What is the difference between SUNION and SUNIONSTORE?
Sep 02, 2025 am 12:12 AMThe main difference between SUNION and SUNIONSTORE is the way of processing results. 1. SUNION is used to return union elements of multiple collections, without modifying data, and is suitable for temporary viewing of results; 2. SUNIONSTORE stores union results in a new key, and is suitable for scenarios where persistent results are required. Both times have O(N), but SUNIONSTORE may have a greater impact on performance due to write operations. You should pay attention to memory usage and execution timing when using it.
SUNION and SUNIONSTORE are both Redis commands used to compute the union of multiple sets, but they serve slightly different purposes. The main difference lies in how they handle the result.
What Does Each Command Do?
SUNION returns the union of multiple sets as a list of elements. It doesn't modify your data—it's read-only in nature. You give it one or more set keys, and it gives you back all the unique elements across those sets.
For example:
SUNION set1 set2
This would return all the distinct members from set1
and set2
.
SUNIONSTORE , on the other hand, takes the result of the union and stores it in a new key. This is useful when you want to persist the result for later use.
Example:
SUNIONSTORE destination_set set1 set2
After this command runs, you'll have a new set called destination_set
that contains the union of set1
and set2
.
One thing to note: if destination_set
already exists, it will be overwritten.
When to Use Each One
Use SUNION when you just need to see the combined elements without saving them. It's great for quick checks or temporary results.
Use SUNIONSTORE when you want to save the union result for future operations. This avoids recalculating the union every time you need it.
- If you're doing a one-time check → go with SUNION
- If you're building a cached or computed dataset → use SUNIONSTORE
Also, keep in mind memory usage: storing large unions can take up space, so only use SUNIONSTORE when necessary.
Performance Considerations
Both commands have a time complexity of O(N), where N is the total number of elements in all the sets. So performance depends heavily on the size of your sets.
A few things to remember:
- Running SUNION on very large sets can block the Redis server momentarily.
- SUNIONSTORE writes data, so it may impact performance more than SUNION, especially if done frequently.
- If you're using Redis in a production environment, consider running these during off-peak hours if dealing with huge datasets.
If you're working with massive sets and concerned about latency, test these commands in a staging environment first.
Basically that's it.
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