HLEN for field count in Redis - Step-by-Step Execution

Concept Flow - HLEN for field count

Start with a Redis hash key

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Send HLEN command with the key

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Redis checks the hash stored at the key

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Count the number of fields in the hash

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Return the count as an integer

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End

HLEN command counts how many fields are inside a Redis hash stored at a given key and returns that number.

Execution Sample

Redis

HSET user:1 name "Alice" age 30 city "NY"
HLEN user:1

This sets three fields in a hash 'user:1' and then counts how many fields it has.

Execution Table

Step	Command	Action	Hash Fields	Result
1	HSET user:1 name "Alice" age 30 city "NY"	Create hash user:1 with fields name, age, city	{"name":"Alice", "age":"30", "city":"NY"}	3
2	HLEN user:1	Count fields in hash user:1	{"name":"Alice", "age":"30", "city":"NY"}	3
3	HLEN user:2	Key user:2 does not exist	{}	0

💡 HLEN returns the number of fields in the hash or 0 if the key does not exist.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
user:1 hash fields	none	{"name":"Alice", "age":"30", "city":"NY"}	{"name":"Alice", "age":"30", "city":"NY"}	{"name":"Alice", "age":"30", "city":"NY"}
user:2 hash fields	none	none	none	none

Key Moments - 2 Insights

Why does HLEN return 0 when the key does not exist?

Because Redis treats a non-existing hash as empty, so the field count is zero, as shown in execution_table row 3.

Does HLEN count the values or the fields in the hash?

HLEN counts the number of fields (keys) in the hash, not the values, as shown in execution_table rows 1 and 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result of HLEN user:1 at step 2?

A 0

B 3

C 1

D Error

💡 Hint

Check the 'Result' column in execution_table row 2.

At which step does Redis return 0 because the key does not exist?

A Step 1

B Step 2

C Step 3

D Never

💡 Hint

Look at the 'Action' and 'Result' columns in execution_table row 3.

If we add a new field to user:1, how will the HLEN result change?

A It will increase by 1

B It will stay the same

C It will decrease by 1

D It will return an error

💡 Hint

Refer to variable_tracker showing how fields count changes after adding fields.

Concept Snapshot

HLEN key
Returns the number of fields in the hash stored at key.
If key does not exist, returns 0.
Used to quickly count how many fields a hash has.

Full Transcript

The HLEN command in Redis counts how many fields are inside a hash stored at a given key. When you run HLEN with a key, Redis looks up the hash and returns the number of fields it contains. If the key does not exist, Redis returns 0 because it treats the missing hash as empty. For example, if you create a hash with three fields and then run HLEN on it, you get 3. If you check a key that does not exist, you get 0. This helps you quickly know how many fields are in a hash without retrieving all data.