MySQL-为什么在这种情况下，二级索引不能减​​少查询时间？

Question

As part of an experiment I'm trying to optimise this query, and for this example I am using [The Employee database][1] provided by MySql. The strategy I'm using is creating a secondary index on the birth_date attribute since theory texts recommend doing this on columns that are used in the WHERE clause.

By creating such index I was expecting a reduction in time of at least 40% but that is not the case with this query. There doesn't seem to be an improvement in time, in fact it takes a bit longer on average. Could anyone tell me why this is happening?

SELECT employees.emp_no,
        employees.departments.dept_name,
        employees.first_name,
        employees.last_name,
        employees.birth_date,
        year(curdate()) - year(birth_date) AS yearsOld
FROM employees.employees
INNER JOIN employees.dept_emp ON employees.dept_emp.emp_no = employees.emp_no
INNER JOIN employees.departments ON employees.departments.dept_no = employees.dept_emp.dept_no
WHERE year (birth_date) < 1953
ORDER BY emp_no ASC;

Results:

Without secondary index:

Time taken on average: 0.728 sec

After creating secondary index:

CREATE INDEX myIndex2 ON employees(birth_date);

Time taken on average: 0.731 sec

Answer 1

You are not using the raw column value of birth_date . Only then an index can be used. When using a function like year() indexes are useless.

To make use of the index you can do this instead:

where birth_date >= '1953-01-01'
  and birth_date <  '1954-01-01'