title: Firestore where clause with big dataset
body: <p>I have the following structure in my firestore database:</p>
  
  <pre><code>messages:
      m1:
          title: "Message 1"
          ...
          archived: false
      m2:
          title: "Message 2"
          ...
          archived: true
  </code></pre>
  
  <p>Let's say I have 20k messages and I want to get archived messages using a "where" clause, <strong>will my query be slower than if I structured my database as following</strong> ?</p>
  
  <pre><code>nonArchivedMessages:
      m1:
          title: "Message 1"
          ...
  archivedMessages:
      m2:
          title: "Message 2"
          ...
  </code></pre>
  
  <p>Using the second structure seems, to me, more adapted for large datasets but implies issues in some cases, such as getting a message without knowing whether it is archived or not.</p>