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.