LUCENE-7065: Fix the explain for the global ordinals join query. Before the

explain would also indicate that non matching documents would match.
  On top of that with score mode average, the explain would fail with a NPE.

lucene/CHANGES.txt

@@ -155,6 +155,13 @@ Tests
   expression to encapsulate a statement that is expected to throw an exception.
   (Ryan Ernst)
 
+Bug Fixes
+
+* LUCENE-7065: Fix the explain for the global ordinals join query. Before the
+  explain would also indicate that non matching documents would match.
+  On top of that with score mode average, the explain would fail with a NPE.
+  (Martijn van Groningen)
+
 Other
 
 * LUCENE-7035: Upgrade icu4j to 56.1/unicode 8. (Robert Muir)

lucene/join/src/java/org/apache/lucene/search/join/GlobalOrdinalsQuery.java

@@ -112,16 +112,29 @@ final class GlobalOrdinalsQuery extends Query {
     @Override
     public Explanation explain(LeafReaderContext context, int doc) throws IOException {
       SortedDocValues values = DocValues.getSorted(context.reader(), joinField);
-      if (values != null) {
-        int segmentOrd = values.getOrd(doc);
-        if (segmentOrd != -1) {
-          BytesRef joinValue = values.lookupOrd(segmentOrd);
-          return Explanation.match(score(), "Score based on join value " + joinValue.utf8ToString());
-        }
-      }
+      if (values == null) {
         return Explanation.noMatch("Not a match");
       }
 
+      int segmentOrd = values.getOrd(doc);
+      if (segmentOrd == -1) {
+        return Explanation.noMatch("Not a match");
+      }
+      BytesRef joinValue = values.lookupOrd(segmentOrd);
+
+      int ord;
+      if (globalOrds != null) {
+        ord = (int) globalOrds.getGlobalOrds(context.ord).get(segmentOrd);
+      } else {
+        ord = segmentOrd;
+      }
+      if (foundOrds.get(ord) == false) {
+        return Explanation.noMatch("Not a match, join value " + Term.toString(joinValue));
+      }
+
+      return Explanation.match(score(), "A match, join value " + Term.toString(joinValue));
+    }
+
     @Override
     public Scorer scorer(LeafReaderContext context) throws IOException {
       SortedDocValues values = DocValues.getSorted(context.reader(), joinField);

lucene/join/src/java/org/apache/lucene/search/join/GlobalOrdinalsWithScoreQuery.java

@@ -120,21 +120,28 @@ final class GlobalOrdinalsWithScoreQuery extends Query {
     @Override
     public Explanation explain(LeafReaderContext context, int doc) throws IOException {
       SortedDocValues values = DocValues.getSorted(context.reader(), joinField);
-      if (values != null) {
+      if (values == null) {
+        return Explanation.noMatch("Not a match");
+      }
+
       int segmentOrd = values.getOrd(doc);
-        if (segmentOrd != -1) {
-          final float score;
-          if (globalOrds != null) {
-            long globalOrd = globalOrds.getGlobalOrds(context.ord).get(segmentOrd);
-            score = collector.score((int) globalOrd);
-          } else {
-            score = collector.score(segmentOrd);
+      if (segmentOrd == -1) {
+        return Explanation.noMatch("Not a match");
       }
       BytesRef joinValue = values.lookupOrd(segmentOrd);
-          return Explanation.match(score, "Score based on join value " + joinValue.utf8ToString());
+
+      int ord;
+      if (globalOrds != null) {
+        ord = (int) globalOrds.getGlobalOrds(context.ord).get(segmentOrd);
+      } else {
+        ord = segmentOrd;
       }
+      if (collector.match(ord) == false) {
+        return Explanation.noMatch("Not a match, join value " + Term.toString(joinValue));
       }
-      return Explanation.noMatch("Not a match");
+
+      float score = collector.score(ord);
+      return Explanation.match(score, "A match, join value " + Term.toString(joinValue));
     }
 
     @Override

lucene/join/src/test/org/apache/lucene/search/join/TestJoinUtil.java

@@ -48,6 +48,7 @@ import org.apache.lucene.index.BinaryDocValues;
 import org.apache.lucene.index.DirectoryReader;
 import org.apache.lucene.index.DocValues;
 import org.apache.lucene.index.IndexReader;
+import org.apache.lucene.index.IndexWriter;
 import org.apache.lucene.index.LeafReader;
 import org.apache.lucene.index.LeafReaderContext;
 import org.apache.lucene.index.MultiDocValues;
@@ -297,6 +298,108 @@ public class TestJoinUtil extends LuceneTestCase {
     dir.close();
   }
 
