What is the optimal execution plan for the query to get the employee with the highest salary and their department?

The optimal query execution plan to retrieve the employee with the highest salary and their department typically includes the following steps:

1. Indexing: Ensure there is an index on the salary (Salary) column to quickly find the maximum value.
2. Finding the maximum: Execute an aggregate query to get the maximum salary.
3. Finding the employee: Find the employee with this maximum salary.
4. Joining the department: Perform a JOIN with the departments table on the department key.

Example SQL query:

SELECT e.EmployeeId, e.Name, e.Salary, d.DepartmentName
FROM Employees e
JOIN Departments d ON e.DepartmentId = d.DepartmentId
WHERE e.Salary = (SELECT MAX(Salary) FROM Employees)

The database query planner will first execute the subquery to find the maximum salary, then use the index to quickly find the employee with that salary, and join with the departments table.

In C# using Entity Framework, it might look like this:

var maxSalary = context.Employees.Max(e => e.Salary);
var employeeWithMaxSalary = context.Employees
    .Include(e => e.Department)
    .FirstOrDefault(e => e.Salary == maxSalary);

This approach minimizes the amount of data processed and utilizes indexes for fast access.