Object storage with hashes in Redis - Time & Space Complexity

Time Complexity: Object storage with hashes

O(n)

Understanding Time Complexity

When storing objects using hashes in Redis, it is important to understand how the time to store or retrieve data changes as the object size grows.

We want to know how the number of operations changes when we add more fields to a hash.

Scenario Under Consideration

Analyze the time complexity of the following Redis commands storing an object as a hash.

HSET user:1000 name "Alice"
HSET user:1000 age "30"
HSET user:1000 email "[e-mail]"
HGETALL user:1000

This code stores multiple fields of a user object in a Redis hash and then retrieves all fields at once.

Identify Repeating Operations

Look for repeated actions that affect performance.

• Primary operation: Setting or getting each field in the hash.
• How many times: Once per field stored or retrieved.

How Execution Grows With Input

As the number of fields in the hash grows, the time to store or retrieve all fields grows roughly in direct proportion.

Input Size (n)	Approx. Operations
10 fields	About 10 operations
100 fields	About 100 operations
1000 fields	About 1000 operations

Pattern observation: The time grows linearly as the number of fields increases.

Final Time Complexity

Time Complexity: O(n)

This means the time to store or retrieve all fields grows directly with the number of fields in the hash.

Common Mistake

[X] Wrong: "Storing or getting a hash is always fast and constant time no matter how many fields it has."

[OK] Correct: Each field requires its own operation, so more fields mean more work and longer time.

Interview Connect

Understanding how Redis hashes scale helps you explain data storage choices clearly and shows you know how performance changes with data size.

Self-Check

"What if we used multiple smaller hashes instead of one big hash? How would that affect the time complexity when retrieving all data?"

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