Scan vs Query performance comparison in DynamoDB

Introduction

We want to find data quickly and efficiently in DynamoDB. Understanding the difference between Scan and Query helps us choose the faster way.

When you know the exact key of the item you want to find.

When you want to get all items but only a few match your condition.

When you want to read the whole table without filtering.

When you want to save time and reduce cost by reading less data.

When you want to avoid reading unnecessary data from the database.

Syntax

DynamoDB

Query:
  Use KeyConditionExpression to specify the key.
  Optionally use FilterExpression to narrow results.

Scan:
  Reads every item in the table.
  Optionally use FilterExpression to narrow results after reading.

Query is faster because it looks only at items with matching keys.

Scan reads the whole table, so it is slower and costs more.

Examples

This finds all songs by the artist 'No One You Know' quickly using the key.

DynamoDB

Query example:

aws dynamodb query \
  --table-name Music \
  --key-condition-expression "Artist = :artist" \
  --expression-attribute-values '{":artist":{"S":"No One You Know"}}'

This reads the whole Music table and then filters to show only Rock songs.

DynamoDB

Scan example:

aws dynamodb scan \
  --table-name Music \
  --filter-expression "Genre = :genre" \
  --expression-attribute-values '{":genre":{"S":"Rock"}}'

Sample Program

The Query example uses the Artist key to find matching songs fast.

The Scan example reads all songs and then filters, which takes more time.

DynamoDB

SELECT * FROM Music WHERE Artist = 'No One You Know'; -- Query example

-- This returns only songs by 'No One You Know' quickly

-- Scan example (conceptual):
-- Scan the whole Music table and filter Genre = 'Rock'
-- This is slower and reads all items

Important Notes

Query uses the primary key or index to find data fast.

Scan reads every item, so it is slower and costs more.

Always prefer Query when you can specify the key.

Summary

Query is fast and efficient for known keys.

Scan reads the whole table and is slower.

Use Query to save time and money whenever possible.

Practice

1. Which DynamoDB operation is generally faster when you know the partition key of the item you want to retrieve?

easy

A. Scan

B. UpdateItem

C. Query

D. DeleteItem

Solution

1. Step 1: Understand Query operation

   Query uses the partition key to directly find matching items, making it efficient.

2. Step 2: Compare with Scan operation

   Scan reads the entire table, which is slower and less efficient.

3. Final Answer:

   Query -> Option C

4. Quick Check:

   Query is faster for known keys [OK]

Hint: Use Query when you know the partition key for speed [OK]

Common Mistakes:

• Thinking Scan is faster because it reads all data
• Confusing Query with Scan
• Assuming UpdateItem is for reading data

2. Which of the following is the correct syntax to perform a Query operation in DynamoDB using AWS SDK for JavaScript?

easy

A. dynamoDbClient.query({ TableName: 'MyTable', KeyConditionExpression: '#pk = :pkval', ExpressionAttributeNames: { '#pk': 'PartitionKey' }, ExpressionAttributeValues: { ':pkval': '123' } })

B. dynamoDbClient.scan({ TableName: 'MyTable', KeyConditionExpression: 'PartitionKey = 123' })

C. dynamoDbClient.query({ TableName: 'MyTable', FilterExpression: 'PartitionKey = 123' })

D. dynamoDbClient.getItem({ TableName: 'MyTable', Key: { PartitionKey: '123' } })

Solution

1. Step 1: Identify correct Query syntax

   Query requires KeyConditionExpression with placeholders and attribute names/values.

2. Step 2: Check options for correct usage

   dynamoDbClient.query({ TableName: 'MyTable', KeyConditionExpression: '#pk = :pkval', ExpressionAttributeNames: { '#pk': 'PartitionKey' }, ExpressionAttributeValues: { ':pkval': '123' } }) uses KeyConditionExpression and ExpressionAttributeNames/Values correctly.

