SOLR-17985: Speed up no-rows distributed queries

changelog/unreleased/SOLR-17985-fix-slow-no-rows-queries.yml

@@ -0,0 +1,10 @@
+# See https://github.com/apache/solr/blob/main/dev-docs/changelog.adoc
+title: Make distributed no-rows queries fast again
+type: fixed # added, changed, fixed, deprecated, removed, dependency_update, security, other
+authors:
+  - name: Houston Putman
+    nick: HoustonPutman
+    url: https://home.apache.org/phonebook.html?uid=houston
+links:
+  - name: SOLR-17985
+    url: https://issues.apache.org/jira/browse/SOLR-17985

solr/core/src/java/org/apache/solr/handler/component/QueryComponent.java

@@ -832,20 +832,24 @@ protected void createMainQuery(ResponseBuilder rb) {
     // perhaps we shouldn't attempt to parse the query at this level?
     // Alternate Idea: instead of specifying all these things at the upper level,
     // we could just specify that this is a shard request.
+    int shardRows;
     if (rb.shards_rows > -1) {
       // if the client set shards.rows set this explicity
-      sreq.params.set(CommonParams.ROWS, rb.shards_rows);
+      shardRows = rb.shards_rows;
     } else {
-      // what if rows<0 as it is allowed for grouped request??
-      sreq.params.set(
-          CommonParams.ROWS, rb.getSortSpec().getOffset() + rb.getSortSpec().getCount());
+      shardRows = rb.getSortSpec().getCount();
+      // If rows = -1 (grouped requests) or rows = 0, then there is no need to add the offset.
+      if (shardRows > 0) {
+        shardRows += rb.getSortSpec().getOffset();
+      }
     }
+    sreq.params.set(CommonParams.ROWS, shardRows);
 
     sreq.params.set(ResponseBuilder.FIELD_SORT_VALUES, "true");
 
     boolean shardQueryIncludeScore =
         (rb.getFieldFlags() & SolrIndexSearcher.GET_SCORES) != 0
-            || rb.getSortSpec().includesScore();
+            || (shardRows != 0 && rb.getSortSpec().includesScore());
     StringBuilder additionalFL = new StringBuilder();
     boolean additionalAdded = false;
     if (rb.onePassDistributedQuery) {

solr/core/src/test/org/apache/solr/TestDistributedSearch.java

@@ -1550,6 +1550,7 @@ public void test() throws Exception {
     // Check Info is added to for each shard
     ModifiableSolrParams q = new ModifiableSolrParams();
     q.set("q", "*:*");
+    q.set("rows", "0");
     q.set(ShardParams.SHARDS_INFO, true);
     setDistributedParams(q);
     rsp = queryRandomShard(q);
@@ -1560,6 +1561,13 @@ public void test() throws Exception {
     assertNotNull("missing shard info", sinfo);
     assertEquals(
         "should have an entry for each shard [" + sinfo + "] " + shards, cnt, sinfo.size());
+    // We are setting rows=0, so scores should not be collected
+    for (int i = 0; i < cnt; i++) {
+      String shard = sinfo.getName(i);
+      assertNull(
+          "should not have any score information (maxScore) in the shard info: " + shard,
+          ((Map<String, Object>) sinfo.get(shard)).get("maxScore"));
+    }
 
     // test shards.tolerant=true