Keyset pagination for performance in Rest API - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is keyset pagination?

Keyset pagination is a method to fetch data pages using a unique key from the last item of the previous page instead of using page numbers. It improves performance by avoiding slow database scans.

beginner

Why is keyset pagination faster than offset pagination?

Keyset pagination uses a fixed key to jump directly to the next set of results, avoiding scanning all previous rows like offset pagination does. This reduces database load and speeds up queries.

intermediate

In keyset pagination, what is typically used as the key?

A unique and indexed column, often a primary key or a timestamp, is used as the key to mark the last seen item and fetch the next page efficiently.

intermediate

How does keyset pagination handle sorting?

Keyset pagination requires a consistent sort order on the key column(s) so that the next page starts exactly after the last item of the previous page.

intermediate

What is a limitation of keyset pagination compared to offset pagination?

Keyset pagination cannot jump to arbitrary pages directly because it depends on the last seen key. It only supports moving forward or backward sequentially.

What does keyset pagination use to fetch the next page?

A A random offset

B The page number

C The last item's unique key

D The total number of items

Keyset pagination uses the last item's unique key to fetch the next page efficiently.

Which of the following is a benefit of keyset pagination?

A Ability to jump to any page instantly

B Works without any database indexes

C Simpler to implement than offset pagination

D Faster queries on large datasets

Keyset pagination speeds up queries by avoiding scanning all previous rows.

What kind of column is best used as a key in keyset pagination?

A A non-unique text column

B A unique and indexed column

C A column with many NULL values

D A column with random values

A unique and indexed column ensures efficient and correct pagination.

What is a common sorting requirement for keyset pagination?

A Sort by the key column in a consistent order

B Sort randomly

C No sorting needed

D Sort by page number

Consistent sorting on the key column is needed to correctly fetch the next page.

Which is a limitation of keyset pagination?

A Cannot jump directly to arbitrary pages

B Slower than offset pagination on small datasets

C Requires full table scans

D Does not work with unique keys

Keyset pagination only supports sequential navigation, not direct jumps to any page.

Explain how keyset pagination improves performance compared to offset pagination.

Think about how the database finds the next set of rows.

Describe a scenario where keyset pagination might not be the best choice.

Consider when you want to go directly to page 10 without going through pages 1 to 9.

Practice

1. What is the main advantage of using keyset pagination over traditional offset-based pagination in REST APIs?

easy

A. It requires less server memory by caching all pages in advance.

B. It allows jumping to any page number directly without sequential access.

C. It improves performance by avoiding slow offset queries on large datasets.

D. It automatically sorts data in descending order without extra code.

Solution

1. Step 1: Understand offset-based pagination issues

   Offset pagination uses a page number and offset, which becomes slow on large datasets because the database must skip many rows.

2. Step 2: Recognize keyset pagination benefits

   Keyset pagination uses a fixed key (like an ID) to fetch the next set of rows, avoiding the costly skip operation and improving performance.

3. Final Answer:

   It improves performance by avoiding slow offset queries on large datasets. -> Option C

4. Quick Check:

   Keyset pagination = better performance [OK]

Hint: Keyset uses keys, not offsets, for faster queries [OK]

Common Mistakes:

• Thinking keyset allows random page jumps
• Assuming keyset caches all data
• Believing keyset changes sort order automatically

2. Which of the following is the correct SQL snippet to implement keyset pagination for a table users ordered by id?

easy

A. SELECT * FROM users ORDER BY id LIMIT 10 OFFSET 20;

B. SELECT * FROM users WHERE id > 20 ORDER BY id LIMIT 10;

C. SELECT * FROM users WHERE id = 20 ORDER BY id LIMIT 10;

D. SELECT * FROM users ORDER BY id DESC LIMIT 10 OFFSET 20;

Solution

1. Step 1: Identify keyset pagination syntax

   Keyset pagination uses a WHERE clause with a key (like id > last_seen_id) and a LIMIT to fetch the next page.

