Second-Highest Salary at Scale

Write a query to find the second-highest salary from an employee table, and then optimize it for a dataset containing millions of records.

Assume the result means the second-highest distinct, non-null salary. Address how the access path can avoid unnecessary full-table work.

Output

1. Return one row with second_highest_salary.
2. Exclude null salaries and duplicate salary values when determining rank.
3. Return no row if fewer than two distinct non-null salaries exist.