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Redis - Hashes

Hashes in Redis work like objects in programming languages; it is a single key that can contain multiple field-value pairs inside it. The fields in the hash are unique.

Learn more

Limitation with hashes

Hashes can only contains strings. You cannot nest more hashes inside hashes or have any other data types inside hashes.

Setting a field inside hash (HSET)

To set a field inside a hash (and to create the key if it does not not exist yet), the command HSET can be used. Learn more

Time Complexity: O(1) for each field / value pair added. O(N) if adding multiple.

Example usage:

Text Only
HSET myapp.user#1 token "abc123"
HSET myapp.user#1 firstname "Anne" lastname "Example"
Example Output 
(integer) 1
(integer) 2

Getting Hash Fields and their Values (HSCAN)

HKEYS

The HKEYS command was used in the past for listing the keys. It is no longer recommended to use the HKEYS command, as it blocks the whole database operation until it outputs the data and can cause massive problems in production environments with lots of data (it's slow and takes large amount of memory with large dataset). Use HSCAN instead.

The command HSCAN works like the SCAN, thus can be used to list all fields and their values inside a hash. It can take in the MATCH (filter) and COUNT (how many results to return) parameters for further customization. Learn more

Time Complexity: O(1) for the call. O(N) where N is the number of elements in the specified hash collection.

Example usage:

Text Only
HSCAN myapp.user#1 0
Example Output 
(cursor) 0
1) "token"
"abc123"
2) "firstname"
"Anne"
3) "lastname"
"Example"

Defaults to 10

By default, when not passing the COUNT parameter, it will list approximately 10 keys.

Text Only
HSCAN myapp.user#1 0 COUNT 10
Example Output 
(cursor) 0
1) "token"
"abc123"
2) "firstname"
"Anne"
3) "lastname"
"Example"

Hint, or requirement

Note that the COUNT parameter can return more or less keys than specified. The COUNT is merely a recommendation for Redis and it can return back different amount of elements that you wanted it to, but it tries to be very close to the specified value.

Text Only
HSCAN myapp.user#1 0 MATCH *name*
Example Output 
(cursor) 0
1) "firstname"
"Anne"
2) "lastname"
"Example"

Defaults to 10

By default, when not passing the COUNT parameter, it will list approximately 10 keys.

Getting Value for a Hash Field (HGET)

The command HGET can be used to get a value for the given field. Learn more

Time Complexity: O(1)

Example usage:

Text Only
HGET myapp.user#1 token
Example Output 
"abc123"

Removing a Hash Field (HDEL)

The command HDEL can be used to remove a field from the hash. Learn more

Time Complexity: O(1) for single field removal, O(N) if there are multiple fields where N denotes the amount of fields.

Example usage:

Text Only
HDEL myapp.user#1 lastname
Example Output 
(integer) 1

Verifying the type and object encoding

To verify that the created key is a hashmap and to determine it's object encoding, we can use the basic TYPE and OBJECT ENCODING commands:

Example usage:

Text Only
TYPE myapp.user#1
Example Output
> hash

Text Only
OBJECT ENCODING myapp.user#1
Example Output
> ziplist

Removing the Hash

To finally remove the whole hash (key), the normal DEL command can be used. Learn more

Time Complexity: O(1) for removing a hash.

Text Only
DEL myapp.user#1
Example Output 
(integer) 1

Usage Example of Hashes

In an example where you would have an online store where products are stored, you could have hash created for each product. For example:

Text Only
HSET myapp.product#1 name "T-shirt" price 19 description "Nice shirt"
HSET myapp.product#2 name "Branded Hoodie" price 50 description "Brand Amidas on the front."
Example Output 
(integer) 3
(integer) 3
Text Only
HGET myapp.product#1 name
HGET myapp.product#1 price
HGET myapp.product#2 name
Example Output 
"T-shirt"
"19"
"Branded Hoodie"