title: PHP memory usage goes up 50mb but result set is only ~7mb
body: <p>I've stumbles upon some interesting memory things concerning PHP
  Consider this code (sql-result is around 18000 rows)</p>
  
  <pre><code>echo memory_get_usage() . PHP_EOL;
  
  echo 'Querying mysql' . PHP_EOL;
  $result = $wpdb-&gt;get_results($sql);
  
  echo memory_get_usage() . PHP_EOL;
  
  echo PHP_EOL;
  echo 'Size of result set' . PHP_EOL;
  echo strlen(serialize($result)) . PHP_EOL;
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>33309232
  Querying mysql
  86203976
  
  Size of result set
  6838278
  </code></pre>
  
  <p>Why does the memory usage peak like this? Isn't it a little weird?</p>
  
  <p>And maybe the most important question, is there something I can do to lower the usage?</p>