Refer to the figure: Three Optimization Layers

A system processes database queries using Python, with no indexes, and SELECT * retrieving all 15 columns. Three independent optimisations target different layers:

• Option A (Language Layer): Rewrite from Python to Go, reducing CPU overhead per query
• Option B (Data Layer): Add a database index, reducing pages read by 98%
• Option C (Query Layer): Change SELECT * to 3 specific columns, reducing data volume by 85%

If only one optimisation can be applied, which would have the largest impact on energy consumption, and what is the reasoning?

A company operates a web API that runs continuously and processes thousands of requests per second. The service is currently written in Python. A team proposes rewriting it in Go. A separate team proposes keeping Python but moving 60% of the workload to a cloud region with lower carbon intensity. Which statement best characterises the trade-off between these two approaches?

An e-commerce platform handles product search queries. Each search filters on category, price range, and brand. Currently, the database has a separate index on each filter column. The database retrieves matching row IDs from each index and intersects the results, examining approximately 25,000 rows per query. A proposed change creates a single combined index on (category, price_range, brand), narrowing to approximately 800 rows per query in a single traversal. Why does the combined index produce fewer rows examined?

A database stores sensor readings in a table where the sensor identifier is stored as a 36-character text field. A proposal changes the identifier to a compact numeric format (significantly fewer bytes per row). The table contains millions of rows. Which of the following is the primary mechanism by which this change reduces query energy?

A video streaming platform must choose between two encoding formats. Format A encodes quickly with moderate quality, producing larger files. Format B takes 2.7 times longer to encode but produces files half the size of Format A. Each time a video is streamed, the smaller file transfers less data over the network.

For a video that is expected to be viewed only twice, which format consumes less total energy (encoding plus all transmissions), and why?

A web application retrieves product data from a database and then sorts the results before displaying them. Currently, the database returns all 50,000 products (no index, full table scan), and the application sorts them using a method where doubling the input quadruples the work. Two changes are proposed:

• Change A: Switch to a sorting method where doubling the input only doubles the work
• Change B: Add a database index so only the 200 matching products are returned

Which change addresses the larger source of wasted energy?

An IoT platform stores sensor data in a single large table containing 90 days of readings. Most queries filter on a specific date range (such as retrieving data from the last 24 hours). A database administrator proposes partitioning the table by day, creating 90 separate daily segments. How does this reduce query energy for date-filtered queries?

A university database uses a normalised schema with separate Students, Majors, and Departments tables. A report query joins all three tables. The IT department proposes denormalising by copying the major name and department building directly into the Students table. Which statement correctly describes the trade-off? 

A

A company operates four microservices, all written in Python. The figure shows how each service divides its request time between CPU processing and waiting for external I/O. The company can afford to rewrite only one service in a compiled language. Which service would benefit most from the rewrite, and why?

A messaging application checks the server for new messages every 10 seconds, regardless of whether any messages have arrived. An alternative design uses push notifications, where the server notifies the application only when a new message exists. Both designs deliver messages to the user. Why does the push-based design consume significantly less energy?