HLEN for field count in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of HLEN on a hash with three fields?

Consider a Redis hash stored with key user:1000 having fields name, age, and email. What will the command HLEN user:1000 return?

Redis

HMSET user:1000 name "Alice" age "30" email "[e-mail]"
HLEN user:1000

A 3

B Error: key does not exist

C 0

D 1

💡 Hint

HLEN returns the number of fields in the hash stored at the key.

HLEN returns the count of fields in the hash. Since there are three fields, the output is 3.

❓ query_result

intermediate

What does HLEN return for a non-existing key?

If you run HLEN on a key that does not exist in Redis, what is the output?

Redis

HLEN non_existing_key

A 0

B Error: key does not exist

C nil

D -1

💡 Hint

Redis treats non-existing hashes as empty.

HLEN returns 0 for keys that do not exist because it treats them as empty hashes.

📝 Syntax

advanced

Which HLEN command syntax is correct?

Identify the correct syntax to get the number of fields in a Redis hash named session:123.

A HLEN(session:123)

B HLEN {session:123}

C HLEN session:123

D HLEN 'session:123'

💡 Hint

Redis commands do not use parentheses or braces.

The correct syntax is just the command followed by the key name without quotes or parentheses.

🧠 Conceptual

advanced

What happens if HLEN is run on a key holding a string?

If a Redis key mykey holds a string value, what will happen when you run HLEN mykey?

Redis

SET mykey "hello"
HLEN mykey

A 1

B Error: WRONGTYPE Operation against a key holding the wrong kind of value

C 0

D nil

💡 Hint

HLEN only works on hashes, not strings.

HLEN on a key holding a string returns a WRONGTYPE error because the command expects a hash.

❓ optimization

expert

Efficiently counting fields in a large hash

You have a very large Redis hash with millions of fields. You want to get the number of fields quickly. Which approach is best?

A Store the field count separately and update it on each modification

B Use HGETALL and count the fields in the client

C Use HSCAN to iterate and count fields manually

D Use HLEN command directly on the hash key

💡 Hint

HLEN is optimized for counting fields in a hash.

HLEN is a fast command that returns the number of fields without transferring all data. Other methods are slower or require extra work.