SOLR-14329: support choosing expand field from multiple collapse group

* The collapse group with low cost is given higher priority. If
  there are multiple groups  with same cost then, first such
  group is chosen

solr/CHANGES.txt

@@ -69,6 +69,8 @@ Improvements
 * SOLR-14342: Load cores in an order that makes collections available sooner and reduces leaderVoteWait timeouts in
   large SolrCloud clusters. (David Smiley)
 
+* SOLR-14329: Add support to choose collapse group to expand in ExpandComponent based on cost (Munendra S N)
+
 Optimizations
 ---------------------
 * SOLR-8306: Do not collect expand documents when expand.rows=0 (Marshall Sanders, Amelia Henderson)

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

@@ -31,6 +31,7 @@ import com.carrotsearch.hppc.cursors.IntObjectCursor;
 import com.carrotsearch.hppc.cursors.LongCursor;
 import com.carrotsearch.hppc.cursors.LongObjectCursor;
 import com.carrotsearch.hppc.cursors.ObjectCursor;
+import org.apache.commons.lang3.StringUtils;
 import org.apache.lucene.index.DocValues;
 import org.apache.lucene.index.LeafReader;
 import org.apache.lucene.index.LeafReaderContext;
@@ -92,6 +93,8 @@ import org.apache.solr.util.plugin.SolrCoreAware;
  * The CollapsingPostFilter collapses a result set on a field.
  * <p>
  * The ExpandComponent expands the collapsed groups for a single page.
+ * When multiple collapse groups are specified then, the field is chosen from collapse group with min cost.
+ * If the cost are equal then, the field is chosen from first collapse group.
  * <p>
  * http parameters:
  * <p>
@@ -100,7 +103,7 @@ import org.apache.solr.util.plugin.SolrCoreAware;
  * expand.sort=field asc|desc<br>
  * expand.q=*:* (optional, overrides the main query)<br>
  * expand.fq=type:child (optional, overrides the main filter queries)<br>
- * expand.field=field (mandatory if the not used with the CollapsingQParserPlugin)<br>
+ * expand.field=field (mandatory, if the not used with the CollapsingQParserPlugin. This is given higher priority when both are present)<br>
  */
 public class ExpandComponent extends SearchComponent implements PluginInfoInitialized, SolrCoreAware {
   public static final String COMPONENT_NAME = "expand";
@@ -143,11 +146,17 @@ public class ExpandComponent extends SearchComponent implements PluginInfoInitia
     if (field == null) {
       List<Query> filters = rb.getFilters();
       if (filters != null) {
+        int cost = Integer.MAX_VALUE;
         for (Query q : filters) {
           if (q instanceof CollapsingQParserPlugin.CollapsingPostFilter) {
             CollapsingQParserPlugin.CollapsingPostFilter cp = (CollapsingQParserPlugin.CollapsingPostFilter) q;
-            field = cp.getField();
+            // if there are multiple collapse pick the low cost one
-            hint = cp.hint;
+            // if cost are equal then first one is picked
+            if (cp.getCost() < cost) {
+              cost = cp.getCost();
+              field = cp.getField();
+              hint = cp.hint;
+            }
           }
         }
       }
@@ -189,7 +198,7 @@ public class ExpandComponent extends SearchComponent implements PluginInfoInitia
         }
       } else {
         for (String fq : fqs) {
-          if (fq != null && fq.trim().length() != 0 && !fq.equals("*:*")) {
+          if (StringUtils.isNotBlank(fq) && !fq.equals("*:*")) {
             QParser fqp = QParser.getParser(fq, req);
             newFilters.add(fqp.getQuery());
           }

solr/core/src/test/org/apache/solr/handler/component/DistributedExpandComponentTest.java

@@ -90,6 +90,19 @@ public class DistributedExpandComponentTest extends BaseDistributedSearchTestCas
     //Test page 2
     query("q", "*:*", "start","1", "rows", "1", "fq", "{!collapse field="+group+"}", "defType", "edismax", "bf", "field(test_i)", "expand", "true", "fl","*,score");
 
+    // multiple collapse and equal cost
+    ModifiableSolrParams baseParams = params("q", "*:*", "defType", "edismax", "expand", "true", "fl", "*,score",
+        "bf", "field(test_i)", "expand.sort", "id asc");
+    baseParams.set("fq", "{!collapse field="+group+"}", "{!collapse field=test_i}");
+    query(baseParams);
+
+    // multiple collapse and unequal cost case1
+    baseParams.set("fq", "{!collapse cost=1000 field="+group+"}", "{!collapse cost=2000 field=test_i}");
+    query(baseParams);
+
+    // multiple collapse and unequal cost case2
+    baseParams.set("fq", "{!collapse cost=1000 field="+group+"}", "{!collapse cost=200 field=test_i}");
+    query(baseParams);
 
