SOLR-17801: Use TotalHitCountCollector to collect count when no rows needed (#2655)

Author: risdenk

solr/CHANGES.txt

@@ -272,6 +272,8 @@ Optimizations
 
 * SOLR-17775: Speed up function queries in 'fl' param. (Yura Korolov)
 
+* SOLR-17801: Use TotalHitCountCollector to collect count when no rows needed (Kevin Risden)
+
 Bug Fixes
 ---------------------
 * SOLR-17629: If SQLHandler failed to open the underlying stream (e.g. Solr returns an error; could be user/syntax problem),

solr/core/src/java/org/apache/solr/search/SolrIndexSearcher.java

@@ -1895,18 +1895,7 @@ private void getDocListNC(QueryResult qr, QueryCommand cmd) throws IOException {
       final Collector collector;
 
       if (!needScores) {
-        collector =
-            new SimpleCollector() {
-              @Override
-              public void collect(int doc) {
-                numHits[0]++;
-              }
-
-              @Override
-              public ScoreMode scoreMode() {
-                return ScoreMode.COMPLETE_NO_SCORES;
-              }
-            };
+        collector = new TotalHitCountCollector();
       } else {
         collector =
             new SimpleCollector() {
@@ -1934,6 +1923,11 @@ public ScoreMode scoreMode() {
       buildAndRunCollectorChain(qr, query, collector, cmd, pf.postFilter);
 
       totalHits = numHits[0];
+      if (collector instanceof TotalHitCountCollector) {
+        totalHits = ((TotalHitCountCollector) collector).getTotalHits();
+      } else {
+        totalHits = numHits[0];
+      }
       maxScore = totalHits > 0 ? topscore[0] : 0.0f;
       docList =
           new DocSlice(