Redis 集
Redis 集简介
Redis 集是唯一字符串（成员）的无序集合。 您可以使用 Redis 集来高效地：
- 跟踪唯一项目（例如，跟踪访问给定博客文章的所有唯一 IP 地址）。
- 表示关系（例如，具有给定角色的所有用户的集合）。
- 执行常见的集作，例如交集、并集和差集。
基本命令
SADD将新成员添加到 SET 中。SREM从 Set 中删除指定的成员。SISMEMBER测试 Set 成员资格的字符串。SINTER返回两个或多个集共有的成员集（即交集）。SCARD返回集合的大小（也称为 cardinality）。
请参阅 set 命令的完整列表。
例子
-
存放法国和美国的自行车组。请注意， 如果添加已存在的成员，则将忽略该成员。
-
Check whether bike:1 or bike:2 are racing in the US.
-
Which bikes are competing in both races?
-
How many bikes are racing in France?
Tutorial
The SADD command adds new elements to a set. It's also possible
to do a number of other operations against sets like testing if a given element
already exists, performing the intersection, union or difference between
multiple sets, and so forth.
Here I've added three elements to my set and told Redis to return all the
elements. There is no order guarantee with a set. Redis is free to return the
elements in any order at every call.
Redis has commands to test for set membership. These commands can be used on single as well as multiple items:
We can also find the difference between two sets. For instance, we may want
to know which bikes are racing in France but not in the USA:
There are other non trivial operations that are still easy to implement
using the right Redis commands. For instance we may want a list of all the
bikes racing in France, the USA, and some other races. We can do this using
the SINTER command, which performs the intersection between different
sets. In addition to intersection you can also perform
unions, difference, and more. For example
if we add a third race we can see some of these commands in action:
You'll note that the SDIFF command returns an empty array when the
difference between all sets is empty. You'll also note that the order of sets
passed to SDIFF matters, since the difference is not commutative.
When you want to remove items from a set, you can use the SREM command to
remove one or more items from a set, or you can use the SPOP command to
remove a random item from a set. You can also return a random item from a
set without removing it using the SRANDMEMBER command:
Limits
The max size of a Redis set is 2^32 - 1 (4,294,967,295) members.
Performance
Most set operations, including adding, removing, and checking whether an item is a set member, are O(1).
This means that they're highly efficient.
However, for large sets with hundreds of thousands of members or more, you should exercise caution when running the SMEMBERS command.
This command is O(n) and returns the entire set in a single response.
As an alternative, consider the SSCAN, which lets you retrieve all members of a set iteratively.
Alternatives
Sets membership checks on large datasets (or on streaming data) can use a lot of memory.
If you're concerned about memory usage and don't need perfect precision, consider a Bloom filter or Cuckoo filter as an alternative to a set.
Redis sets are frequently used as a kind of index.
If you need to index and query your data, consider the JSON data type and the Redis Query Engine features.
Learn more
- Redis Sets Explained and Redis Sets Elaborated are two short but thorough video explainers covering Redis sets.
- Redis University's RU101 explores Redis sets in detail.
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