DynamoDB CRUD operations and Query vs Scan

Using Data Stores in AWS

Dunieski Otano

AWS Solutions Architect

The expensive mistake

• Developer uses Scan to find one user
• Reads entire 1 million item table
• Bill: $500 for a single lookup
• Query would have cost $0.01

CRUD operations in dynamoDB

• PutItem: Create or replace item
• GetItem: Read item by primary key
• UpdateItem: Modify specific attributes
• DeleteItem: Remove item
• Query: Retrieve items by partition key (efficient)
• Scan: Read entire table (expensive)

Query vs Scan: the critical difference

• Query: Requires partition key, reads specific items
  • Fast, efficient, low cost
• Scan: Reads entire table, filters after
  • Slow, expensive, avoid when possible
• Rule: If you know the key, Query. Otherwise, rethink your design.

Preventing race conditions with conditional expressions

• Problem: Two users update same item simultaneously
• Solution: Conditional expressions
• Example: Update only if current value matches expected

UpdateItem expressions: modifying attributes

• SET: Set attribute value or create new attribute
• ADD: Increment/decrement numbers, add to sets
• REMOVE: Delete attributes from item
• DELETE: Remove elements from sets

Pagination with Query and Scan

• Problem: Large result sets exceed 1MB limit
• Solution: Pagination with LastEvaluatedKey
• Limit: Control items per page
• Continue until LastEvaluatedKey is null

Let's practice!

Using Data Stores in AWS