title: What indexing should I apply to user ID column?
body: <p>I have a table like this:</p>
  
  <pre><code>| id | user_id | ... | ... | ...
  </code></pre>
  
  <p>Where <code>id</code> is the primary key. What indexing should I apply to the <code>user_id</code> column (note that there can be multiple rows in this table from the same user)? Just a regular index?</p>