When Scan is acceptable in DynamoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Scan operation in DynamoDB?

A Scan operation reads every item in a DynamoDB table and returns all data attributes by default. It examines the entire table.

beginner

When is it acceptable to use Scan in DynamoDB?

Scan is acceptable when the table is small, or when you need to retrieve all items without filtering by keys, or during development/testing.

beginner

Why should Scan be avoided on large DynamoDB tables?

Because Scan reads every item, it can be slow and expensive on large tables, consuming a lot of read capacity and increasing latency.

intermediate

How can you reduce the cost of a Scan operation?

You can reduce cost by using FilterExpression to return fewer items, paginating results, or using parallel scans to speed up the process.

beginner

What is a better alternative to Scan when you know the partition key?

Using Query is better because it retrieves items based on the partition key efficiently without scanning the whole table.

When is it acceptable to use a Scan operation in DynamoDB?

A When you know the partition key

B When the table is small or you need all items

C When you want to update a single item

D When you want to delete an item

Scan is acceptable when the table is small or when you need to retrieve all items without filtering by keys.

What is a downside of using Scan on large tables?

A It is very fast

B It updates items automatically

C It consumes a lot of read capacity and is slow

D It only returns one item

Scan reads every item, which can be slow and costly on large tables.

Which DynamoDB operation is more efficient than Scan when you know the partition key?

A PutItem

B DeleteItem

C UpdateItem

D Query

Query retrieves items based on the partition key efficiently without scanning the whole table.

How can you reduce the cost of a Scan operation?

A Use FilterExpression and pagination

B Scan the table multiple times

C Avoid using indexes

D Use Scan only on large tables

Using FilterExpression and pagination reduces the amount of data returned and cost.

Which scenario is NOT a good use case for Scan?

A Large tables with known keys

B Development and testing

C Small tables

D Retrieving all items without filters

Scan should be avoided on large tables when keys are known; Query is better.

Explain when it is acceptable to use a Scan operation in DynamoDB and why.

Think about table size and use cases where filtering by keys is not needed.

Describe the differences between Scan and Query in DynamoDB and when to prefer each.

Focus on efficiency and use cases.

Practice

1. Which situation is best suited for using a Scan operation in DynamoDB?

easy

A. When you want to update an item quickly.

B. When you want to retrieve a single item by its primary key.

C. When you need to read all items from a small table.

D. When you want to delete an item by its key.

Solution

1. Step 1: Understand Scan operation purpose

   Scan reads every item in the table, which is simple but slow for large tables.

2. Step 2: Match Scan use case

   Scan is acceptable when the table is small and you need all data, not a specific item by key.

3. Final Answer:

   When you need to read all items from a small table. -> Option C

4. Quick Check:

   Scan = small table full read [OK]

Hint: Use Scan only for small tables or full data needs [OK]

Common Mistakes:

• Using Scan to get a single item by key
• Using Scan for updates or deletes
• Ignoring performance impact on large tables

2. Which of the following is the correct syntax to perform a Scan operation with a filter expression in DynamoDB?

easy

A. Scan(TableName='Users', ConditionExpression='Age > :age')

B. Scan(TableName='Users', KeyConditionExpression='Age > :age', ExpressionAttributeValues={':age': 30})

C. Scan(TableName='Users', Filter='Age > 30')

D. Scan(TableName='Users', FilterExpression='Age > :age', ExpressionAttributeValues={':age': 30})

Solution

1. Step 1: Identify correct Scan parameters

   Scan uses FilterExpression to filter results after scanning all items.

2. Step 2: Check syntax correctness

   Scan(TableName='Users', FilterExpression='Age > :age', ExpressionAttributeValues={':age': 30}) uses FilterExpression and ExpressionAttributeValues correctly for filtering.

3. Final Answer:

   Scan(TableName='Users', FilterExpression='Age > :age', ExpressionAttributeValues={':age': 30}) -> Option D

4. Quick Check:

   FilterExpression syntax = Scan(TableName='Users', FilterExpression='Age > :age', ExpressionAttributeValues={':age': 30}) [OK]

Hint: FilterExpression filters after Scan, not KeyConditionExpression [OK]

Common Mistakes:

• Using KeyConditionExpression with Scan
• Using wrong parameter names like Filter or ConditionExpression
• Not providing ExpressionAttributeValues for placeholders

3. Given a DynamoDB table 'Products' with 3 items: {"ID":1, "Category":"Book"}, {"ID":2, "Category":"Toy"}, {"ID":3, "Category":"Book"}, what will be the result of this Scan operation?

Scan(TableName='Products', FilterExpression='Category = :cat', ExpressionAttributeValues={':cat': 'Book'})

medium

A. [{"ID":1, "Category":"Book"}, {"ID":3, "Category":"Book"}]

B. [{"ID":2, "Category":"Toy"}]

C. [{"ID":1, "Category":"Book"}, {"ID":2, "Category":"Toy"}, {"ID":3, "Category":"Book"}]

D. []

Solution

1. Step 1: Understand Scan with FilterExpression

   Scan reads all items, then returns only those matching the filter Category = 'Book'.

2. Step 2: Apply filter to items

   Items with Category 'Book' are ID 1 and ID 3, so only these are returned.

3. Final Answer:

   [{"ID":1, "Category":"Book"}, {"ID":3, "Category":"Book"}] -> Option A

4. Quick Check:

   Filter returns matching items only [OK]

Hint: Scan returns all, filter narrows results after [OK]

Common Mistakes:

• Expecting Scan to return only filtered items without FilterExpression
• Confusing Scan with Query operation
• Ignoring that Scan reads all items first

4. You wrote this Scan code but it returns all items without filtering:

Scan(TableName='Orders', FilterExpression='Status = :s')

What is the most likely error?

medium

A. Missing ExpressionAttributeValues for ':s'.

B. Using FilterExpression instead of KeyConditionExpression.

C. Scan does not support FilterExpression.

D. Table name is incorrect.

Solution

1. Step 1: Check FilterExpression usage

   FilterExpression uses placeholders like ':s' which must be defined in ExpressionAttributeValues.

2. Step 2: Identify missing ExpressionAttributeValues

   Code lacks ExpressionAttributeValues, so filter cannot apply and returns all items.

3. Final Answer:

   Missing ExpressionAttributeValues for ':s'. -> Option A

4. Quick Check:

   Filter needs ExpressionAttributeValues [OK]

Hint: Always provide ExpressionAttributeValues for FilterExpression [OK]

Common Mistakes:

• Confusing FilterExpression with KeyConditionExpression
• Forgetting ExpressionAttributeValues
• Assuming Scan ignores missing values silently

5. You have a large DynamoDB table with millions of items but no suitable key for your query. You need to find all items where Status = 'Active'. Which approach is best?

hard

A. Use Scan with FilterExpression 'Status = :status' and paginate results.

B. Create a Global Secondary Index on 'Status' and use Query.

C. Use Query with KeyConditionExpression on 'Status'.

D. Use Scan without any filter to get all items.

Solution

1. Step 1: Understand limitations of Scan on large tables

   Scan reads all items and is slow and costly on large tables, even with filters.

2. Step 2: Use Global Secondary Index (GSI) for efficient queries

   Creating a GSI on 'Status' allows Query operation, which is fast and efficient for filtering by 'Status'.

3. Final Answer:

   Create a Global Secondary Index on 'Status' and use Query. -> Option B

4. Quick Check:

   GSI + Query = efficient large table filter [OK]

Hint: Use GSI for filtering large tables, not Scan [OK]

Common Mistakes:

• Using Scan on large tables causing slow performance
• Trying Query without suitable key or index
• Ignoring cost and speed implications