Projection support in Transactional Gets at the DynamoDBContext level

[aanton-git]

Description

Add projection support for DynamoDB transactional get operations exposed through DynamoDBContext in the AWS SDK for .NET.

When using DynamoDBContext to perform transactional get operations, it is currently not possible to specify a projection to retrieve only a subset of item attributes. All attributes are always returned.

This capability is supported by DynamoDB and exposed in other SDKs, but is missing from the high-level DynamoDBContext transactional APIs.

Motivation and Context

I need this feature to reduce payload size and improve efficiency when performing transactional reads via DynamoDBContext.

Without projection support:

Transactional gets always return full items, even when only a few attributes are required
Network transfer and deserialization costs are higher than necessary
Transactional reads are less efficient than non-transactional DynamoDBContext reads, which already support projections
This makes it harder to optimize read-heavy transactional workflows.

Testing

• to add benchmarks runs screenshots

Dry-runs

Breaking Changes Assessment

1. Identify all breaking changes including the following details:
   • What functionality was changed?
   • How will this impact customers?
   • Why does this need to be a breaking change and what are the most notable non-breaking alternatives?
   • Are best practices being followed?
   • How have you tested this breaking change?
2. Has a senior/+ engineer been assigned to review this PR?

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• [ x] New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• [x ] I have added tests to cover my changes
• [x ] All new and existing tests passed