Index: CHANGES.txt
===================================================================
--- CHANGES.txt	(revision 888320)
+++ CHANGES.txt	(working copy)
@@ -50,6 +50,11 @@
   lots of seeking (Mike McCandless, Uwe Schindler, Robert Muir, Yonik
   Seeley)
 
+* LUCENE-2136: If the multi reader (DirectoryReader or MultiReader)
+  only has a single sub-reader, delegate all enum requests to it.
+  This avoid the overhead of using a PQ unecessarily.  (Mike
+  McCandless)
+
 Build
 
 Test Cases
Index: src/java/org/apache/lucene/index/MultiReader.java
===================================================================
--- src/java/org/apache/lucene/index/MultiReader.java	(revision 888320)
+++ src/java/org/apache/lucene/index/MultiReader.java	(working copy)
@@ -103,7 +103,12 @@
 
   @Override
   public Fields fields() throws IOException {
-    return fields;
+    if (subReaders.length == 1) {
+      // Optimize the single reader case
+      return subReaders[0].fields();
+    } else {
+      return fields;
+    }
   }
 
   /**
Index: src/java/org/apache/lucene/index/DirectoryReader.java
===================================================================
--- src/java/org/apache/lucene/index/DirectoryReader.java	(revision 888320)
+++ src/java/org/apache/lucene/index/DirectoryReader.java	(working copy)
@@ -763,10 +763,15 @@
     ensureOpen();
     return new MultiTermDocs(this, subReaders, starts);
   }
-  
+
   @Override
   public Fields fields() throws IOException {
-    return fields;
+    if (subReaders.length == 1) {
+      // Optimize the single reader case
+      return subReaders[0].fields();
+    } else {
+      return fields;
+    }
   }
 
   @Override
Index: src/java/org/apache/lucene/index/ParallelReader.java
===================================================================
--- src/java/org/apache/lucene/index/ParallelReader.java	(revision 888320)
+++ src/java/org/apache/lucene/index/ParallelReader.java	(working copy)
@@ -151,7 +151,12 @@
     @Override
     public TermsEnum terms() throws IOException {
       assert currentReader != null;
-      return currentReader.fields().terms(currentField).iterator();
+      Terms terms = currentReader.fields().terms(currentField);
+      if (terms != null) {
+        return terms.iterator();
+      } else {
+        return null;
+      }
     }
   }
 
Index: contrib/regex/src/test/org/apache/lucene/search/regex/TestRegexQuery.java
===================================================================
--- contrib/regex/src/test/org/apache/lucene/search/regex/TestRegexQuery.java	(revision 888320)
+++ contrib/regex/src/test/org/apache/lucene/search/regex/TestRegexQuery.java	(working copy)
@@ -83,7 +83,6 @@
   public void testMatchAll() throws Exception {
     RegexTermsEnum terms = (RegexTermsEnum) new RegexQuery(new Term(FN, "jum.")).getTermsEnum(searcher.getIndexReader());
     // no term should match
-    assertNull(terms.term());
     assertNull(terms.next());
   }