title: Performance of the "SELECT WHERE WEEKDAY() = something" statement
body: <p>A really simple question.
  I have a table representing receipts with ~1M+ rows, and must filter/group it by the day of the week too.
  For example: show me statistically what is the day of the week with more receipts emitted
  and the one where people spend more money in total? and per receipt?</p>
  
  <p>I know how to do it, my only concern regards performance.
  I was thinking about adding an entirely new fiel "Day_of_the_week", but this way I have to modify and retest a lot of code.</p>
  
  <p>So, WEEKDAY(Date_field) is faster/same than a dedicated field, or is better to rework the table?</p>