title: When are rows overwritten in cassandra
body: <p>My understanding that rows are overwritten when another row with identical primary keys is inserted.</p>
  
  <p>For example:</p>
  
  <p>I have columns <code>(user_id int, item_id int, site_id int)</code>, and my <code>PRIMARY KEY(user_id, item_id)</code></p>
  
  <p>If I had the following table:</p>
  
  <pre><code>user_id, item_id, site_id
     2       3        4
  </code></pre>
  
  <p>and I insert <code>user_id : 2, item_id : 3, site_id : 10</code>, my new table would be:</p>
  
  <pre><code>user_id, item_id, site_id
     2       3        10
  </code></pre>
  
  <p>not</p>
  
  <pre><code>user_id, item_id, site_id
     2       3        4
     2       3        10
  </code></pre>
  
  <p>Is this simple case hold in all cases? Are any subtleties that I likely not aware off? Also, I could not find this in the docs and came to this conclusion by playing around with cassandra, can anyone provide a doc source?</p>