A retail company uses Cloud Spanner to store product inventory data. The table structure is:

CREATE TABLE Inventory ( ProductId INT64 NOT NULL, WarehouseId INT64 NOT NULL, StockLevel INT64 NOT NULL, LastUpdated TIMESTAMP NOT NULL OPTIONS (allow_commit_timestamp=true) ) PRIMARY KEY (ProductId, WarehouseId);

The application frequently runs the query: SELECT ProductId, SUM(StockLevel) AS TotalStock FROM Inventory WHERE WarehouseId = 123 GROUP BY ProductId. The query is slow and scans many rows. The index used is:

CREATE INDEX InventoryByWarehouse ON Inventory (WarehouseId);

What is the most effective schema change to improve query performance?