Hashes

What a Redis hash holds

Redis hashes store a set of field-value pairs under a single key. If a string is one box with one thing in it, a hash is one box with labelled compartments inside.

That shape fits an object or a database row exactly: one user, with a name, an email, and a role, all under one key.

user:42
  name  -> "Ada"
  email -> "ada@example.com"
  role  -> "admin"

HSET: write fields

HSET sets one or more fields on a hash. If the key does not exist, Redis creates it.

HSET user:42 name "Ada" email "ada@example.com" role "admin"

(integer) 3

The number returned is how many new fields were added.

HGET: read one field

HGET pulls back a single field.

HGET user:42 email

"ada@example.com"

You get just the field you asked for, not the whole object. That is the big win over a JSON string.

HGETALL: read the whole object

HGETALL returns every field and value in the hash.

HGETALL user:42

1) "name"
2) "Ada"
3) "email"
4) "ada@example.com"
5) "role"
6) "admin"

Fields and values come back alternating: field, value, field, value. That is fine for a small user record. On a hash with thousands of fields it is a different story, since HGETALL walks every field and ships them all back at once. When you only need two or three, HGET each one instead and leave the rest on the server.

HDEL: remove a field

HDEL removes one or more fields from the hash.

HDEL user:42 role

(integer) 1

The rest of the hash stays exactly as it was. When you delete the last field, Redis removes the whole key.

When to use a hash over many string keys

You could store the same data as separate string keys:

SET user:42:name "Ada"
SET user:42:email "ada@example.com"

That works, but a hash is usually better when the fields belong to one thing:

• One key to manage. Set a TTL on user:42 and the whole object expires together.
• Read it all at once. HGETALL fetches the object in one command instead of several.
• Less memory. Redis stores small hashes very compactly.

Reach for a hash when you have an object or a row. Reach for separate string keys when the values are truly independent.

Common mistake

Do not set an expiry on individual fields. In Redis, TTL lives on the key, not on each field inside a hash. If you need different fields to expire at different times, they probably should not share one hash. Give them their own keys.

FAQ

How is a hash different from a JSON string?

With a JSON string you must read and rewrite the whole value to change one field. With a hash you can read or update a single field directly using HGET and HSET.

Can a hash field hold another hash?

No. Hash values are flat strings. For nested data you either flatten the keys or store a JSON string in the field.

What does HGETALL return if the key does not exist?

An empty result. Redis treats a missing hash the same as an empty one.