Object storage with hashes in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Retrieve a field from a Redis hash

Given a Redis hash stored with key user:1001 containing fields name, email, and age, what is the output of the command HGET user:1001 email if the stored email is [e-mail]?

Redis

HSET user:1001 name Alice email [e-mail] age 30
HGET user:1001 email

A "[e-mail]"

B "Alice"

C 30

D nil

💡 Hint

Remember that HGET returns the value of the specified field in the hash.

The HGET command fetches the value of the given field from the hash stored at the key. Since the field is 'email', it returns the stored email string.

📝 Syntax

intermediate

Identify the correct Redis command to add multiple fields to a hash

Which of the following commands correctly adds multiple fields title and author to a Redis hash with key book:1?

A HMSET book:1 title "Redis Basics" author "John Doe"

B HADD book:1 title "Redis Basics" author "John Doe"

C HSET book:1 {title: "Redis Basics", author: "John Doe"}

D HSET book:1 title "Redis Basics" author "John Doe"

💡 Hint

Use the modern command that supports multiple field-value pairs.

The HSET command in modern Redis versions supports multiple field-value pairs. HMSET is deprecated, HADD does not exist, and the syntax with curly braces is invalid.

❓ optimization

advanced

Efficiently retrieving multiple fields from a Redis hash

You want to retrieve the fields name, email, and age from a Redis hash user:1002. Which command is the most efficient to get all three fields in a single call?

A HMGET user:1002 name email age

B HGETALL user:1002

C

HGET user:1002 name
HGET user:1002 email
HGET user:1002 age

D GET user:1002

💡 Hint

Choose the command that fetches specific fields in one request.

HMGET fetches specified fields in one command efficiently. HGETALL returns all fields which may be unnecessary. Multiple HGET calls are less efficient. GET is for strings, not hashes.

🔧 Debug

advanced

Identify the error in this Redis hash command

What error will the following Redis command produce?

HSET user:1003 name "Bob" age

Redis

HSET user:1003 name "Bob" age

A Sets 'age' field to the string 'age'

B Sets 'age' field to empty string

C Syntax error: wrong number of arguments for 'HSET' command

D No error, command runs successfully

💡 Hint

HSET requires field-value pairs; check if all pairs are complete.

HSET expects pairs of field and value. Here, 'age' is given without a value, causing a syntax error due to wrong number of arguments.

🧠 Conceptual

expert

Understanding Redis hash memory efficiency

Why are Redis hashes considered memory efficient for storing many small objects compared to storing each object as a separate key?

A Because hashes allow storing only string values, which are smaller

B Because hashes internally use a ziplist or hashtable optimized for small fields, reducing overhead per field

C Because hashes store data on disk instead of memory

D Because Redis compresses all hash keys automatically

💡 Hint

Think about how Redis stores small hashes internally.

Redis uses a special encoding (ziplist or listpack) for small hashes that packs fields tightly, reducing memory overhead compared to many separate keys.

Practice

1. What does a Redis hash store?

easy

A. A set of unique keys without duplicates

B. Only a single string value per key

C. A list of keys without values

D. Multiple fields and their values under one key

Solution

1. Step 1: Understand Redis hash structure

   Redis hashes store multiple field-value pairs grouped under a single key.

2. Step 2: Compare options with hash definition

   Only Multiple fields and their values under one key correctly describes storing multiple fields and values under one key.

3. Final Answer:

   Multiple fields and their values under one key -> Option D

4. Quick Check:

   Hashes = multiple fields + values [OK]

Hint: Hashes group fields and values under one key [OK]

Common Mistakes:

• Thinking hashes store only one value
• Confusing hashes with lists or sets
• Assuming keys are stored without values

2. Which Redis command correctly adds a field 'name' with value 'Alice' to a hash key 'user:1'?

easy

A. HGET user:1 name Alice

B. HSET user:1 name Alice

C. SET user:1 name Alice

D. HMGET user:1 name Alice

Solution

1. Step 1: Identify command to add/update hash field

   HSET is the Redis command to set a field in a hash.

2. Step 2: Check command syntax

   HSET key field value is correct; so HSET user:1 name Alice is valid.

3. Final Answer:

   HSET user:1 name Alice -> Option B

4. Quick Check:

   HSET adds/updates hash fields [OK]

Hint: Use HSET to add or update hash fields [OK]

Common Mistakes:

• Using HGET instead of HSET to add data
• Using SET which is for strings, not hashes
• Confusing HMGET (retrieves multiple fields) with HSET

3. Given the following Redis commands executed in order:
HSET user:2 age 30
HSET user:2 name Bob
HGETALL user:2
What is the output of HGETALL user:2?

medium

A. ["age", 30, "name", "Bob"]

B. ["user:2", "age", "30", "name", "Bob"]

C. ["age", "30", "name", "Bob"]

D. ["name", "Bob"]

Solution

1. Step 1: Understand HSET commands

   HSET adds fields 'age' with '30' and 'name' with 'Bob' to hash 'user:2'.

2. Step 2: Understand HGETALL output format

   HGETALL returns an array of field-value pairs as strings in order added.

3. Final Answer:

   ["age", "30", "name", "Bob"] -> Option C

4. Quick Check:

   HGETALL returns all fields and values as string pairs [OK]

Hint: HGETALL returns flat list of field-value strings [OK]

Common Mistakes:

• Expecting key name in output
• Mixing data types (number vs string)
• Only getting one field instead of all

4. You run the command HGET user:3 email but get a null response. What could be the reason?

medium

A. The field 'email' does not exist in the hash 'user:3'

B. The key 'user:3' is a string, not a hash

C. The Redis server is down

D. The command syntax is incorrect

Solution

1. Step 1: Understand HGET behavior

   HGET returns null if the field does not exist in the hash.

2. Step 2: Check other options

   Key being a string would cause an error, not null; server down or syntax error would cause different errors.

3. Final Answer:

   The field 'email' does not exist in the hash 'user:3' -> Option A

4. Quick Check:

   Missing field in hash returns null [OK]

Hint: Null means field missing, not syntax or server error [OK]

Common Mistakes:

• Assuming null means server or syntax error
• Confusing key type errors with missing field
• Expecting error instead of null for missing field

5. You want to store user profiles with fields 'name', 'age', and 'email' in Redis hashes. Which approach is best to retrieve all user data efficiently?

hard

A. Store each user as a separate hash with key 'user:<id>' and use HGETALL to get all fields

B. Store all users in one hash with fields like 'user1_name', 'user1_age', 'user2_name', etc.

C. Store each field as a separate key like 'name:<id>', 'age:<id>', 'email:<id>'

D. Store user data as JSON strings in simple keys

Solution

1. Step 1: Analyze storage options

   Storing each user as a separate hash groups related fields logically and allows efficient access.

2. Step 2: Evaluate retrieval efficiency

   Using HGETALL on 'user:<id>' fetches all fields at once, simpler and faster than scattered keys or complex parsing.

3. Final Answer:

   Store each user as a separate hash with key 'user:<id>' and use HGETALL to get all fields -> Option A

4. Quick Check:

   Separate hashes per user = organized and efficient [OK]

Hint: Use one hash per user for grouped fields and easy retrieval [OK]

Common Mistakes:

• Mixing all users in one hash causing complex keys
• Using separate keys for each field complicates retrieval
• Storing JSON loses Redis hash benefits