title: Can MYSQL use an index for a WHERE ... IN query?
body: <p>We currently have a query in our app like this:</p>
  
  <pre><code>SELECT * FROM combos WHERE text = '[text]'
  </code></pre>
  
  <p>We're putting a <code>category_id</code> column on <code>combos</code>, and the query will change to this:</p>
  
  <pre><code>SELECT * FROM combos WHERE category_id in ([c1],[c2],[...]) AND text = '[text]'
  </code></pre>
  
  <p>At the moment, there's an index on <code>combos(text)</code>.  We're thinking of removing it and putting one on <code>combos(category_id, text)</code>.  Will MYSQL be able to use the new index like it did the old one?  If the answer is 'it depends', what does it depend on?  Should we keep the old one in case it can't?</p>