     ignoreException("missing expand field");
     SolrException e = expectThrows(SolrException.class, () -> query("q", "*:*", "expand", "true"));

solr/core/src/test/org/apache/solr/handler/component/TestExpandComponent.java

@@ -419,6 +419,48 @@ public class TestExpandComponent extends SolrTestCaseJ4 {
         "/response/lst[@name='expanded']/result[@name='1"+floatAppend+"']/doc[1]/str[@name='id'][.='1']",
         "/response/lst[@name='expanded']/result[@name='2"+floatAppend+"']/doc[1]/str[@name='id'][.='5']"
     );
+
+    // With multiple collapse
+
+    // with different cost
+    params = params("q", "*:*", "defType", "edismax", "expand", "true",  "bf", "field(test_i)", "expand.sort", "id asc");
+    params.set("fq", "{!collapse cost=1000 field="+group+"}", "{!collapse cost=2000 field=test_f}");
+    assertQ(req(params),
+        "*[count(/response/result/doc)=1]",
+        "/response/result/doc[1]/str[@name='id'][.='2']",
+        "/response/lst[@name='expanded']/result[@name='1"+floatAppend+"']/doc[1]/str[@name='id'][.='1']"
+    );
+
+    // with same cost (default cost)
+    params.set("fq", "{!collapse field="+group+"}", "{!collapse field=test_f}");
+    assertQ(req(params),
+        "*[count(/response/result/doc)=1]",
+        "/response/result/doc[1]/str[@name='id'][.='2']",
+        "/response/lst[@name='expanded']/result[@name='1"+floatAppend+"']/doc[1]/str[@name='id'][.='1']"
+    );
+
+    // with different cost but choose the test_f
+    params.set("fq", "{!collapse cost=3000 field="+group+"}", "{!collapse cost=2000 field=test_f}");
+    assertQ(req(params),
+        "*[count(/response/result/doc)=1]",
+        "/response/result/doc[1]/str[@name='id'][.='2']",
+        "/response/lst[@name='expanded']/result[@name='200.0']/doc[1]/str[@name='id'][.='3']",
+        "/response/lst[@name='expanded']/result[@name='200.0']/doc[2]/str[@name='id'][.='6']",
+        "/response/lst[@name='expanded']/result[@name='200.0']/doc[3]/str[@name='id'][.='8']"
+    );
+
+    // with different cost and nullPolicy
+    params.set("bf", "ord(id)");
+    params.set("fq", "{!collapse cost=1000 field="+group+" nullPolicy=collapse}", "{!collapse cost=2000 field=test_f}");
+    assertQ(req(params),
+        "*[count(/response/result/doc)=2]",
+        "/response/result/doc[1]/str[@name='id'][.='8']",
+        "/response/result/doc[2]/str[@name='id'][.='7']",
+        "/response/lst[@name='expanded']/result[@name='2"+floatAppend+"']/doc[1]/str[@name='id'][.='5']",
+        "/response/lst[@name='expanded']/result[@name='2"+floatAppend+"']/doc[2]/str[@name='id'][.='6']",
+        "/response/lst[@name='expanded']/result[@name='1"+floatAppend+"']/doc[1]/str[@name='id'][.='1']",
+        "/response/lst[@name='expanded']/result[@name='1"+floatAppend+"']/doc[2]/str[@name='id'][.='2']"
+    );
   }
 
   @Test

solr/solr-ref-guide/src/collapse-and-expand-results.adoc

@@ -116,6 +116,12 @@ Collapse on `group_field` with a hint to use the top level field cache:
 fq={!collapse field=group_field hint=top_fc}
 ----
 
+Collapse with custom `cost` which defaults to `100`
+[source,text]
+----
+fq={!collapse cost=1000 field=group_field}
+----
+
 The CollapsingQParserPlugin fully supports the QueryElevationComponent.
 
 == Expand Component
@@ -138,12 +144,22 @@ The ExpandComponent can now be used to expand the results so you can see the doc
 q=foo&fq={!collapse field=ISBN}&expand=true
 ----
 
+[IMPORTANT]
+====
+When used with CollapsingQParserPlugin and there are multiple collapse groups then, the field is chosen from group with least cost.
+If there are multiple collapse groups with same cost then, the first specified one is chosen
+====
+
 The “expand=true” parameter turns on the ExpandComponent. The ExpandComponent adds a new section to the search output labeled “expanded”.
 
 Inside the expanded section there is a _map_ with each group head pointing to the expanded documents that are within the group. As applications iterate the main collapsed result set, they can access the _expanded_ map to retrieve the expanded groups.
 
 The ExpandComponent has the following parameters:
 
+`expand.field`::
+Field on which expand documents need to be populated. When `expand=true`, either this parameter needs to be specified or should be used with CollapsingQParserPlugin.
+When both are specified, this parameter is given higher priority.
+
 `expand.sort`::
 Orders the documents within the expanded groups. The default is `score desc`.