title: Innodb buffer pool and multiple joins
body: <p>assume we have 1 Billion records and want to store them in <code>One</code> or <code>Three</code> tables by post_type.</p>
  
  <pre><code>post_id   -&gt; primary key  
  post_type -&gt; ( page, group, user )
  user_id   -&gt; index
  date      -&gt; index
  </code></pre>
  
  <p>Note: we have enough RAM to store all data in buffer pool. ( about 40 GB )<br>
  In our situation, <code>Reading</code> from database is too much</p>
  
  <p>the question is: <strong>WHILE</strong> the whole data is ON buffer pool ( RAM ), is there any difference <code>in performance</code> to have <code>3</code> separated tables and <code>Join</code> them, or have <code>1</code> big table and <code>Select</code> from it without any <code>Join</code>.</p>
  
  <p>In some cases it's better for us to have separated tables for other kind of light queries.<br>
  But we fear of <code>JOINS</code>.</p>