OR conditions are normally difficult to optimize when used on different columns, a pain when the columns are of range conditions and worst when done between 2 or more different tables. Look at my example below, the original query  is trying to find rows which conditions are based on columns from the JOIN’ed tables.

mysql [localhost] {msandbox} (employees) > EXPLAIN SELECT
    ->      e.emp_no, birth_date, first_name,
    ->      last_name, gender, hire_date,
    ->      salary, from_date, to_date
    -> FROM employees e
    -> INNER JOIN salaries s ON (e.emp_no = s.emp_no)
    -> WHERE e.hire_date BETWEEN '1990-06-01 00:00:00' AND '1990-07-01 00:00:00' OR
    ->      s.from_date BETWEEN '1990-06-01 00:00:00' AND '1990-07-01 00:00:00'
    -> ORDER BY e.emp_no, hire_date, from_date \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: e
         type: ALL …