Limit and pagination in DynamoDB - Mini Project: Build & Apply

Limit and Pagination in DynamoDB Queries

📖 Scenario: You are building a simple product catalog using DynamoDB. The catalog contains many products, but you want to show only a few products at a time to users. This is like flipping pages in a book, where each page shows a limited number of products.

🎯 Goal: Build a DynamoDB query that retrieves products with a limit on how many items to fetch, and supports pagination by using the last evaluated key to get the next set of products.

📋 What You'll Learn

Create a dictionary called query_params with the table name and key condition expression.

Add a Limit key to query_params to restrict the number of items returned.

Use a variable called last_evaluated_key to store the pagination token.

Add ExclusiveStartKey to query_params when last_evaluated_key is not None.

Write the DynamoDB query call using query_params.

💡 Why This Matters

🌍 Real World

Pagination is essential in real-world applications to avoid loading too much data at once and to improve user experience by showing data in manageable chunks.

💼 Career

Understanding DynamoDB pagination is important for backend developers working with AWS to build scalable and efficient data-driven applications.

1

Set up the initial query parameters

Create a dictionary called query_params with 'TableName': 'Products', 'KeyConditionExpression': 'Category = :cat', and 'ExpressionAttributeValues': {':cat': 'Books'}.

DynamoDB

# Create query_params dictionary with TableName, KeyConditionExpression, and ExpressionAttributeValues
# Your code here

Hint

Remember to create query_params as a dictionary with the exact keys and values.

2

Add a limit to the query

Add a key 'Limit' with the value 5 to the existing query_params dictionary.

DynamoDB

query_params = {
    'TableName': 'Products',
    'KeyConditionExpression': 'Category = :cat',
    'ExpressionAttributeValues': {':cat': 'Books'}
}
# Add a Limit of 5 to query_params
# Your code here

Hint

Use query_params['Limit'] = 5 or add it directly in the dictionary.

3

Set up pagination with last evaluated key

Create a variable called last_evaluated_key and set it to None. Then add an if statement that checks if last_evaluated_key is not None. Inside the if, add 'ExclusiveStartKey' to query_params with the value last_evaluated_key.

DynamoDB

query_params = {
    'TableName': 'Products',
    'KeyConditionExpression': 'Category = :cat',
    'ExpressionAttributeValues': {':cat': 'Books'},
    'Limit': 5
}
# Create last_evaluated_key variable and add ExclusiveStartKey if it is not None
# Your code here

Hint

Use last_evaluated_key = None and an if to add ExclusiveStartKey only when needed.

4

Write the DynamoDB query call

Assuming you have a DynamoDB client called dynamodb_client, write a line that calls dynamodb_client.query() with **query_params and assign the result to a variable called response.

DynamoDB

query_params = {
    'TableName': 'Products',
    'KeyConditionExpression': 'Category = :cat',
    'ExpressionAttributeValues': {':cat': 'Books'},
    'Limit': 5
}
last_evaluated_key = None
if last_evaluated_key is not None:
    query_params['ExclusiveStartKey'] = last_evaluated_key

# Call dynamodb_client.query with query_params and save to response
# Your code here

Hint

Use the query method of dynamodb_client with unpacked query_params.

Practice

1. What does the Limit parameter do in a DynamoDB query or scan?

easy

A. It filters items based on a condition.

B. It sets the maximum size of each item returned.

C. It limits the number of items returned in one response.

D. It sorts the items in ascending order.

Solution

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

   The Limit parameter controls how many items DynamoDB returns in a single query or scan response.

2. Step 2: Differentiate Limit from other parameters

   Limit does not filter or sort items; it only restricts the count of items returned at once.

3. Final Answer:

   It limits the number of items returned in one response. -> Option C

4. Quick Check:

   Limit = number of items returned [OK]

Hint: Limit sets max items per response, not filtering or sorting [OK]

Common Mistakes:

• Confusing Limit with filtering conditions
• Thinking Limit sorts the results
• Assuming Limit changes item size

2. Which of the following is the correct way to specify a limit of 5 items in a DynamoDB query using AWS SDK for JavaScript?

easy

A. query({ TableName: 'MyTable', MaxResults: 5 })

B. query({ TableName: 'MyTable', Limit: 5 })

C. query({ TableName: 'MyTable', MaxItems: 5 })

D. query({ TableName: 'MyTable', limit: 5 })

Solution

1. Step 1: Check parameter case sensitivity in AWS SDK

   In AWS SDK for JavaScript, parameter names are case-sensitive and 'Limit' must be capitalized.

