Memory-efficient data structures in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a memory-efficient Redis string key with a small value.

Redis

SET mykey [1]

Drag options to blanks, or click blank then click option'

A""

B"this is a very long string value that uses a lot of memory"

C1000000

D"small"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a very long string wastes memory.

Using an empty string might not be useful.

Using a small string value helps Redis store it efficiently in memory.

2fill in blank

medium

Complete the code to add multiple small elements efficiently to a Redis list.

Redis

LPUSH mylist [1]

Drag options to blanks, or click blank then click option'

A"element1"

B"a very long element that wastes memory"

C1000000

D""

Attempts:

3 left

💡 Hint

Common Mistakes

Adding large elements increases memory usage.

Adding empty strings might not be useful.

Adding small elements keeps the list memory-efficient.

3fill in blank

hard

Fix the error in the command to create a memory-efficient Redis hash with small fields.

Redis

HSET myhash field1 [1]

Drag options to blanks, or click blank then click option'

A"this is a very long value that wastes memory"

B""

C"smallvalue"

D1000000

Attempts:

3 left

💡 Hint

Common Mistakes

Using large values increases memory usage.

Using empty strings might not be useful.

Using small field values helps Redis store hashes efficiently using ziplists.

4fill in blank

hard

Fill both blanks to create a memory-efficient Redis sorted set with small scores and members.

Redis

ZADD myzset [1] [2]

Drag options to blanks, or click blank then click option'

A1

B"member1"

C1000000

D"averylongmembername"

Attempts:

3 left

💡 Hint

Common Mistakes

Using large scores or long member names wastes memory.

Using small numeric scores and short member names keeps sorted sets memory-efficient.

5fill in blank

hard

Fill all three blanks to create a memory-efficient Redis set with small members and add a new member.

Redis

SADD myset [1] [2] [3]

Drag options to blanks, or click blank then click option'

A"a"

B"b"

C"c"

D"averylongmembername"

Attempts:

3 left

💡 Hint

Common Mistakes

Adding long member names wastes memory.

Using short member names keeps Redis sets memory-efficient.

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