Scan pagination in DynamoDB - Step-by-Step Execution

Concept Flow - Scan pagination

Start Scan

↓

Fetch Page of Items

↓

Check for LastEvaluatedKey

Yes No↓

Save LastEvaluatedKey

↓

Use LastEvaluatedKey to Fetch Next Page

↓

Fetch Page of Items

Scan reads items page by page. If more items exist, it returns a LastEvaluatedKey to fetch the next page.

Execution Sample

DynamoDB

response = table.scan(Limit=2)
items = response['Items']
last_key = response.get('LastEvaluatedKey')
if last_key:
  response = table.scan(Limit=2, ExclusiveStartKey=last_key)

Scan DynamoDB table with a limit of 2 items per page, then fetch next page if more items exist.

Execution Table

Step	Action	Items Fetched	LastEvaluatedKey Present?	Next Step
1	Scan with Limit=2, no start key	Item1, Item2	Yes	Save LastEvaluatedKey, fetch next page
2	Scan with Limit=2, ExclusiveStartKey=last_key	Item3, Item4	Yes	Save LastEvaluatedKey, fetch next page
3	Scan with Limit=2, ExclusiveStartKey=last_key	Item5	No	End scan, no more pages

💡 No LastEvaluatedKey returned at step 3, scan completed all items.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
items	[]	[Item1, Item2]	[Item1, Item2, Item3, Item4]	[Item1, Item2, Item3, Item4, Item5]
last_key	null	key after step 1	key after step 2	null

Key Moments - 2 Insights

Why do we need to check for LastEvaluatedKey after each scan?

Because LastEvaluatedKey tells us if there are more items to fetch. Without it, we don't know if the scan is complete (see execution_table rows 1 and 2).

What happens if we don't use ExclusiveStartKey for the next scan?

The scan will start from the beginning again, causing repeated items and an infinite loop (refer to concept_flow loop back).

Visual Quiz - 3 Questions

Look at the execution_table, what items are fetched at step 2?

A Item3, Item4

B Item1, Item2

C Item5

D No items

💡 Hint

Check the 'Items Fetched' column in execution_table row 2.

At which step does the scan end because no LastEvaluatedKey is returned?

A Step 2

B Step 3

C Step 1

D Scan never ends

💡 Hint

Look at the 'LastEvaluatedKey Present?' column in execution_table.

If the Limit is increased to 5, how would the execution_table change?

A More steps with fewer items per step

B Same number of steps and items

C Fewer steps with more items per step

D Scan would not return LastEvaluatedKey

💡 Hint

Increasing Limit fetches more items per scan, reducing total steps (see concept_flow).

Concept Snapshot

Scan pagination in DynamoDB:
- Scan returns items in pages limited by 'Limit'.
- If more items exist, 'LastEvaluatedKey' is returned.
- Use 'ExclusiveStartKey' with LastEvaluatedKey to fetch next page.
- Repeat until no LastEvaluatedKey is returned.
- Prevents loading all items at once, useful for large tables.

Full Transcript

Scan pagination in DynamoDB works by fetching items in small pages. Each scan request returns a limited number of items and may include a LastEvaluatedKey if more items remain. This key is used as ExclusiveStartKey in the next scan to continue from where the last scan ended. This process repeats until no LastEvaluatedKey is returned, indicating all items have been scanned. This method helps manage large datasets efficiently by loading data in chunks.

Practice

1. What is the main purpose of using Scan pagination in DynamoDB?

easy

A. To speed up writes to the database

B. To delete items in batches

C. To create indexes automatically

D. To break a large scan into smaller parts for easier processing

Solution

1. Step 1: Understand Scan Pagination Concept

   Scan pagination is used to divide a large scan operation into smaller chunks to avoid timeouts and manage resource use.

2. Step 2: Identify the Purpose

   The main goal is to process big scans in parts, not to speed writes or create indexes.

3. Final Answer:

   To break a large scan into smaller parts for easier processing -> Option D

4. Quick Check:

   Scan pagination = break large scan into parts [OK]

Hint: Scan pagination splits big scans into smaller chunks [OK]

Common Mistakes:

• Thinking pagination speeds up writes
• Confusing scan with index creation
• Assuming pagination deletes items

2. Which of the following is the correct way to continue a paginated scan in DynamoDB?

easy

A. Use LastEvaluatedKey as the ExclusiveStartKey in the next scan

B. Use Limit as the ExclusiveStartKey

C. Use ScanIndexForward to continue scanning

D. Use StartKey to begin the scan

Solution

1. Step 1: Identify Pagination Parameters

   In DynamoDB, LastEvaluatedKey from the previous scan is used as ExclusiveStartKey to continue scanning.