+  public void testOrdinalsJoinExplainNoMatches() throws Exception {
+    final String idField = "id";
+    final String productIdField = "productId";
+    // A field indicating to what type a document belongs, which is then used to distinques between documents during joining.
+    final String typeField = "type";
+    // A single sorted doc values field that holds the join values for all document types.
+    // Typically during indexing a schema will automatically create this field with the values
+    final String joinField = idField + productIdField;
+
+    Directory dir = newDirectory();
+    IndexWriter w = new IndexWriter(
+        dir, newIndexWriterConfig(new MockAnalyzer(random())).setMergePolicy(NoMergePolicy.INSTANCE)
+    );
+
+    // 0
+    Document doc = new Document();
+    doc.add(new TextField(idField, "1", Field.Store.NO));
+    doc.add(new TextField(typeField, "product", Field.Store.NO));
+    doc.add(new TextField("description", "random text", Field.Store.NO));
+    doc.add(new TextField("name", "name1", Field.Store.NO));
+    doc.add(new SortedDocValuesField(joinField, new BytesRef("1")));
+    w.addDocument(doc);
+
+    // 1
+    doc = new Document();
+    doc.add(new TextField(idField, "2", Field.Store.NO));
+    doc.add(new TextField(typeField, "product", Field.Store.NO));
+    doc.add(new TextField("description", "random text", Field.Store.NO));
+    doc.add(new TextField("name", "name2", Field.Store.NO));
+    doc.add(new SortedDocValuesField(joinField, new BytesRef("2")));
+    w.addDocument(doc);
+
+    // 2
+    doc = new Document();
+    doc.add(new TextField(productIdField, "1", Field.Store.NO));
+    doc.add(new TextField(typeField, "price", Field.Store.NO));
+    doc.add(new TextField("price", "10.0", Field.Store.NO));
+    doc.add(new SortedDocValuesField(joinField, new BytesRef("1")));
+    w.addDocument(doc);
+
+    // 3
+    doc = new Document();
+    doc.add(new TextField(productIdField, "2", Field.Store.NO));
+    doc.add(new TextField(typeField, "price", Field.Store.NO));
+    doc.add(new TextField("price", "20.0", Field.Store.NO));
+    doc.add(new SortedDocValuesField(joinField, new BytesRef("1")));
+    w.addDocument(doc);
+
+    if (random().nextBoolean()) {
+      w.flush();
+    }
+
+    // 4
+    doc = new Document();
+    doc.add(new TextField(productIdField, "3", Field.Store.NO));
+    doc.add(new TextField(typeField, "price", Field.Store.NO));
+    doc.add(new TextField("price", "5.0", Field.Store.NO));
+    doc.add(new SortedDocValuesField(joinField, new BytesRef("2")));
+    w.addDocument(doc);
+
+    // 5
+    doc = new Document();
+    doc.add(new TextField("field", "value", Field.Store.NO));
+    w.addDocument(doc);
+
+    IndexReader r = DirectoryReader.open(w);
+    IndexSearcher indexSearcher = new IndexSearcher(r);
+    SortedDocValues[] values = new SortedDocValues[r.leaves().size()];
+    for (int i = 0; i < values.length; i++) {
+      LeafReader leafReader =  r.leaves().get(i).reader();
+      values[i] = DocValues.getSorted(leafReader, joinField);
+    }
+    MultiDocValues.OrdinalMap ordinalMap = MultiDocValues.OrdinalMap.build(
+        r.getCoreCacheKey(), values, PackedInts.DEFAULT
+    );
+
+    Query toQuery = new TermQuery(new Term("price", "5.0"));
+    Query fromQuery = new TermQuery(new Term("name", "name2"));
+
+    for (ScoreMode scoreMode : ScoreMode.values()) {
+      Query joinQuery = JoinUtil.createJoinQuery(joinField, fromQuery, toQuery, indexSearcher, scoreMode, ordinalMap);
+      TopDocs result = indexSearcher.search(joinQuery, 10);
+      assertEquals(1, result.totalHits);
+      assertEquals(4, result.scoreDocs[0].doc); // doc with price: 5.0
+      Explanation explanation = indexSearcher.explain(joinQuery, 4);
+      assertTrue(explanation.isMatch());
+      assertEquals(explanation.getDescription(), "A match, join value 2");
+
+      explanation = indexSearcher.explain(joinQuery, 3);
+      assertFalse(explanation.isMatch());
+      assertEquals(explanation.getDescription(), "Not a match, join value 1");
+
+      explanation = indexSearcher.explain(joinQuery, 5);
+      assertFalse(explanation.isMatch());
+      assertEquals(explanation.getDescription(), "Not a match");
+    }
+
+    w.close();
+    indexSearcher.getIndexReader().close();
+    dir.close();
+  }
+
   public void testRandomOrdinalsJoin() throws Exception {
     IndexIterationContext context = createContext(512, false, true);
     int searchIters = 10;