Pagination with SDK in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Understanding DynamoDB SDK Pagination Output

You run a DynamoDB query using the AWS SDK with a limit of 2 items per page. The table has 5 items matching the query. What will the LastEvaluatedKey value be after the first page is fetched?

DynamoDB

const params = { TableName: 'Products', KeyConditionExpression: 'Category = :cat', ExpressionAttributeValues: { ':cat': 'Books' }, Limit: 2 };
const data = await dynamodb.query(params).promise();
console.log(data.LastEvaluatedKey);

A The primary key of the last item in the first page (second item)

B The primary key of the second item in the result set

C null (no more pages)

D The primary key of the last item in the entire table

💡 Hint

Think about what LastEvaluatedKey represents in pagination.

LastEvaluatedKey is the key of the last item returned in the current page. It is used to fetch the next page starting after this item.

📝 Syntax

intermediate

Correct SDK Parameter for Pagination

Which of the following is the correct way to specify the starting point for the next page in a DynamoDB query using the AWS SDK?

A ExclusiveStartKey: lastEvaluatedKey

B StartKey: lastEvaluatedKey

C LastKey: lastEvaluatedKey

D NextKey: lastEvaluatedKey

💡 Hint

Check the official AWS SDK documentation for the exact parameter name.

The correct parameter to continue a query from a specific point is ExclusiveStartKey.

❓ optimization

advanced

Optimizing Pagination for Large Result Sets

You want to fetch all items matching a query but avoid loading too many items into memory at once. Which approach is best when using DynamoDB SDK pagination?

A Use Scan instead of Query to get all items faster

B Set Limit to a very large number to get all items in one request

C Set a small Limit and use ExclusiveStartKey to fetch pages one by one in a loop

D Fetch all pages in parallel using multiple queries with different ExclusiveStartKeys

💡 Hint

Think about memory usage and API limits.

Using a small Limit and paginating with ExclusiveStartKey avoids loading too many items at once and respects DynamoDB limits.

🔧 Debug

advanced

Why Does Pagination Stop Early?

You use the DynamoDB SDK to paginate through query results with Limit=3. After fetching the first page, LastEvaluatedKey is null, but you know more items exist. What is the likely cause?

A The table has only 3 items matching the query

B Limit parameter is ignored by DynamoDB

C You forgot to set ExclusiveStartKey for the second page

D The query filter excludes remaining items after the first page

💡 Hint

Consider how filters affect the result count.

Filters are applied after fetching items, so if many items are filtered out, fewer results are returned and pagination may stop early.

🧠 Conceptual

expert

Understanding Pagination Consistency in DynamoDB

When paginating query results in DynamoDB, which statement about consistency is true?

A Each page reflects a consistent snapshot of the data at the query start time

B Pages may reflect different data if items are added or deleted during pagination

C Pagination guarantees strongly consistent reads across all pages by default

D Pagination disables eventual consistency for the query

💡 Hint

Think about how DynamoDB handles eventual consistency and pagination.

DynamoDB queries are eventually consistent by default, so data can change between pages if items are added or deleted.

Practice

1. What does the Limit parameter do when paginating results in DynamoDB SDK?

easy

A. It specifies the starting point for the next page.

B. It sets the maximum number of items returned in one page.

C. It filters items based on a condition.

D. It sorts the results in ascending order.

Solution

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

   The Limit parameter controls how many items DynamoDB returns in a single request, effectively setting the page size.

2. Step 2: Differentiate from other parameters

   ExclusiveStartKey sets the starting point for the next page, not Limit. Filtering and sorting are handled differently.

3. Final Answer:

   It sets the maximum number of items returned in one page. -> Option B

4. Quick Check:

   Limit = page size [OK]

Hint: Limit controls page size, not start or filter [OK]

Common Mistakes:

• Confusing Limit with ExclusiveStartKey
• Thinking Limit filters data
• Assuming Limit sorts results

2. Which of the following is the correct way to continue fetching the next page of results in DynamoDB SDK?

easy

A. Set StartKey to zero.

B. Use Limit with the last item key.

C. Set ExclusiveStartKey to the last evaluated key from previous response.

D. Use NextPageToken parameter.

Solution

1. Step 1: Identify the parameter for pagination continuation

   DynamoDB uses ExclusiveStartKey to tell the query where to start for the next page.

