Pagination patterns (cursor, offset) in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Offset Pagination Limitations

Which of the following is a common drawback of using offset-based pagination in large datasets?

A It cannot be used with relational databases.

B It requires storing a cursor token on the client side.

C It can cause inconsistent results if data changes between requests.

D It always returns results faster than cursor pagination.

💡 Hint

Think about what happens if new items are added or removed while paginating.

Offset pagination uses page numbers and offsets, so if data changes between requests, the same item might appear on different pages or be skipped.

❓ Architecture

intermediate

Choosing Pagination Strategy for a Social Feed

You are designing a social media feed that updates frequently. Which pagination pattern is best to ensure users see a consistent and smooth scrolling experience?

A Cursor pagination using a unique timestamp or ID.

B Offset pagination using page numbers.

C Load all data at once without pagination.

D Random access pagination by jumping to random pages.

💡 Hint

Consider how to handle new posts appearing while the user scrolls.

Cursor pagination uses a stable cursor like a timestamp or unique ID, which helps maintain consistency even if new posts are added.

❓ scaling

advanced

Scaling Pagination for Millions of Records

When scaling a system with millions of records, which pagination method is more efficient to reduce database load and why?

A Offset pagination because it allows jumping to any page quickly.

B Cursor pagination because it loads all data into the client at once.

C Offset pagination because it caches all pages in memory.

D Cursor pagination because it uses indexed columns to fetch next items efficiently.

💡 Hint

Think about how the database uses indexes to find rows.

Cursor pagination uses indexed columns (like IDs or timestamps) to fetch the next set of rows without scanning large offsets, reducing load.

❓ tradeoff

advanced

Tradeoffs Between Cursor and Offset Pagination

Which statement best describes a tradeoff when choosing between cursor and offset pagination?

A Offset pagination is simpler but can cause inconsistent data views; cursor pagination is consistent but harder to implement.

B Cursor pagination is simpler and faster but cannot handle large datasets; offset pagination handles large datasets well.

C Offset pagination requires client to store cursor tokens; cursor pagination uses page numbers only.

D Cursor pagination always returns results in random order; offset pagination returns sorted results.

💡 Hint

Consider implementation complexity and user experience consistency.

Offset pagination is easy to implement but can show inconsistent results if data changes. Cursor pagination is more complex but provides stable views.

❓ estimation

expert

Estimating Maximum Page Number with Offset Pagination

A database table has 10 million records. If each page shows 100 records, what is the maximum page number a user can request using offset pagination?

A 10,000 pages

B 100,000 pages

C 1,000,000 pages

D 100 pages

💡 Hint

Divide total records by records per page.

10,000,000 records / 100 records per page = 100,000 pages maximum.

Practice

1. What is the main advantage of cursor pagination over offset pagination in system design?

easy

A. Cursor pagination is faster and more reliable for large datasets.

B. Cursor pagination uses page numbers to navigate pages.

C. Cursor pagination requires less memory than offset pagination.

D. Cursor pagination always returns the total number of pages.

Solution

1. Step 1: Understand offset pagination limitations

   Offset pagination uses page numbers and offsets, which can be slow and inconsistent on large or changing datasets.

2. Step 2: Analyze cursor pagination benefits

   Cursor pagination uses a unique cursor (like an ID) to fetch the next set of results, making it faster and more reliable for large data.

3. Final Answer:

   Cursor pagination is faster and more reliable for large datasets. -> Option A

4. Quick Check:

   Cursor pagination = faster, reliable [OK]

Hint: Cursor uses unique IDs, offset uses page numbers [OK]

Common Mistakes:

• Confusing cursor with page number
• Thinking cursor pagination returns total pages
• Assuming offset is always faster

2. Which of the following is the correct way to represent an offset pagination request in an API call?

easy

A. /items?start=abc123&count=10

B. /items?cursor=abc123&limit=10

C. /items?page=2&limit=10

D. /items?offset=abc123&limit=10

Solution

1. Step 1: Identify offset pagination syntax

   Offset pagination typically uses page numbers and limit, e.g., page=2 and limit=10.

