title: Understanding MySQL explain, `rows`-wise
body: <p>I'm trying to figure out how should I take into account the <code>rows</code> column of MySQL <code>explain</code>'s output. Here's what <a href="https://dev.mysql.com/doc/refman/5.0/en/explain-output.html#explain_rows" rel="noreferrer">MySQL documentation</a> says about it:</p>
  
  <blockquote>
    <p>The rows column indicates the number of rows MySQL believes it must
    examine to execute the query.</p>
  </blockquote>
  
  <p>So here are my questions:</p>
  
  <ol>
  <li>Regardless of its exactness, is this the number of records that are going to be examined after the indices are used or before?</li>
  <li>Is it correct that I need to value the optimization of tables with high <code>rows</code>?</li>
  <li>Is it true that the total number of records MySQL will examine is the product of <code>rows</code> column?</li>
  <li>What are the strategies to reduce the <code>rows</code>?</li>
  </ol>