Scan with filter expressions in DynamoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does a Scan operation do in DynamoDB?

A Scan reads every item in a table and returns all data attributes by default. It examines all items, which can be slow for large tables.

beginner

What is a Filter Expression in a DynamoDB Scan?

A Filter Expression lets you specify conditions to filter the results returned by a Scan. It does not reduce the read workload but filters data after reading.

beginner

How does a Filter Expression affect the data returned by a Scan?

It filters out items that don't match the condition after scanning all items. Only matching items are returned to you.

intermediate

Why might using a Filter Expression still be costly in DynamoDB?

Because Scan reads all items before filtering, it consumes read capacity for every item scanned, even if many are filtered out.

beginner

Give an example of a simple Filter Expression in a DynamoDB Scan.

Example: To find items where attribute 'status' equals 'active', use FilterExpression: "#s = :val" with ExpressionAttributeNames: {"#s": "status"} and ExpressionAttributeValues: {":val": "active"}.

What does a Filter Expression do in a DynamoDB Scan?

A Indexes items for faster search

B Filters items after scanning all items

C Deletes items that don't match

D Prevents scanning items that don't match

Filter Expressions filter the results after scanning all items; they do not prevent scanning.

Which statement is true about Scan with Filter Expression?

A Filter Expression reduces read capacity used

B Scan only reads items matching the filter

C Scan reads all items, then filters results

D Filter Expression changes table data

Scan reads all items in the table, then applies the Filter Expression to filter results.

Why might Scan with Filter Expression be inefficient?

A Because it reads all items before filtering

B Because it writes data multiple times

C Because it uses indexes incorrectly

D Because it deletes filtered items

Scan reads every item, which can be slow and costly, even if many are filtered out.

What is needed to use a Filter Expression in DynamoDB Scan?

A No extra parameters

B Only FilterExpression

C Only ExpressionAttributeNames

D FilterExpression and ExpressionAttributeValues

You need both FilterExpression and ExpressionAttributeValues to specify conditions and values.

If you want to find items where 'age' > 30 using Scan, what would you use?

A FilterExpression: "age > :val", ExpressionAttributeValues: {":val": 30}

B KeyConditionExpression: "age > 30"

C ProjectionExpression: "age > 30"

D UpdateExpression: "age > 30"

FilterExpression with ExpressionAttributeValues is used to filter items after scanning.

Explain how a Scan with Filter Expression works in DynamoDB and why it might be costly.

Think about when filtering happens during the Scan process.

Describe a simple example of using a Filter Expression in a DynamoDB Scan operation.

Focus on the syntax and purpose of FilterExpression.

Practice

1. What does a Scan operation with a filter expression do in DynamoDB?

easy

A. It deletes items that do not match the filter condition.

B. It reads only the items that match the filter condition directly.

C. It updates items that match the filter condition.

D. It reads all items but returns only those matching the filter condition.

Solution

1. Step 1: Understand Scan operation

   A Scan reads every item in the table regardless of any condition.

2. Step 2: Apply filter expression effect

   The filter expression is applied after reading all items, so only matching items are returned.

3. Final Answer:

   It reads all items but returns only those matching the filter condition. -> Option D

4. Quick Check:

   Scan + filter = read all, return filtered [OK]

Hint: Scan reads all, filter returns matching items only [OK]

Common Mistakes:

• Thinking Scan reads only filtered items
• Confusing Scan with Query
• Assuming filter modifies data

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

easy

A. scan(TableName='MyTable', Filter='Name = "John"')

B. scan(TableName='MyTable', ConditionExpression='Name = "John"')

C. scan(TableName='MyTable', FilterExpression='attribute_exists(Name)')

D. scan(TableName='MyTable', FilterCondition='Name = "John"')

Solution

1. Step 1: Identify correct parameter name

   The correct parameter for filtering in Scan is FilterExpression.

2. Step 2: Check syntax correctness

   scan(TableName='MyTable', FilterExpression='attribute_exists(Name)') uses FilterExpression with a valid condition attribute_exists(Name).