3. Final Answer:

   dynamoDbClient.query({ TableName: 'MyTable', KeyConditionExpression: '#pk = :pkval', ExpressionAttributeNames: { '#pk': 'PartitionKey' }, ExpressionAttributeValues: { ':pkval': '123' } }) -> Option A

4. Quick Check:

   Query needs KeyConditionExpression [OK]

Hint: Query needs KeyConditionExpression, not FilterExpression [OK]

Common Mistakes:

• Using FilterExpression instead of KeyConditionExpression for Query
• Using scan method with KeyConditionExpression
• Confusing getItem with query syntax

3. Given a DynamoDB table with 1000 items, what will be the main difference in performance between these two operations?
dynamoDbClient.scan({ TableName: 'MyTable' })
and
dynamoDbClient.query({ TableName: 'MyTable', KeyConditionExpression: '#pk = :pk', ExpressionAttributeNames: { '#pk': 'PartitionKey' }, ExpressionAttributeValues: { ':pk': '123' } })

medium

A. Scan reads all 1000 items; Query reads only matching items, so Query is faster.

B. Scan is faster because it reads all items at once; Query is slower due to filtering.

C. Both operations have the same speed because they access the same table.

D. Query reads all items; Scan reads only matching items.

Solution

1. Step 1: Understand Scan operation

   Scan reads every item in the table, so it processes all 1000 items.

2. Step 2: Understand Query operation

   Query uses the partition key to read only matching items, which is faster.

3. Final Answer:

   Scan reads all items; Query reads only matching items, so Query is faster. -> Option A

4. Quick Check:

   Scan reads all; Query reads matching [OK]

Hint: Scan reads whole table; Query reads only matching keys [OK]

Common Mistakes:

• Thinking Scan is faster because it reads all data at once
• Confusing Query reading all items
• Assuming both have same speed

4. You wrote this DynamoDB Query code but it returns no results:

const params = { TableName: 'MyTable', KeyConditionExpression: 'PartitionKey = :pk', ExpressionAttributeValues: { ':pk': '123' } };
const data = await dynamoDbClient.query(params);

What is the most likely error?

medium

A. Missing ExpressionAttributeNames for reserved word PartitionKey

B. Using Scan instead of Query

C. TableName is misspelled

D. Incorrect KeyConditionExpression syntax; should use #pk = :pk

Solution

1. Step 1: Check KeyConditionExpression syntax

   KeyConditionExpression requires placeholders for attribute names like #pk defined in ExpressionAttributeNames.

2. Step 2: Identify missing ExpressionAttributeNames

   The code uses 'PartitionKey' directly without ExpressionAttributeNames, causing no matches.

3. Final Answer:

   Incorrect KeyConditionExpression syntax; should use #pk = :pk -> Option D

4. Quick Check:

   Reserved words need placeholders in KeyConditionExpression [OK]

Hint: Use placeholders for reserved words in KeyConditionExpression [OK]

Common Mistakes:

• Not using ExpressionAttributeNames for reserved words
• Confusing Scan and Query methods
• Misspelling TableName

5. You want to retrieve all items where the attribute 'Status' equals 'Active' from a large DynamoDB table. The table's partition key is 'UserId'. Which approach is best for performance and cost?

hard

A. Use Query with KeyConditionExpression on 'UserId' and FilterExpression on 'Status' = 'Active'.

B. Create a Global Secondary Index (GSI) on 'Status' and Query the GSI for 'Active' items.

C. Use Scan with a FilterExpression on 'Status' = 'Active' to get all matching items.

D. Use Scan without any filters to get all items and then filter in application code.

Solution

1. Step 1: Understand limitations of Scan and Query

   Scan reads entire table and is costly; Query requires partition key, but 'Status' is not the partition key.

2. Step 2: Use GSI for efficient querying

   Creating a GSI on 'Status' allows Query on 'Status' attribute efficiently without scanning.

3. Final Answer:

   Create a Global Secondary Index (GSI) on 'Status' and Query the GSI for 'Active' items. -> Option B

4. Quick Check:

   GSI enables efficient queries on non-key attributes [OK]

Hint: Use GSI to query non-key attributes efficiently [OK]

Common Mistakes:

• Using Scan with filters on large tables
• Trying to Query without partition key
• Filtering in application instead of database