2 Ways to Sort the Elements in a Redis List, Set, or Sorted Set

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Redis provides a couple of commands that allow us to sort the elements in a list, set, or sorted set.

The SORT command is probably the most obvious, and does exactly what its namesake promises. However, there’s also a SORT_RO command, which is a read-only variant of the SORT command.

Example

If you just want to do a sort operation without storing the result, you can use either command – SORT or SORT_RO. However, bear in mind that there may be implications when using the SORT command (see “The Difference Between SORT and SORT_RO” below).

Here’s an example of using SORT_RO:

SORT_RO animals ALPHA

Result:

1) "Bird"
2) "Cat"
3) "Cow"
4) "Dog"
5) "Horse"
6) "Zebra"

Here, I included the ALPHA modifier, which is used to sort string values. In this case, the animals key contains a set, and that set contains string values.

We can get the same result with the SORT command:

SORT animals ALPHA

Result:

1) "Bird"
2) "Cat"
3) "Cow"
4) "Dog"
5) "Horse"
6) "Zebra"

As mentioned, there could be implications with using SORT over SORT_RO (see below).

The Difference Between SORT and SORT_RO

The main difference between SORT and SORT_RO is that SORT provides a STORE option. This enables us to store the sorted list/set/sorted set in a new key:

SORT animals DESC ALPHA STORE animals_desc

Result:

(integer) 6

Here, I sorted the list in descending order, then stored it in a key called animals_desc.

Let’s check the contents of the new key:

LRANGE animals_desc 0 -1

Result:

1) "Zebra"
2) "Horse"
3) "Dog"
4) "Cow"
5) "Cat"
6) "Bird"

We used the LRANGE command because the SORT command stores the result as a list, and LRANGE returns the elements of a list.

Note that because of the STORE option, SORT is technically flagged as a writing command in the Redis command table. This means that read-only replicas in a Redis Cluster will redirect it to the master instance even if the connection is in read-only mode.

Therefore, if this is a concern for you, you can use SORT_RO command in place of the SORT command. The SORT_RO variant was introduced in order to allow SORT behaviour in read-only replicas without breaking compatibility on command flags.