Scan pagination in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this DynamoDB scan pagination code?

Consider a DynamoDB table with 25 items. You run a scan with a limit of 10 items per page. The code below fetches the first page and then the second page using the LastEvaluatedKey.

response1 = table.scan(Limit=10)
items_page1 = response1['Items']
last_key = response1.get('LastEvaluatedKey')
response2 = table.scan(Limit=10, ExclusiveStartKey=last_key)
items_page2 = response2['Items']
total_items = len(items_page1) + len(items_page2)

What is the value of total_items after running this code?

DynamoDB

response1 = table.scan(Limit=10)
items_page1 = response1['Items']
last_key = response1.get('LastEvaluatedKey')
response2 = table.scan(Limit=10, ExclusiveStartKey=last_key)
items_page2 = response2['Items']
total_items = len(items_page1) + len(items_page2)

A 20

B 25

C 10

D 15

💡 Hint

Each scan call returns up to the limit number of items. The second scan starts after the last key from the first scan.

The first scan returns 10 items. The second scan also returns 10 items starting after the last key of the first scan. So total items fetched are 20.

🧠 Conceptual

intermediate

Why is LastEvaluatedKey important in DynamoDB scan pagination?

When paginating scan results in DynamoDB, what role does LastEvaluatedKey play?

A It sorts the scan results in ascending order

B It marks the last item returned so the next scan can continue from there

C It limits the number of items returned in a scan

D It filters items based on a condition

💡 Hint

Think about how to continue scanning from where you left off.

LastEvaluatedKey tells DynamoDB where to start the next scan to avoid returning duplicate items.

📝 Syntax

advanced

Which option correctly uses scan pagination to fetch all items?

You want to scan all items in a DynamoDB table using pagination. Which code snippet correctly implements this?

A

items = []
last_key = None
while True:
    response = table.scan(Limit=5, ExclusiveStartKey=last_key) if last_key else table.scan(Limit=5)
    items.extend(response['Items'])
    last_key = response.get('LastEvaluatedKey')
    if not last_key:
        break

B

items = []
last_key = None
while last_key:
    response = table.scan(Limit=5, ExclusiveStartKey=last_key)
    items.extend(response['Items'])
    last_key = response.get('LastEvaluatedKey')

C

items = []
last_key = None
while True:
    response = table.scan(Limit=5)
    items.extend(response['Items'])
    last_key = response.get('LastEvaluatedKey')
    if last_key is None:
        break

D

items = []
last_key = None
while True:
    response = table.scan(Limit=5, ExclusiveStartKey=last_key)
    items.extend(response['Items'])
    if 'LastEvaluatedKey' not in response:
        break

💡 Hint

Remember to handle the first scan call without ExclusiveStartKey.

Option A correctly handles the first scan without ExclusiveStartKey and uses it for subsequent scans until no LastEvaluatedKey is returned.

❓ optimization

advanced

How to optimize scan pagination to reduce read capacity usage?

You want to paginate scan results but reduce the read capacity units consumed. Which approach helps achieve this?

A Use a smaller Limit value and filter results client-side

B Use ExclusiveStartKey to skip items

C Use ProjectionExpression to return only needed attributes

D Increase the Limit value to fetch more items per scan

💡 Hint

Think about reducing the amount of data read per item.

Using ProjectionExpression reduces the data returned and read capacity used by only fetching necessary attributes.

🔧 Debug

expert

Why does this scan pagination code cause an infinite loop?

Review this code snippet that paginates a DynamoDB scan. It causes an infinite loop. What is the cause?

items = []
last_key = None
while True:
    response = table.scan(Limit=10, ExclusiveStartKey=last_key)
    items.extend(response['Items'])
    if 'LastEvaluatedKey' not in response:
        break
    # Missing update of last_key here

A Items are not extended properly causing an error

B Limit is too high causing the scan to never finish

C The break condition is incorrect and never triggers

D last_key is never updated, so ExclusiveStartKey is always None causing repeated scans

💡 Hint

Check if the pagination key is updated inside the loop.

The code never updates last_key with LastEvaluatedKey, so the scan always starts at the same place, causing an infinite loop.