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.

Practice

1. In Redis, when should you prefer using a hash over a string to store data?

easy

A. When you want to store multiple related fields under one key, like an object.

B. When you only need to store a single simple value per key.

C. When you want to store large binary data efficiently.

D. When you want to store data that changes very rarely.

Solution

1. Step 1: Understand Redis data types

   Strings store single values per key, while hashes can store multiple fields under one key.

2. Step 2: Match use case to data type

   If you want to store an object with multiple fields (like name, age, email), hashes are better suited.

3. Final Answer:

   When you want to store multiple related fields under one key, like an object. -> Option A

4. Quick Check:

   Hash = multiple fields per key [OK]

Hint: Use hashes for objects with many fields, strings for single values [OK]

Common Mistakes:

• Thinking strings can store multiple fields easily
• Using hashes for simple single values
• Confusing hashes with lists or sets

2. Which of the following Redis commands correctly sets a field age to 30 in a hash named user:1?

easy

A. SET user:1 age 30

B. HSET user:1 age 30

C. HMSET user:1 age: 30

D. SETFIELD user:1 age 30

Solution

1. Step 1: Identify command for setting a single field in a hash

   The HSET command sets a field in a hash key.

2. Step 2: Check other options

   SET is for strings, HMSET is deprecated but similar to HSET, SETFIELD is invalid.

3. Final Answer:

   HSET user:1 age 30 -> Option B

4. Quick Check:

   HSET sets hash fields [OK]

Hint: Use HSET to set one field in a hash [OK]

Common Mistakes:

• Using SET for hash fields
• Using deprecated HMSET instead of HSET
• Inventing commands like SETFIELD

3. Given the following Redis commands executed in order:

HSET user:2 name "Alice" age 25
HSET user:2 city "Paris"
GET user:2

What will be the output of the GET user:2 command?

medium

A. null (no value)

B. "Alice"

C. A list of all fields and values in user:2.

D. An error because user:2 is a hash, not a string.

Solution

1. Step 1: Understand data types and commands

   HSET creates a hash key user:2. GET works only on string keys.

2. Step 2: Predict command behavior

   Using GET on a hash key causes a type error in Redis.

3. Final Answer:

   An error because user:2 is a hash, not a string. -> Option D

4. Quick Check:

   GET on hash key = error [OK]

Hint: GET only works on strings, not hashes [OK]

Common Mistakes:

• Expecting GET to return hash fields
• Confusing GET with HGETALL
• Assuming null means no error

4. You have this Redis command sequence:

HSET user:3 name "Bob" age 40
SET user:3 "simple string"

What problem will occur and how to fix it?

medium

A. Error because user:3 key type conflicts; delete key before SET.

B. No problem; Redis allows both hash and string on same key.

C. The SET command will overwrite the hash silently.

D. The HSET command will fail because key exists as string.

Solution

1. Step 1: Understand Redis key type rules

   A key can only hold one data type at a time. Setting a string on a key that is a hash causes an error.

2. Step 2: Identify fix

   To set a string on the same key, delete the existing hash key first using DEL.

3. Final Answer:

   Error because user:3 key type conflicts; delete key before SET. -> Option A

4. Quick Check:

   Key type conflict = error, DEL key first [OK]

Hint: Delete key before changing type to avoid errors [OK]

Common Mistakes:

• Assuming Redis allows multiple types on one key
• Expecting SET to overwrite hash without error
• Not deleting key before changing type

5. You want to store user profiles with fields name, age, and email in Redis. You also want to efficiently update individual fields without rewriting the whole object. Which approach is best and why?

hard

A. Store each user as multiple string keys, one per field, like user:1:name, user:1:age.

B. Store each user as a string with JSON and rewrite entire string on update.

C. Store each user as a hash with fields for name, age, and email for easy updates.

D. Store all users in one big hash with user IDs as fields and JSON strings as values.

Solution

1. Step 1: Consider update efficiency

   Updating individual fields is easiest with hashes because you can update one field without rewriting others.

2. Step 2: Compare options

   Strings with JSON require rewriting whole string. Multiple string keys increase complexity. One big hash mixes users and complicates access.

3. Final Answer:

   Store each user as a hash with fields for name, age, and email for easy updates. -> Option C

4. Quick Check:

   Hashes = efficient field updates [OK]

Hint: Use hashes for objects needing partial field updates [OK]

Common Mistakes:

• Using strings with JSON and rewriting whole data
• Splitting fields into many string keys unnecessarily
• Storing all users in one hash causing complexity