2. Step 2: Verify correct parameter name for limiting items

   The correct parameter to limit items is 'Limit', not 'limit', 'MaxItems', or 'MaxResults'.

3. Final Answer:

   query({ TableName: 'MyTable', Limit: 5 }) -> Option B

4. Quick Check:

   Correct case and name for limit is 'Limit' [OK]

Hint: Use exact 'Limit' with capital L in query parameters [OK]

Common Mistakes:

• Using lowercase 'limit' instead of 'Limit'
• Using wrong parameter names like MaxItems
• Confusing limit with max results

3. Given a DynamoDB table with 10 items, what will be the result of this scan operation?

scan({ TableName: 'MyTable', Limit: 3 })

medium

A. Returns all 10 items in one response.

B. Returns exactly 3 items in the response.

C. Returns an error because Limit is too small.

D. Returns 3 items and a LastEvaluatedKey for next page.

Solution

1. Step 1: Understand Limit effect on scan results

   Limit restricts the number of items returned in one response to 3.

2. Step 2: Recognize pagination behavior with LastEvaluatedKey

   Since there are more items (10 total), DynamoDB returns 3 items plus a LastEvaluatedKey to continue scanning next items.

3. Final Answer:

   Returns 3 items and a LastEvaluatedKey for next page. -> Option D

4. Quick Check:

   Limit 3 + more items = partial results + LastEvaluatedKey [OK]

Hint: Limit returns partial items plus LastEvaluatedKey if more exist [OK]

Common Mistakes:

• Assuming all items return ignoring Limit
• Thinking Limit causes error if small
• Missing LastEvaluatedKey for pagination

4. You run this DynamoDB query but get an error:

query({ TableName: 'MyTable', Limit: 5, ExclusiveStartKey: { id: '123' } })

What is the most likely cause?

medium

A. ExclusiveStartKey is missing required key attributes.

B. Limit cannot be used with ExclusiveStartKey.

C. TableName is incorrect.

D. Limit value must be greater than 10.

Solution

1. Step 1: Understand ExclusiveStartKey requirements

   ExclusiveStartKey must include the full primary key attributes exactly as stored in the table.

2. Step 2: Identify error cause

   If the provided ExclusiveStartKey lacks required key attributes or has wrong format, DynamoDB returns an error.

3. Final Answer:

   ExclusiveStartKey is missing required key attributes. -> Option A

4. Quick Check:

   ExclusiveStartKey needs full primary key [OK]

Hint: ExclusiveStartKey must have full primary key attributes [OK]

Common Mistakes:

• Thinking Limit and ExclusiveStartKey cannot be combined
• Assuming TableName typo causes this error
• Believing Limit must be > 10

5. You want to fetch all items from a large DynamoDB table but only 4 items at a time to avoid timeouts. Which approach correctly implements pagination?

hard

A. Use Limit: 4 and pass LastEvaluatedKey from previous response as ExclusiveStartKey in next query.

B. Use Limit: 4 and always start query with ExclusiveStartKey: null.

C. Use Limit: 4 and increase Limit by 4 each time to get next items.

D. Use Limit: 4 and scan the whole table once without ExclusiveStartKey.

Solution

1. Step 1: Understand pagination with Limit and ExclusiveStartKey

   Limit controls how many items are returned per request. To get next items, use ExclusiveStartKey with LastEvaluatedKey from previous response.

2. Step 2: Confirm correct pagination process

   Use Limit: 4 and pass LastEvaluatedKey from previous response as ExclusiveStartKey in next query to fetch items in pages. Restarting from the beginning, increasing Limit, or scanning without pagination leads to repeated or incomplete results.

3. Final Answer:

   Use Limit: 4 and pass LastEvaluatedKey from previous response as ExclusiveStartKey in next query. -> Option A

4. Quick Check:

   Pagination = Limit + ExclusiveStartKey chaining [OK]

Hint: Pass LastEvaluatedKey as ExclusiveStartKey to paginate [OK]

Common Mistakes:

• Restarting query without ExclusiveStartKey
• Increasing Limit instead of paginating
• Scanning whole table ignoring pagination