LUCENE-9446: In boolean rewrite, remove MatchAllDocsQuery filter clau…

…ses (#1709)

Previously, we only removed 'match all' FILTER clauses if there was at least one
MUST clause. Now they're also removed if there is another distinct FILTER clause.

This lets boolean queries like `#field:value #*:*` be written to `#field:value`.

lucene/CHANGES.txt

@@ -31,6 +31,9 @@ Improvements
 * LUCENE-9440: FieldInfo#checkConsistency called twice from Lucene50(60)FieldInfosFormat#read;
   Removed the (redundant?) assert and do these checks for real. (Yauheni Putsykovich)
 
+* LUCENE-9446: In BooleanQuery rewrite, always remove MatchAllDocsQuery filter clauses
+  when possible. (Julie Tibshirani)
+
 Optimizations
 ---------------------
 

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

@@ -315,11 +315,13 @@ public Query rewrite(IndexReader reader) throws IOException {
       }
     }
 
-    // remove FILTER clauses that are also MUST clauses
-    // or that match all documents
-    if (clauseSets.get(Occur.MUST).size() > 0 && clauseSets.get(Occur.FILTER).size() > 0) {
-      final Set<Query> filters = new HashSet<Query>(clauseSets.get(Occur.FILTER));
-      boolean modified = filters.remove(new MatchAllDocsQuery());
+    // remove FILTER clauses that are also MUST clauses or that match all documents
+    if (clauseSets.get(Occur.FILTER).size() > 0) {
+      final Set<Query> filters = new HashSet<>(clauseSets.get(Occur.FILTER));
+      boolean modified = false;
+      if (filters.size() > 1 || clauseSets.get(Occur.MUST).isEmpty() == false) {
+        modified = filters.remove(new MatchAllDocsQuery());
+      }
       modified |= filters.removeAll(clauseSets.get(Occur.MUST));
       if (modified) {
         BooleanQuery.Builder builder = new BooleanQuery.Builder();

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

@@ -312,12 +312,28 @@ public void testRemoveMatchAllFilter() throws IOException {
         .add(new TermQuery(new Term("foo", "baz")), Occur.MUST)
         .add(new MatchAllDocsQuery(), Occur.FILTER)
         .build();
-    BooleanQuery expected = new BooleanQuery.Builder()
+    Query expected = new BooleanQuery.Builder()
         .setMinimumNumberShouldMatch(bq.getMinimumNumberShouldMatch())
         .add(new TermQuery(new Term("foo", "bar")), Occur.MUST)
         .add(new TermQuery(new Term("foo", "baz")), Occur.MUST)
         .build();
     assertEquals(expected, searcher.rewrite(bq));
+
+    bq = new BooleanQuery.Builder()
+            .add(new TermQuery(new Term("foo", "bar")), Occur.FILTER)
+            .add(new MatchAllDocsQuery(), Occur.FILTER)
+            .build();
+    expected = new BoostQuery(new ConstantScoreQuery(
+            new TermQuery(new Term("foo", "bar"))), 0.0f);
+    assertEquals(expected, searcher.rewrite(bq));
+
+    bq = new BooleanQuery.Builder()
+            .add(new MatchAllDocsQuery(), Occur.FILTER)
+            .add(new MatchAllDocsQuery(), Occur.FILTER)
+            .build();
+    expected = new BoostQuery(new ConstantScoreQuery(
+            new MatchAllDocsQuery()), 0.0f);
+    assertEquals(expected, searcher.rewrite(bq));
   }
 
   public void testRandom() throws IOException {