2. Step 2: Check options for correct syntax

   /items?page=2&limit=10 uses page=2 and limit=10, which matches offset pagination style.

3. Final Answer:

   /items?page=2&limit=10 -> Option C

4. Quick Check:

   Offset pagination = page + limit [OK]

Hint: Offset uses page and limit parameters [OK]

Common Mistakes:

• Using cursor parameter for offset pagination
• Confusing offset with cursor values
• Using invalid parameter names like start or offset with strings

3. Given the following cursor pagination code snippet, what will be the next cursor value returned?

items = [
  {"id": 101, "name": "A"},
  {"id": 102, "name": "B"},
  {"id": 103, "name": "C"}
]

last_cursor = 101
page_size = 2

next_page = [item for item in items if item["id"] > last_cursor][:page_size]
next_cursor = next_page[-1]["id"] if next_page else None
print(next_cursor)

medium

A. 102

B. 101

C. None

D. 103

Solution

1. Step 1: Filter items with id greater than last_cursor

   Items with id > 101 are 102 and 103.

2. Step 2: Take first 2 items as page size is 2

   Next page items are [{"id": 102, "name": "B"}, {"id": 103, "name": "C"}].

3. Step 3: Get last item's id as next cursor

   Last item id is 103, so next_cursor = 103.

4. Final Answer:

   103 -> Option D

5. Quick Check:

   Next cursor = last id in next page = 103 [OK]

Hint: Next cursor = last item's unique id in current page [OK]

Common Mistakes:

• Choosing last_cursor as next cursor
• Selecting first item id instead of last
• Ignoring page size limit

4. A developer implemented offset pagination but users report inconsistent results when data changes rapidly. What is the likely cause and fix?

medium

A. Cause: Using offset with changing data causes duplicates or skips; Fix: Switch to cursor pagination.

B. Cause: Using cursor pagination causes slow queries; Fix: Use offset pagination instead.

C. Cause: Not caching pages; Fix: Add caching layer for offset pages.

D. Cause: Using limit too high; Fix: Reduce limit to 1.

Solution

1. Step 1: Understand offset pagination issues with changing data

   Offset pagination can skip or repeat items if data changes between requests because offset is position-based.

2. Step 2: Identify solution with cursor pagination

   Cursor pagination uses unique IDs, avoiding duplicates or skips even if data changes.

3. Final Answer:

   Cause: Using offset with changing data causes duplicates or skips; Fix: Switch to cursor pagination. -> Option A

4. Quick Check:

   Offset pagination = inconsistent with changing data [OK]

Hint: Offset breaks with data changes; cursor stays consistent [OK]

Common Mistakes:

• Thinking caching fixes offset pagination issues
• Confusing cursor and offset pagination roles
• Reducing limit does not solve data inconsistency

5. You need to design a scalable API for a social media feed with millions of posts. Which pagination pattern should you choose and why?

hard

A. Offset pagination, because it is simple and supports jumping to any page.

B. Cursor pagination, because it handles large, changing datasets efficiently and avoids duplicates.

C. Offset pagination, because it requires less backend logic and is faster for large datasets.

D. Cursor pagination, because it returns total page count quickly.

Solution

1. Step 1: Analyze requirements for large, changing datasets

   Social media feeds have millions of posts and frequent updates, so pagination must be efficient and consistent.

2. Step 2: Compare offset and cursor pagination suitability

   Offset pagination is simple but slow and inconsistent with large, changing data. Cursor pagination efficiently handles these issues by using unique cursors.

3. Final Answer:

   Cursor pagination, because it handles large, changing datasets efficiently and avoids duplicates. -> Option B

4. Quick Check:

   Large dynamic data = cursor pagination best [OK]

Hint: For big changing data, use cursor pagination [OK]

Common Mistakes:

• Choosing offset for large dynamic datasets
• Assuming offset is faster for big data
• Thinking cursor returns total pages