title: How to use join in solr in order to get result from both the cores?
body: <p>I have two cores:</p>
  
  <p>core 1: BookID, Venue, Title, Authors, PubDate</p>
  
  <p>core 2: BookID, Text, Referenecs, Size, LastUpdated</p>
  
  <blockquote>
    <p><a href="http://localhost:8983/solr/core1/select?fl=Title,%20Venue,%20Authors,%20PubDate&amp;q=" rel="nofollow noreferrer">http://localhost:8983/solr/core1/select?fl=Title,%20Venue,%20Authors,%20PubDate&amp;q=</a>{!join%20from=PaperID%20to=fileName%20fromIndex=core2%20}size:15870</p>
  </blockquote>
  
  <p>The above query returns zero results as:</p>
  
  <pre><code>&lt;response&gt;
     &lt;lst name="responseHeader"&gt;
        &lt;int name="status"&gt;0&lt;/int&gt;
        &lt;int name="QTime"&gt;0&lt;/int&gt;
        &lt;lst name="params"&gt;
           &lt;str name="q"&gt;{!join from=PaperID to=fileName fromIndex=PapersIndex }size:15870&lt;/str&gt;
           &lt;str name="fl"&gt;Title, Venue&lt;/str&gt;
        &lt;/lst&gt;
     &lt;/lst&gt;
     &lt;result name="response" numFound="0" start="0" /&gt;
  &lt;/response&gt;
  </code></pre>
  
  <p>I don't know how the solr join works. Why does it give zero results?</p>