title: When does Cassandra fetches full rows when a client sends a update query
body: <p>Lets take an example table:</p>
  <pre><code>CREATE TABLE student (
      id int PRIMARY KEY,
      name text,
      phone text
  );
  </code></pre>
  <p>And a clients sends a update query like: <code>update student set name='name_temp' where id in (1, 2);</code></p>
  <p>My question is what gets saved into memtable, does it save the whole row for ids 1 and 2 (which means it has to fetch the whole row first) with updated value for <code>name</code> column or just the delta? When does the whole row gets fetched as I assume when it writes to SSTable it has to write the whole row with the latest 'name` column value.</p>
  <p><strong>EDIT:</strong></p>
  <p>For complete understanding please read the comments as part of the selected answer.</p>