Basic scan operation in DynamoDB - Step-by-Step Execution

Concept Flow - Basic scan operation

Start Scan Request

↓

Read First Page of Items

↓

Return Items to User

↓

Check for More Pages?

No→End Scan

Yes↓

Read Next Page of Items

↩Back to Return Items

The scan operation reads all items in a table page by page, returning them until no more pages remain.

Execution Sample

DynamoDB

ScanRequest = {'TableName': 'Books'}
response = dynamodb.scan(**ScanRequest)
items = response['Items']
while 'LastEvaluatedKey' in response:
  response = dynamodb.scan(TableName='Books', ExclusiveStartKey=response['LastEvaluatedKey'])
  items.extend(response['Items'])

This code scans the entire 'Books' table, fetching all items page by page.

Execution Table

Step	Action	Page Number	Items Returned	LastEvaluatedKey Present	Next Step
1	Send scan request to 'Books' table	1	5 items	Yes	Fetch next page
2	Send scan request with ExclusiveStartKey	2	5 items	Yes	Fetch next page
3	Send scan request with ExclusiveStartKey	3	3 items	No	End scan
4	No more pages	-	0 items	No	Stop

💡 No LastEvaluatedKey after page 3 means all items have been scanned.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
response.Items	empty	5 items	5 items	3 items	13 items total
response.LastEvaluatedKey	none	present	present	none	none
items	empty list	5 items	10 items	13 items	13 items

Key Moments - 2 Insights

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

Because DynamoDB returns data in pages, LastEvaluatedKey tells us if more pages exist. Without checking it (see execution_table rows 1-3), we might miss items.

What happens if we ignore LastEvaluatedKey and only scan once?

We only get the first page of items (row 1), missing the rest of the table data.

Visual Quiz - 3 Questions

Look at the execution table, how many items are returned after step 2?

A 10 items

B 3 items

C 5 items

D 13 items

💡 Hint

Check the 'Items Returned' column for step 2 and the 'items' variable in variable_tracker after step 2.

At which step does the scan operation know it has reached the last page?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'LastEvaluatedKey Present' column in execution_table; it is 'No' at step 3.

If the table had 20 items and each page returned 5 items, how many steps would the scan take?

A 3 steps

B 4 steps

C 5 steps

D 6 steps

💡 Hint

Each step returns 5 items; 20 items total means 4 full pages plus no more pages after the 4th.

Concept Snapshot

Basic Scan Operation in DynamoDB:
- Reads all items in a table, page by page.
- Each scan returns a page of items and a LastEvaluatedKey if more pages exist.
- Use LastEvaluatedKey to fetch next page.
- Repeat until no LastEvaluatedKey.
- Returns all items but can be costly for large tables.

Full Transcript

The Basic Scan Operation in DynamoDB reads all items from a table by fetching pages of data. Each scan request returns some items and a LastEvaluatedKey if there are more pages. The client checks this key to continue scanning until all items are retrieved. This process is shown step-by-step in the execution table, where each step fetches a page of items. Variables like response.Items and response.LastEvaluatedKey change after each scan. Beginners often wonder why checking LastEvaluatedKey is necessary; it ensures no items are missed. The visual quiz tests understanding of how many items are returned at each step and when the scan ends.