Pagination with first and after in GraphQL - Time & Space Complexity

Time Complexity: Pagination with first and after

O(n)

Understanding Time Complexity

When using pagination with first and after in GraphQL, it's important to understand how the time to get results changes as the data grows.

We want to know how the number of operations grows when we ask for pages of data.

Scenario Under Consideration

Analyze the time complexity of the following GraphQL query snippet.

query GetItems($first: Int, $after: String) {
  items(first: $first, after: $after) {
    edges {
      node {
        id
        name
      }
    }
    pageInfo {
      endCursor
      hasNextPage
    }
  }
}
    

This query fetches a page of items, starting after a cursor, limited by the number first.

Identify Repeating Operations

Look for repeated work inside the query execution.

• Primary operation: Scanning items to find the start position after the cursor.
• How many times: Depends on the position of after in the list and the number first requested.

How Execution Grows With Input

As the total number of items grows, finding the start point after the cursor may require scanning more items.

Input Size (n)	Approx. Operations
10	Scan up to 10 items to find start, then return first items.
100	Scan up to 100 items to find start, then return first items.
1000	Scan up to 1000 items to find start, then return first items.

Pattern observation: The work grows roughly in proportion to how far into the list the after cursor is, plus the number of items requested.

Final Time Complexity

Time Complexity: O(n)

This means the time to get a page grows linearly with how far you have to scan to reach the cursor position.

Common Mistake

[X] Wrong: "Pagination with first and after always takes the same time no matter how big the data is."

[OK] Correct: The server often needs to scan items up to the cursor position, so if the cursor is far in the list, it takes longer.

Interview Connect

Understanding how pagination scales helps you design efficient queries and explain performance trade-offs clearly in real projects.

Self-Check

What if the data was indexed by cursor so the server could jump directly to the after position? How would the time complexity change?

Practice

1.

What does the first argument do in GraphQL pagination?

easy

A. Limits the number of items returned starting from the beginning or after a cursor

B. Specifies the cursor to start fetching items from

C. Returns all items without limit

D. Sorts the items in ascending order

Solution

1. Step 1: Understand the role of first in pagination

   The first argument tells GraphQL how many items to return in the current page.

2. Step 2: Differentiate from after

   after sets the starting point, but first limits the count of items fetched.

3. Final Answer:

   Limits the number of items returned starting from the beginning or after a cursor -> Option A

4. Quick Check:

   first = limit count [OK]

Hint: Remember: first = how many items to get [OK]

Common Mistakes:

• Confusing first with after
• Thinking first sets the start position
• Assuming first returns all items

2.

Which of the following is the correct syntax to fetch the first 5 items after a cursor "abc123" in GraphQL?

{ posts(first: 5, after: "abc123") { edges { node { id } } } }

easy

A. { posts(first: "5", after: abc123) { edges { node { id } } } }

B. { posts(after: 5, first: "abc123") { edges { node { id } } } }

C. { posts(first: 5, after: "abc123") { edges { node { id } } } }

D. { posts(first: 5 after: "abc123") { edges { node { id } } } }

Solution

1. Step 1: Check argument order and types

   The correct syntax uses first: 5 (number) and after: "abc123" (string) with a comma separating them.

2. Step 2: Identify syntax errors in other options

   { posts(after: 5, first: "abc123") { edges { node { id } } } } swaps argument names incorrectly. { posts(first: "5", after: abc123) { edges { node { id } } } } uses quotes incorrectly and misses quotes around cursor. { posts(first: 5 after: "abc123") { edges { node { id } } } } misses comma between arguments.

3. Final Answer:

   { posts(first: 5, after: "abc123") { edges { node { id } } } } -> Option C

4. Quick Check:

   Correct argument names and types with commas [OK]

Hint: Use commas between args and quotes for cursor strings [OK]

Common Mistakes:

• Missing commas between arguments
• Using wrong argument order
• Not quoting cursor strings

3.

Given this GraphQL query:

{ users(first: 3) { edges { node { name } cursor } pageInfo { hasNextPage endCursor } } }

And the data has 5 users: Alice, Bob, Carol, Dave, Eve. What will be the edges returned?

medium

A. [{name: "Dave"}, {name: "Eve"}]

B. [{name: "Alice"}, {name: "Bob"}, {name: "Carol"}]

C. [{name: "Alice"}, {name: "Bob"}, {name: "Carol"}, {name: "Dave"}, {name: "Eve"}]

D. [] (empty list)

Solution

1. Step 1: Understand first: 3 effect

   The query requests the first 3 users from the start of the list.

2. Step 2: Identify first 3 users in data

   The users are Alice, Bob, Carol, Dave, Eve. The first 3 are Alice, Bob, and Carol.

3. Final Answer:

   [{name: "Alice"}, {name: "Bob"}, {name: "Carol"}] -> Option B

4. Quick Check:

   first 3 users = Alice, Bob, Carol [OK]

Hint: first = number of items from start [OK]

Common Mistakes:

• Returning all users instead of limited
• Confusing first with after
• Returning empty list by mistake

4.

Identify the error in this GraphQL pagination query:

{ comments(first: 10 after: "cursor123") { edges { node { text } } } }

medium

A. Missing comma between first and after arguments

B. Cursor value should not be in quotes

C. The first argument must be a string

D. The edges field is invalid here

Solution

1. Step 1: Check argument syntax

   GraphQL requires commas between arguments. The query misses a comma between first: 10 and after: "cursor123".

2. Step 2: Validate other parts

   Cursor should be a quoted string (correct). first is a number (correct). edges is valid in pagination.

3. Final Answer:

   Missing comma between first and after arguments -> Option A

4. Quick Check:

   Arguments need commas [OK]

Hint: Always separate arguments with commas [OK]

Common Mistakes:

• Omitting commas between arguments
• Not quoting cursor strings
• Misunderstanding argument types

5.

You have a list of 12 products. You want to fetch products in pages of 4 using first and after. After fetching the first 4 products, you get an endCursor value of "cursor4". What query should you use to fetch the next 4 products?

hard

A. { products(first: 4, after: cursor4) { edges { node { id name } } } }

B. { products(after: 4, first: "cursor4") { edges { node { id name } } } }

C. { products(first: 8) { edges { node { id name } } } }

D. { products(first: 4, after: "cursor4") { edges { node { id name } } pageInfo { hasNextPage endCursor } } }

Solution

1. Step 1: Use first to limit items per page

   You want 4 items per page, so first: 4 is correct.

2. Step 2: Use after with the last page's endCursor

   To fetch the next page, use after: "cursor4" with quotes as it is a string cursor.

3. Step 3: Check syntax correctness

   { products(first: 4, after: "cursor4") { edges { node { id name } } pageInfo { hasNextPage endCursor } } } uses correct argument order, types, and includes pageInfo for navigation.

4. Final Answer:

   { products(first: 4, after: "cursor4") { edges { node { id name } } pageInfo { hasNextPage endCursor } } } -> Option D

5. Quick Check:

   Next page = first 4 after last cursor [OK]

Hint: Use last page's endCursor as after with first count [OK]

Common Mistakes:

• Swapping first and after argument values
• Not quoting cursor strings
• Requesting more items than page size