Hash vs string for objects in Redis - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why choose a Redis hash over a string for storing user profiles?

Consider you want to store user profile data in Redis. Which reason best explains why using a Redis hash is better than a single string for this?

A Strings automatically compress data, saving memory compared to hashes.

B Hashes allow updating individual fields without rewriting the entire object.

C Hashes can only store numeric values, making them faster than strings.

D Strings support nested data structures, while hashes do not.

💡 Hint

Think about how you update parts of a user profile frequently.

Redis hashes let you update or retrieve individual fields without rewriting the whole object, which is efficient for partial updates. Strings require rewriting the entire value.

❓ query_result

intermediate

What is the output of this Redis command sequence?

Given these commands executed in order, what is the output of the final command?

HSET user:1 name "Alice" age "30"
HSET user:1 city "Paris"
HGETALL user:1

A ["name", "Alice", "age", "30", "city", "Paris"]

B ["user:1", "name", "Alice", "age", "30", "city", "Paris"]

C Error: key user:1 does not exist

D ["name", "Alice", "city", "Paris"]

💡 Hint

HGETALL returns all fields and values of a hash as a flat list.

HGETALL returns all fields and values in the hash as a list of alternating keys and values.

📝 Syntax

advanced

Which Redis command syntax correctly sets multiple fields in a hash?

Select the correct Redis command to set the fields 'name' to 'Bob' and 'age' to '25' in the hash 'user:2'.

A HMSET user:2 name Bob age 25

B SET user:2 name Bob age 25

C HSET user:2 {name: 'Bob', age: 25}

D HSET user:2 name Bob age 25

💡 Hint

Recent Redis versions recommend a specific command for setting multiple hash fields.

HSET supports multiple field-value pairs in recent Redis versions. HMSET is deprecated. SET is for strings, not hashes. Option D is invalid syntax.

❓ optimization

advanced

Which approach is more memory efficient for storing 1000 small user objects in Redis?

You have 1000 user objects each with 5 small fields. Which Redis data structure is generally more memory efficient?

A Store each user as a separate Redis string with JSON data.

B Store all users in a single Redis list as JSON strings.

C Store each user as a Redis hash with 5 fields.

D Store each user as a Redis set with field values as members.

💡 Hint

Consider Redis internal encoding and overhead for small objects.

Redis hashes are optimized for small objects and use less memory than storing JSON strings in separate keys or lists. Sets are not suitable for key-value pairs.

🔧 Debug

expert

Why does this Redis command fail with a WRONGTYPE error?

Given the commands below, why does the last command produce a WRONGTYPE error?

SET user:3 "{\"name\":\"Eve\", \"age\": 28}"
HGET user:3 name

A Because 'user:3' is a string key, not a hash, so HGET cannot be used.

B Because the JSON string is invalid and cannot be parsed by HGET.

C Because HGET requires the key to be a string, not a hash.

D Because the field 'name' does not exist in the string value.

💡 Hint

Check the data type of the key before using hash commands.

HGET only works on hash keys. Here, 'user:3' is a string key, so using HGET causes a WRONGTYPE error.