2. Step 2: Eliminate Incorrect Options

   Limit sets chunk size, not start key. ScanIndexForward is for queries, not scans. StartKey is not a valid parameter.

3. Final Answer:

   Use LastEvaluatedKey as the ExclusiveStartKey in the next scan -> Option A

4. Quick Check:

   Continue scan = ExclusiveStartKey = LastEvaluatedKey [OK]

Hint: Use LastEvaluatedKey as ExclusiveStartKey to continue scan [OK]

Common Mistakes:

• Using Limit as ExclusiveStartKey
• Confusing scan with query parameters
• Using invalid parameter names

3. Given the following scan code snippet, what will be the output if the table has 15 items and Limit is set to 5?

response = table.scan(Limit=5)
items = response['Items']
last_key = response.get('LastEvaluatedKey')
print(len(items), last_key is not None)

medium

A. 5 False

B. 15 False

C. 5 True

D. 15 True

Solution

1. Step 1: Understand Limit Effect on Scan

   Setting Limit=5 returns up to 5 items in one scan call, not all 15.

2. Step 2: Check LastEvaluatedKey Presence

   Since there are more items after 5, LastEvaluatedKey will be present (not None), indicating more data.

3. Final Answer:

   5 True -> Option C

4. Quick Check:

   Limit=5 returns 5 items and LastEvaluatedKey exists [OK]

Hint: Limit controls items returned; LastEvaluatedKey shows more data [OK]

Common Mistakes:

• Assuming all items return ignoring Limit
• Expecting LastEvaluatedKey to be None always
• Confusing item count with total table size

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

response = table.scan(Limit=3)
items = response['Items']
while 'LastEvaluatedKey' in response:
    response = table.scan(Limit=3)
    items.extend(response['Items'])
print(len(items))

What is the error?

medium

A. Missing ExclusiveStartKey in the subsequent scan calls

B. Limit should be increased to get all items

C. Items list should be reset inside the loop

D. LastEvaluatedKey should be deleted after each scan

Solution

1. Step 1: Analyze Pagination Loop

   The loop calls scan repeatedly but does not pass ExclusiveStartKey, so it always scans from start.

2. Step 2: Identify Missing Parameter

   To continue scanning, ExclusiveStartKey must be set to previous LastEvaluatedKey in each call.

3. Final Answer:

   Missing ExclusiveStartKey in the subsequent scan calls -> Option A

4. Quick Check:

   Pagination needs ExclusiveStartKey to continue [OK]

Hint: Pass ExclusiveStartKey to continue scan pages [OK]

Common Mistakes:

• Not passing ExclusiveStartKey in loop
• Increasing Limit instead of fixing pagination
• Resetting items list inside loop

5. You want to scan a DynamoDB table with 1000 items but only want to process 100 items at a time to avoid timeouts. Which approach correctly implements scan pagination to achieve this?

hard

A. Set ExclusiveStartKey to null and scan with Limit=100 once

B. Use a loop with Limit=100 and pass ExclusiveStartKey from LastEvaluatedKey until no more keys

C. Use ScanIndexForward=true with Limit=100 to paginate

D. Scan once with Limit=1000 and split results in code

Solution

1. Step 1: Understand Pagination Requirements

   To process 100 items at a time, scan must be done in chunks with Limit=100 and continue using ExclusiveStartKey.

2. Step 2: Evaluate Options

   Use a loop with Limit=100 and pass ExclusiveStartKey from LastEvaluatedKey until no more keys correctly loops with Limit=100 and uses ExclusiveStartKey from LastEvaluatedKey. Scan once with Limit=1000 and split results in code fetches all at once, risking timeout. Use ScanIndexForward=true with Limit=100 to paginate uses wrong parameter for scan. Set ExclusiveStartKey to null and scan with Limit=100 once scans only once without continuation.

3. Final Answer:

   Use a loop with Limit=100 and pass ExclusiveStartKey from LastEvaluatedKey until no more keys -> Option B

4. Quick Check:

   Paginate with Limit and ExclusiveStartKey loop [OK]

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

Common Mistakes:

• Fetching all items at once risking timeout
• Using query parameters for scan
• Not looping to continue scan