection to Redis, then combines 'game1_scores' and 'game2_scores' into a new sorted set called 'combined_scores'.

Using Weights and Aggregate Options

You can also apply weights to the scores from different sets and choose an aggregation method (sum, min, or max).

result = r.zunionstore('weighted_combined_scores', {'game1_scores': 1, 'game2_scores': 2}, aggregate='max') print(f"Number of elements in 'weighted_combined_scores': {result}")

This example creates a combined score list where scores from 'game2_scores' are twice as significant as 'game1_scores'. The maximum score between the two for each player is stored using the aggregate='max' option.

Best Practices

  • When using ZUNIDONSTORE, carefully consider the memory usage, especially when dealing with large number of keys or very large sorted sets.
  • Ensure that the data types of the keys involved are all sorted sets. Attempting to use ZUNIONSTORE with other data types will result in an error.

Common Mistakes

  • Forgetting to handle the scenario where one or more keys do not exist. Redis treats them as empty sets, but depending on your application logic, this might need special handling.
  • Not specifying weights correctly: Weights must be numbers and applied consistently if you are trying to achieve a weighted union.


What happens if one of the sorted sets does not exist?

Redis treats non-existing keys as empty sorted sets during the operation.

How does Redis handle ties in scores within sorted sets?

Redis ranks items with the same score by lexicographical order of their members.

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