3. Final Answer:

   scan(TableName='MyTable', FilterExpression='attribute_exists(Name)') -> Option C

4. Quick Check:

   FilterExpression is correct parameter [OK]

Hint: Use FilterExpression parameter for scan filters [OK]

Common Mistakes:

• Using ConditionExpression instead of FilterExpression
• Using Filter or FilterCondition which are invalid
• Incorrect syntax for filter condition

3. Given a DynamoDB table with items: [{"Name": "Alice", "Age": 30}, {"Name": "Bob", "Age": 25}, {"Name": "Carol", "Age": 35}], what will be the result of a Scan with filter expression Age > 30?

medium

A. [{"Name": "Alice", "Age": 30}]

B. [{"Name": "Carol", "Age": 35}]

C. [{"Name": "Bob", "Age": 25}]

D. [{"Name": "Alice", "Age": 30}, {"Name": "Carol", "Age": 35}]

Solution

1. Step 1: Understand filter condition

   The filter expression Age > 30 means only items with Age greater than 30 are returned.

2. Step 2: Check each item against condition

   Alice has Age 30 (not greater), Bob 25 (not greater), Carol 35 (greater). Only Carol matches.

3. Final Answer:

   [{"Name": "Carol", "Age": 35}] -> Option B

4. Quick Check:

   Age > 30 returns Carol only [OK]

Hint: Filter returns only items strictly matching condition [OK]

Common Mistakes:

• Including items with Age equal to 30
• Confusing greater than with greater or equal
• Returning all items ignoring filter

4. You wrote this Scan code with filter expression: FilterExpression='Age > :val' and ExpressionAttributeValues={':val': 30}, but it returns no items. What is the likely error?

medium

A. ExpressionAttributeValues must be a dictionary with DynamoDB types, e.g., {':val': {'N': '30'}}.

B. FilterExpression should use '>' not '>'.

C. ExpressionAttributeValues key should be ':val' as a string, not a number.

D. Scan does not support FilterExpression.

Solution

1. Step 1: Check ExpressionAttributeValues format

   DynamoDB expects attribute values in a typed format, e.g., numbers as {'N': '30'}.

2. Step 2: Identify why no items returned

   Providing raw number 30 instead of typed value causes filter to fail matching any item.

3. Final Answer:

   ExpressionAttributeValues must be a dictionary with DynamoDB types, e.g., {':val': {'N': '30'}}. -> Option A

4. Quick Check:

   Use typed values in ExpressionAttributeValues [OK]

Hint: Use DynamoDB typed values in ExpressionAttributeValues [OK]

Common Mistakes:

• Passing raw Python values instead of typed dict
• Using HTML entities like > in code instead of >
• Thinking Scan does not support filters

5. You want to scan a DynamoDB table to find items where Status is 'Active' and Score is greater than 80. Which filter expression and attribute values are correct?

hard

A. FilterExpression='Status = :s AND Score > :sc', ExpressionAttributeValues={':s': {'S': 'Active'}, ':sc': {'N': '80'}}

B. FilterExpression='Status == :s AND Score > :sc', ExpressionAttributeValues={':s': 'Active', ':sc': 80}

C. FilterExpression='Status = :s OR Score > :sc', ExpressionAttributeValues={':s': {'S': 'Active'}, ':sc': {'N': '80'}}

D. FilterExpression='Status = :s AND Score > :sc', ExpressionAttributeValues={':s': 'Active', ':sc': '80'}

Solution

1. Step 1: Check filter expression logic

   The condition requires both Status equals 'Active' AND Score greater than 80, so use 'AND' and '=' operators correctly.

2. Step 2: Verify ExpressionAttributeValues format

   Values must be typed: strings as {'S': 'Active'} and numbers as {'N': '80'}.

3. Final Answer:

   FilterExpression='Status = :s AND Score > :sc', ExpressionAttributeValues={':s': {'S': 'Active'}, ':sc': {'N': '80'}} -> Option A

4. Quick Check:

   Correct logic and typed values [OK]

Hint: Use AND and typed values for multiple conditions [OK]

Common Mistakes:

• Using '==' instead of '=' in filter expression
• Using OR instead of AND
• Passing untyped values in ExpressionAttributeValues