LUCENE-7874: DisjunctionMaxQuery rewrites to a BooleanQuery when tiebreaker is set to 1.

lucene/CHANGES.txt

@@ -112,6 +112,8 @@ Optimizations
   query is a point (for 2D) or a is a simple date interval (e.g. 1 month).  When
   the strategy is marked as pointsOnly, the results is a TermQuery. (David Smiley)
 
+* LUCENE-7874: DisjunctionMaxQuery rewrites to a BooleanQuery when tiebreaker is set to 1. (Jim Ferenczi)
+
 Other
 
 * LUCENE-7328: Remove LegacyNumericEncoding from GeoPointField. (Nick Knize)

lucene/core/src/java/org/apache/lucene/search/DisjunctionMaxQuery.java

@@ -178,6 +178,14 @@ public final class DisjunctionMaxQuery extends Query implements Iterable<Query>
       return disjuncts[0];
     }
 
+    if (tieBreakerMultiplier == 1.0f) {
+      BooleanQuery.Builder builder = new BooleanQuery.Builder();
+      for (Query sub : disjuncts) {
+        builder.add(sub, BooleanClause.Occur.SHOULD);
+      }
+      return builder.build();
+    }
+
     boolean actuallyRewritten = false;
     List<Query> rewrittenDisjuncts = new ArrayList<>();
     for (Query sub : disjuncts) {

lucene/core/src/test/org/apache/lucene/search/TestDisjunctionMaxQuery.java

@@ -524,6 +524,21 @@ public class TestDisjunctionMaxQuery extends LuceneTestCase {
     }
   }
 
+  public void testRewriteBoolean() throws Exception {
+    Query sub1 = tq("hed", "albino");
+    Query sub2 = tq("hed", "elephant");
+    DisjunctionMaxQuery q = new DisjunctionMaxQuery(
+        Arrays.asList(
+            sub1, sub2
+        ), 1.0f);
+    Query rewritten = s.rewrite(q);
+    assertTrue(rewritten instanceof BooleanQuery);
+    BooleanQuery bq = (BooleanQuery) rewritten;
+    assertEquals(bq.clauses().size(), 2);
+    assertEquals(bq.clauses().get(0), new BooleanClause(sub1, BooleanClause.Occur.SHOULD));
+    assertEquals(bq.clauses().get(1), new BooleanClause(sub2, BooleanClause.Occur.SHOULD));
+  }
+  
   /** macro */
   protected Query tq(String f, String t) {
     return new TermQuery(new Term(f, t));