2. Step 2: Analyze each option

   SELECT * FROM users WHERE id > 20 ORDER BY id LIMIT 10; uses WHERE id > 20 with ORDER BY id LIMIT 10, which matches keyset pagination logic.

3. Final Answer:

   SELECT * FROM users WHERE id > 20 ORDER BY id LIMIT 10; -> Option B

4. Quick Check:

   Keyset uses WHERE key > last_key [OK]

Hint: Keyset uses WHERE with key comparison, not OFFSET [OK]

Common Mistakes:

• Using OFFSET instead of WHERE for pagination
• Using equality (=) instead of greater than (>)
• Not ordering results by the key column

3. Given this REST API endpoint using keyset pagination:

GET /items?last_id=50&limit=5

And the database table items with IDs: [45, 47, 50, 52, 55, 60, 65], what will be the IDs returned by this request?

medium

A. [52, 55, 60, 65]

B. [45, 47, 50, 52, 55]

C. [52, 55, 60, 65, 70]

D. [51, 52, 53, 54, 55]

Solution

1. Step 1: Understand keyset pagination with last_id=50

   The API returns items with IDs greater than 50, limited to 5 results.

2. Step 2: Select IDs greater than 50 from the list

   IDs greater than 50 are [52, 55, 60, 65]. There are only 4 such items, so all are returned.

3. Final Answer:

   [52, 55, 60, 65] -> Option A

4. Quick Check:

   IDs > 50 limited to 5 = [52, 55, 60, 65] [OK]

Hint: Return items with ID > last_id, up to limit [OK]

Common Mistakes:

• Including items with ID equal to last_id
• Using offset instead of key comparison
• Assuming IDs are continuous numbers

4. You have this SQL query for keyset pagination:

SELECT * FROM orders WHERE order_date > '2024-01-01' ORDER BY order_date LIMIT 10;

But it returns duplicate rows when new orders are added. What is the likely cause?

medium

A. Ordering by a non-unique column causing duplicates

B. LIMIT value is too high causing repeats

C. Missing OFFSET clause to skip duplicates

D. Using > instead of >= in WHERE clause

Solution

1. Step 1: Identify ordering column uniqueness

   Ordering by order_date alone can cause duplicates if multiple rows share the same date.

2. Step 2: Understand keyset pagination requirements

   Keyset pagination requires ordering by a unique column or combination to avoid duplicates and missing rows.

3. Final Answer:

   Ordering by a non-unique column causing duplicates -> Option A

4. Quick Check:

   Order by unique key to avoid duplicates [OK]

Hint: Order by unique keys to prevent duplicates [OK]

Common Mistakes:

• Thinking OFFSET fixes duplicates
• Using >= instead of > causes duplicates
• Assuming LIMIT controls duplicates

5. You want to implement keyset pagination on a REST API that returns products sorted by price ascending, then by id ascending to break ties.
Which SQL WHERE clause correctly fetches the next page after last product with price=100 and id=50?

hard

A. WHERE price > 100 OR id > 50

B. WHERE price >= 100 AND id > 50

C. WHERE price > 100 AND id > 50

D. WHERE price > 100 OR (price = 100 AND id > 50)

Solution

1. Step 1: Understand multi-column keyset pagination

   When ordering by multiple columns, the WHERE clause must handle the first column and then the second to break ties.

2. Step 2: Analyze the correct condition

   The correct condition is price > 100 OR (price = 100 AND id > 50) to get all rows with higher price or same price but higher id.

3. Final Answer:

   WHERE price > 100 OR (price = 100 AND id > 50) -> Option D

4. Quick Check:

   Multi-column keyset uses OR + AND for tie-break [OK]

Hint: Use OR with AND to handle multi-column keys [OK]

Common Mistakes:

• Using AND instead of OR for first column
• Ignoring tie-break column in WHERE clause
• Using >= instead of > causing duplicates