SUNIONSTORE for storing results in Redis - Time & Space Complexity

Time Complexity: SUNIONSTORE for storing results

O(n)

Understanding Time Complexity

We want to understand how the time needed to combine sets grows when using SUNIONSTORE in Redis.

Specifically, how does the work change as the sets get bigger?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

# Combine two sets and store the result
SUNIONSTORE resultSet set1 set2

# Retrieve the combined set
SMEMBERS resultSet
    

This code takes two sets, merges their unique members into a new set, and stores it under a new key.

Identify Repeating Operations

Look for repeated actions that take time.

• Primary operation: Scanning all members of each input set to combine them.
• How many times: Once for each member in each input set.

How Execution Grows With Input

As the sets get bigger, the work grows roughly with the total number of unique members.

Input Size (n)	Approx. Operations
10	About 10 to 20 operations
100	About 100 to 200 operations
1000	About 1000 to 2000 operations

Pattern observation: The work grows roughly in direct proportion to the total number of elements in the input sets combined.

Final Time Complexity

Time Complexity: O(n)

This means the time to combine sets grows linearly with the total number of elements in all input sets.

Common Mistake

[X] Wrong: "SUNIONSTORE runs instantly no matter how big the sets are."

[OK] Correct: The command must look at every element in the input sets to combine them, so bigger sets take more time.

Interview Connect

Understanding how set operations scale helps you reason about performance in real applications using Redis sets.

Self-Check

"What if we used SUNIONSTORE with three or more sets instead of two? How would the time complexity change?"

Practice

1. What does the Redis command SUNIONSTORE destination key1 key2 do?

easy

A. It merges the sets stored at key1 and key2 and stores the result in destination.

B. It deletes the sets at key1 and key2 and creates a new set at destination.

C. It returns the union of sets at key1 and key2 without storing it.

D. It copies the set at key1 to destination without merging.

Solution

1. Step 1: Understand SUNIONSTORE purpose

   The command merges multiple sets into one combined set.

2. Step 2: Identify storage behavior

   The result is stored in the specified destination key, overwriting if it exists.

3. Final Answer:

   It merges the sets stored at key1 and key2 and stores the result in destination. -> Option A

4. Quick Check:

   SUNIONSTORE merges and stores = C [OK]

Hint: SUNIONSTORE merges sets and saves result in destination [OK]

Common Mistakes:

• Confusing SUNIONSTORE with SUNION (which does not store)
• Thinking it deletes original sets
• Assuming it copies only one set

2. Which of the following is the correct syntax to store the union of sets setA and setB into resultSet using Redis?

easy

A. SUNIONSTORE setA setB resultSet

B. SUNIONSTORE resultSet setA setB

C. SUNION resultSet setA setB

D. SUNIONSTORE setA resultSet setB

Solution

1. Step 1: Recall SUNIONSTORE syntax

   The correct syntax is SUNIONSTORE destination key1 key2 ....

2. Step 2: Match keys to syntax

   Here, resultSet is destination, followed by sets setA and setB.

3. Final Answer:

   SUNIONSTORE resultSet setA setB -> Option B

4. Quick Check:

   Destination first, then sets = A [OK]

Hint: Destination key comes first in SUNIONSTORE command [OK]

Common Mistakes:

• Placing destination key last
• Using SUNION instead of SUNIONSTORE for storing
• Mixing order of keys

3. You have three sets:
setX = {1, 2, 3}
setY = {3, 4, 5}
setZ = {5, 6}
You want to create a new set combined that contains the union of all three sets. Which command correctly does this and stores the result in combined?

easy

A. SUNIONSTORE combined setX setY setZ

B. SUNIONSTORE combined setX setY

C. SUNION combined setX setY setZ

D. SUNIONSTORE setX setY setZ combined

Solution

1. Step 1: Recall SUNIONSTORE usage for multiple sets

   SUNIONSTORE can take multiple keys after the destination key.

2. Step 2: Verify correct order and keys

   Destination is combined, followed by setX, setY, and setZ.

3. Final Answer:

   SUNIONSTORE combined setX setY setZ -> Option A

4. Quick Check:

   Destination first, then all sets = B [OK]

Hint: List all sets after destination in SUNIONSTORE [OK]

Common Mistakes:

• Using SUNION instead of SUNIONSTORE to store
• Placing destination key last
• Omitting one or more sets

4. Given the following Redis sets:
SADD set1 a b c
SADD set2 b c d
What will be the members of resultSet after executing SUNIONSTORE resultSet set1 set2?

medium

A. ["a", "d"]

B. ["a", "b", "c"]

C. ["a", "b", "c", "d"]

D. ["b", "c", "d"]

Solution

1. Step 1: Identify members of each set

   set1 has {a, b, c}, set2 has {b, c, d}.

2. Step 2: Compute union of both sets

   The union is {a, b, c, d} combining all unique members.

3. Final Answer:

   ["a", "b", "c", "d"] -> Option C

4. Quick Check:

   Union of set1 and set2 = ["a", "b", "c", "d"] [OK]

Hint: Union combines all unique elements from both sets [OK]

Common Mistakes:

• Forgetting to include all unique elements
• Confusing union with intersection
• Assuming order matters in sets

5. You run the command SUNIONSTORE result set1 set2 but get an error. What is the most likely cause?

medium

A. SUNIONSTORE does not accept more than two sets.

B. You forgot to specify the destination key.

C. The sets set1 and set2 do not exist.

D. The key result already exists as a string, not a set.

Solution

1. Step 1: Understand data type requirements

   SUNIONSTORE requires the destination key to be a set or not exist.

2. Step 2: Identify error cause

   If result exists as a string or other type, Redis throws a type error.

3. Final Answer:

   The key result already exists as a string, not a set. -> Option D

4. Quick Check:

   Destination key type mismatch causes error [OK]

Hint: Destination key must be a set or not exist [OK]

Common Mistakes:

• Assuming missing keys cause errors (they don't)
• Thinking SUNIONSTORE limits number of sets
• Ignoring key data types