Understanding Redis Sets and Their Use Cases

Redis Sets are unordered collections of strings, similar to Java's HashSet . They are implemented with hash tables, giving O(1) time complexity for add, delete, and lookup operations.

Elements in a Set are unique, so duplicate data cannot appear.

Internally, the hash table stores each element as a key with a NULL value. When all elements are 64‑bit integers and the number of elements does not exceed the set-max-intset-entries limit (default 512), Redis switches to an intset (integer array) for lower memory usage.

Typical scenarios include storing multiple items without duplicates and without caring about order, as well as performing set operations such as intersection, union, and difference. Examples: finding common followers, calculating daily new followers, or tagging articles.

Practical example – a social app needs a “common friends” feature. First, create Sets for each user:

SADD user:刘备 赵子龙 张飞 关羽 貂蝉
SADD user:曹操 貂蝉 夏侯惇 典韦 张辽

Then compute the intersection with:

SINTERSTORE user:曹刘好友 user:刘备 user:曹操

Retrieve the result using:

SMEMBERS user:曹刘好友

The command returns the common friend “貂蝉”.