Memory-efficient data structures in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Redis command sequence?

Consider the following Redis commands executed in order:

HMSET user:1 name "Alice" age 30 city "NY"
HGETALL user:1

What is the output of HGETALL user:1?

Redis

HMSET user:1 name "Alice" age 30 city "NY"
HGETALL user:1

A ["user:1", "name", "Alice", "age", "30", "city", "NY"]

B {"name": "Alice", "age": 30, "city": "NY"}

C ["name", "Alice", "age", "30", "city", "NY"]

D ["name", "Alice", "age", 30, "city", "NY"]

💡 Hint

Remember that HGETALL returns a flat list of fields and values as strings.

The HGETALL command returns a flat list of field-value pairs as strings, not a JSON object or dictionary.

🧠 Conceptual

intermediate

Which Redis data structure is most memory-efficient for storing a large set of unique integers?

You need to store millions of unique integers in Redis with minimal memory usage. Which data structure should you choose?

A Redis Bitmap

B Redis Sorted Set

C Redis Set

D Redis List

💡 Hint

Think about bit-level storage for integers.

Redis Bitmaps use bits to represent presence or absence of integers, making them very memory-efficient for large sets of integers.

📝 Syntax

advanced

Which Redis command correctly creates a memory-efficient small hash?

You want to create a small hash with fields 'a' and 'b' having values '1' and '2' respectively, optimized for memory. Which command is correct?

A HSET myhash {a 1} {b 2}

B HSET myhash a 1 b 2

C HMSET myhash a 1 b 2

D HSET myhash a=1 b=2

💡 Hint

Check the correct syntax for HSET with multiple fields.

HSET supports multiple field-value pairs as separate arguments. HMSET is deprecated. Options A and D have invalid syntax.

❓ optimization

advanced

How to optimize memory usage for a Redis list with many small elements?

You have a Redis list with many small string elements. Which approach reduces memory usage the most?

A Use a Redis List as is

B Use Redis Sorted Set instead

C Store elements as separate keys

D Convert the list to a Redis Ziplist-encoded list by keeping it small

💡 Hint

Redis uses special encodings for small lists to save memory.

Redis automatically uses Ziplist encoding for small lists, which is memory efficient. Keeping the list small enables this optimization.

🔧 Debug

expert

Why does this Redis command cause a memory spike?

You run:
LPUSH mylist "a" "b" "c" ... "z"
and notice a sudden memory spike. What is the most likely cause?

Redis

LPUSH mylist "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" "t" "u" "v" "w" "x" "y" "z"

A Redis converted the list from Ziplist to Linkedlist due to size

B LPUSH command syntax is incorrect causing memory leak

C Redis is duplicating the list elements internally

D Redis is compressing the list causing temporary memory spike

💡 Hint

Redis changes internal encoding based on list size.

When a list grows beyond a threshold, Redis switches from Ziplist (compact) to Linkedlist encoding, increasing memory usage.

Practice

1. Which Redis data structure is best suited for storing many small key-value pairs efficiently in memory?

easy

A. List

B. Set

C. Hash

D. String

Solution

1. Step 1: Understand Redis data structures

   Hashes store multiple field-value pairs under one key, saving memory compared to many separate keys.

2. Step 2: Compare with other structures

   Lists and sets store collections but are less memory-efficient for many small pairs; strings store single values only.

3. Final Answer:

   Hash -> Option C

4. Quick Check:

   Hashes = memory-efficient key-value pairs [OK]

Hint: Use hashes for many small key-value pairs to save memory [OK]

Common Mistakes:

• Choosing List or Set for key-value pairs
• Thinking String can store multiple pairs
• Ignoring memory efficiency of Hashes

2. Which of the following is the correct Redis command to add a field-value pair to a hash named user:1?

easy

A. HSET user:1 name "Alice"

B. SADD user:1 name "Alice"

C. LPUSH user:1 name "Alice"

D. SET user:1 name "Alice"

Solution

1. Step 1: Identify the command for hashes

   HSET adds or updates a field in a hash key in Redis.

2. Step 2: Check other commands

   SADD is for sets, LPUSH for lists, SET for strings; none add fields to hashes.

3. Final Answer:

   HSET user:1 name "Alice" -> Option A

4. Quick Check:

   HSET = add field to hash [OK]

Hint: Use HSET to add fields to hashes in Redis [OK]

Common Mistakes:

• Using SADD or LPUSH for hashes
• Confusing SET with HSET
• Incorrect command syntax

3. Given the Redis commands:
HSET user:2 age 30
HSET user:2 city "NYC"
What will HGETALL user:2 return?

medium

A. ["age", "30", "city", "NYC"]

B. ["user:2", "age", "30", "city", "NYC"]

C. ["age", 30, "city", "NYC"]

D. ["age", "30", "city"]

Solution

1. Step 1: Understand HSET commands

   Two fields, 'age' and 'city', are set with values '30' and 'NYC' in hash 'user:2'.

2. Step 2: Check HGETALL output format

   HGETALL returns a flat list of field-value pairs as strings.

3. Final Answer:

   ["age", "30", "city", "NYC"] -> Option A

4. Quick Check:

   HGETALL returns field-value pairs list [OK]

Hint: HGETALL returns flat list of fields and values [OK]

Common Mistakes:

• Expecting key name in output
• Mixing data types (string vs integer)
• Missing value for a field

4. You run the command HSET user:3 name "Bob" but get an error. Which is the most likely cause?

medium

A. Redis does not support hashes with string fields

B. The key user:3 already holds a string value

C. HSET requires a numeric field value

D. The field name is reserved and cannot be used

Solution

1. Step 1: Understand Redis type errors

   If the key 'user:3' holds a string, trying to use HSET (hash command) causes a type error.

2. Step 2: Check other options

   Field names are not reserved, HSET accepts strings, and Redis supports string fields in hashes.

3. Final Answer:

   The key user:3 already holds a string value -> Option B

4. Quick Check:

   Type mismatch causes HSET error [OK]

Hint: Check key type before using hash commands [OK]

Common Mistakes:

• Assuming field names are restricted
• Thinking HSET needs numeric values
• Ignoring Redis data type rules

5. You want to store user preferences with many optional fields but minimize memory. Which Redis approach is best?

hard

A. Use a set with all field names as members

B. Use a list with all possible fields and empty strings for missing

C. Use separate string keys for each field

D. Use a hash with only fields that have values

Solution

1. Step 1: Consider memory efficiency

   Hashes store only existing fields, saving memory by omitting empty or missing fields.

2. Step 2: Evaluate other options

   Lists with empty strings waste memory; sets store unique members but not key-value pairs; separate keys increase overhead.

3. Final Answer:

   Use a hash with only fields that have values -> Option D

4. Quick Check:

   Hashes save memory by storing only present fields [OK]

Hint: Store only present fields in hashes to save memory [OK]

Common Mistakes:

• Using lists with empty placeholders
• Using sets for key-value data
• Creating many separate keys