I use MySQL database (innodb)
I use Visual Basic 2008
I have a table Employees with two fields EmpNo and Managerno
ALTER TABLE Employees ADD PRIMARY KEY (EmpNo
CREATE INDEX index_Managerno ON Employees (ManagerNo)
How do i now IF Mysql using this index is if i do this:
SELECT * FROM Employees WHERE Managerno = 123451

Look at the EXPLAIN command. It will tell you what steps the MySQL engine would take to execute a given SELECT statement.
MySQL Documentation