SET with expiry (EX, PX) in Redis - Time & Space Complexity

Time Complexity: SET with expiry (EX, PX)

O(1)

Understanding Time Complexity

We want to understand how the time it takes to run a Redis SET command with an expiry changes as we add more data.

Specifically, we ask: does setting a key with expiration take longer when the database grows?

Scenario Under Consideration

Analyze the time complexity of the following Redis commands.

SET mykey "Hello" EX 10
SET anotherkey "World" PX 5000
    

These commands set keys with expiration times in seconds (EX) and milliseconds (PX).

Identify Repeating Operations

Look for any repeated work inside the command execution.

• Primary operation: Storing the key and value, and scheduling the expiration.
• How many times: Each command runs once per key set; no loops or repeated traversals inside the command.

How Execution Grows With Input

The time to set a key with expiry stays about the same no matter how many keys are in the database.

Input Size (n)	Approx. Operations
10	Constant time to set and schedule expiry
100	Still constant time per command
1000	Still constant time per command

Pattern observation: The work does not grow with the number of keys; it stays steady.

Final Time Complexity

Time Complexity: O(1)

This means setting a key with an expiry takes the same amount of time regardless of how many keys are stored.

Common Mistake

[X] Wrong: "Setting a key with expiry takes longer as the database grows because Redis must check all keys for expiration."

[OK] Correct: Redis handles expirations efficiently using internal data structures, so setting a key with expiry is a quick, direct operation.

Interview Connect

Knowing that setting keys with expiry is fast and does not slow down as data grows helps you design efficient caching and session systems.

Self-Check

"What if we used a command that scans all keys to set expiry instead? How would the time complexity change?"

Practice

1. What does the EX option do when used with the Redis SET command?

easy

A. Sets the key to expire after a specified number of seconds

B. Sets the key to expire after a specified number of milliseconds

C. Sets the key without any expiration

D. Deletes the key immediately after setting it

Solution

1. Step 1: Understand the EX option in Redis SET

   The EX option sets the expiration time in seconds for the key.

2. Step 2: Compare EX with other expiry options

   PX sets expiry in milliseconds, so EX is specifically for seconds.

3. Final Answer:

   Sets the key to expire after a specified number of seconds -> Option A

4. Quick Check:

   EX = seconds expiry [OK]

Hint: EX means seconds expiry, PX means milliseconds expiry [OK]

Common Mistakes:

• Confusing EX with PX and thinking EX is milliseconds
• Assuming EX sets no expiry
• Thinking EX deletes the key immediately

2. Which of the following is the correct syntax to set a key session with value abc123 that expires in 10 seconds using Redis?

easy

A. SET session abc123 EXPIRE 10

B. SET session EX 10 abc123

C. SET session abc123 PX 10

D. SET session abc123 EX 10

Solution

1. Step 1: Recall Redis SET command syntax with expiry

   The correct syntax is: SET key value EX seconds.

2. Step 2: Check each option for correct order and keyword

   SET session abc123 EX 10 matches the correct order and uses EX for seconds expiry.

3. Final Answer:

   SET session abc123 EX 10 -> Option D

4. Quick Check:

   SET key value EX seconds = correct syntax [OK]

Hint: Remember: SET key value EX seconds [OK]

Common Mistakes:

• Placing EX before the value
• Using PX instead of EX for seconds
• Using EXPIRE keyword inside SET command

3. What will be the output of the following Redis commands executed in order?

SET temp 100 PX 5000
TTL temp

medium

A. An integer close to 5

B. 5

C. 0

D. -1

Solution

1. Step 1: Understand PX expiry in SET command

   PX 5000 sets the key to expire in 5000 milliseconds (5 seconds).

2. Step 2: Interpret TTL command output

   TTL returns time to live in seconds. Since expiry was set in milliseconds, TTL rounds to nearest second.

3. Step 3: Predict TTL value

   TTL will return an integer close to 5 seconds, depending on execution time.

4. Final Answer:

   An integer close to 5 -> Option A

5. Quick Check:

   PX 5000 ms = ~5 seconds TTL [OK]

Hint: TTL returns seconds even if PX used for expiry [OK]

Common Mistakes:

• Expecting TTL to return milliseconds
• Thinking TTL returns 0 immediately
• Assuming TTL returns -1 for keys with expiry

4. Identify the error in this Redis command:

SET user123 "John" EXPIRE 20

medium

A. Value should not be in quotes

B. EXPIRE is not valid inside SET command

C. Missing quotes around key

D. EXPIRE should be PX for milliseconds

Solution

1. Step 1: Check Redis SET command syntax

   SET command uses EX or PX for expiry, not EXPIRE keyword.

2. Step 2: Identify misuse of EXPIRE keyword

   EXPIRE is a separate command, not an option inside SET.

3. Final Answer:

   EXPIRE is not valid inside SET command -> Option B

4. Quick Check:

   Use EX or PX inside SET, not EXPIRE [OK]

Hint: Use EX or PX with SET, EXPIRE is separate command [OK]

Common Mistakes:

• Using EXPIRE inside SET command
• Confusing EXPIRE command with expiry option
• Misplacing quotes around key or value

5. You want to store a temporary API token in Redis that expires exactly after 1500 milliseconds. Which command correctly sets the key api_token with value xyz789 and the required expiry?

hard

A. SET api_token xyz789 EX 1.5

B. SET api_token xyz789 EX 1500

C. SET api_token xyz789 PX 1500

D. SET api_token xyz789 PX 1.5

Solution

1. Step 1: Determine correct expiry option for milliseconds

   PX sets expiry in milliseconds, EX uses seconds only.

2. Step 2: Check value format for PX

   PX requires integer milliseconds, so 1500 is valid, decimals like 1.5 are invalid.

3. Step 3: Validate each option

   SET api_token xyz789 PX 1500 uses PX 1500 correctly; others use wrong units or decimals.

4. Final Answer:

   SET api_token xyz789 PX 1500 -> Option C

5. Quick Check:

   Milliseconds expiry = PX integer value [OK]

Hint: Use PX with integer milliseconds for precise expiry [OK]

Common Mistakes:

• Using EX with decimal seconds
• Using PX with decimal instead of integer
• Confusing EX and PX units