document num_docs_indexed

Author: radu-gheorghe

en/performance/streaming-search.html

@@ -60,7 +60,9 @@ <h2 id="differences-in-streaming-search">Differences in streaming search</h2>
                 <em>nativeRank</em>, <em>nativeFieldMatch</em>, <em>nativeProximity</em> and <em>fieldMatch</em>.</li>
             <li>If using <em>bm25</em>, adjust the
                 <a href="../reference/ranking/rank-feature-configuration.html#properties">averageFieldLength</a>
-                configuration for a more precise <em>bm25</em> score.</li>
+                configuration for a more precise <em>bm25</em> score.
+                The <a href="../reference/ranking/rank-features.html#num_docs_indexed">num_docs_indexed</a>
+                rank feature returns 1 in streaming search.</li>
         </ul>
     </li>
     <li>Even without any indexes, fields must be specified as

en/ranking/bm25.html

@@ -215,6 +215,9 @@ <h2 id="ranking-function">Ranking function</h2>
     </figure>
     <p>
     <em>N</em> is the total number of documents on the content node.
+    The local value BM25 uses in this case is exposed as the
+    <a href="../reference/ranking/rank-features.html#num_docs_indexed">num_docs_indexed</a>
+    rank feature.
     <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline">
       <semantics>
         <mrow>

en/reference/ranking/rank-features.html

@@ -1386,6 +1386,19 @@ <h3 id="rank-scores">Rank scores</h3></td></tr>
   <td>n/a</td>
   <td><p id="now">Time at which the query is executed in unix-time (seconds since epoch)</p></td></tr>
 
+<tr><td>num_docs_indexed</td>
+  <td>n/a</td>
+  <td>
+    <p id="num_docs_indexed">
+    The local document count used as <a href="../../ranking/bm25.html">BM25</a>'s total document count
+    when no per-term document-frequency override is present.
+    This is the <em>N</em> term used by the BM25 inverse document frequency calculation.
+    </p>
+    {% include note.html content="The value is a per-content-node statistic computed locally; it is
+    <em>not</em> cluster-wide and may differ between nodes. For
+    <a href='../../performance/streaming-search.html'>streaming search</a>, this feature returns 1." %}
+  </td></tr>
+
 <tr><td>random</td>
   <td>n/a</td>
   <td>