2. Step 2: Verify other options

   Limit sets page size, StartKey is not a valid parameter, and NextPageToken is not used in DynamoDB SDK.

3. Final Answer:

   Set ExclusiveStartKey to the last evaluated key from previous response. -> Option C

4. Quick Check:

   Use ExclusiveStartKey for next page [OK]

Hint: Use ExclusiveStartKey to continue pagination [OK]

Common Mistakes:

• Using Limit to continue pages
• Using invalid parameter StartKey
• Assuming NextPageToken exists in DynamoDB

3. Given the following DynamoDB SDK code snippet, what will be the output if LastEvaluatedKey is not empty?

const params = { TableName: 'Books', Limit: 2 };
const data = await dynamoDbClient.scan(params);
console.log(data.Items.length);
console.log(data.LastEvaluatedKey !== undefined);

medium

A. Error due to missing ExclusiveStartKey

B. All items and false

C. 2 and false

D. 2 and true

Solution

1. Step 1: Understand Limit effect on Items returned

   With Limit: 2, the scan returns at most 2 items, so data.Items.length will be 2.

2. Step 2: Check LastEvaluatedKey presence

   If LastEvaluatedKey is not empty, it means more pages exist, so data.LastEvaluatedKey !== undefined is true.

3. Final Answer:

   2 and true -> Option D

4. Quick Check:

   Limit = 2 items, LastEvaluatedKey present = true [OK]

Hint: Limit caps items; LastEvaluatedKey true means more pages [OK]

Common Mistakes:

• Assuming all items returned ignoring Limit
• Thinking LastEvaluatedKey is false when present
• Expecting error without ExclusiveStartKey

4. You wrote this code to paginate DynamoDB scan results but it returns only the first page repeatedly:

let params = { TableName: 'Users', Limit: 3 };
let data;
do {
  data = await dynamoDbClient.scan(params);
  console.log(data.Items);
} while (data.LastEvaluatedKey);

What is the main issue?

medium

A. Missing update of ExclusiveStartKey in params inside the loop.

B. Limit should be increased to fetch all items.

C. The loop condition should check data.Items instead.

D. The scan method does not support pagination.

Solution

1. Step 1: Analyze loop behavior

   The code calls scan repeatedly but never updates ExclusiveStartKey, so it fetches the same first page each time.

2. Step 2: Identify correct fix

   To get next pages, params.ExclusiveStartKey must be set to data.LastEvaluatedKey inside the loop.

3. Final Answer:

   Missing update of ExclusiveStartKey in params inside the loop. -> Option A

4. Quick Check:

   Update ExclusiveStartKey to paginate [OK]

Hint: Update ExclusiveStartKey each loop to get next page [OK]

Common Mistakes:

• Not updating ExclusiveStartKey inside loop
• Increasing Limit instead of paginating
• Wrong loop condition on Items
• Thinking scan can't paginate

5. You want to fetch all items from a large DynamoDB table using pagination with the SDK. Which approach correctly handles this scenario?

hard

A. Use a loop that calls scan with Limit, updating ExclusiveStartKey each time until LastEvaluatedKey is undefined.

B. Call scan once with a very large Limit to get all items at once.

C. Use scan without Limit and stop when Items array is empty.

D. Call scan repeatedly without ExclusiveStartKey until Items length is zero.

Solution

1. Step 1: Understand DynamoDB pagination for large tables

   Large tables require multiple scan calls with a Limit to avoid timeouts and memory issues.

2. Step 2: Correctly implement pagination loop

   Each scan call should use ExclusiveStartKey from the previous LastEvaluatedKey until it is undefined, indicating no more pages.

3. Final Answer:

   Use a loop that calls scan with Limit, updating ExclusiveStartKey each time until LastEvaluatedKey is undefined. -> Option A

4. Quick Check:

   Loop with Limit and ExclusiveStartKey until done [OK]

Hint: Loop with Limit and ExclusiveStartKey until no LastEvaluatedKey [OK]

Common Mistakes:

• Using huge Limit causing timeouts
• Ignoring ExclusiveStartKey for next pages
• Stopping on empty Items instead of LastEvaluatedKey
• Calling scan repeatedly without pagination