Index: CHANGES.txt
===================================================================
--- CHANGES.txt	(revision 888322)
+++ CHANGES.txt	(working copy)
@@ -70,6 +70,11 @@
   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 888322)
+++ src/java/org/apache/lucene/index/MultiReader.java	(working copy)
@@ -339,13 +339,23 @@
   @Override
   public TermEnum terms() throws IOException {
     ensureOpen();
-    return new MultiTermEnum(this, subReaders, starts, null);
+    if (subReaders.length == 1) {
+      // Optimize single segment case:
+      return subReaders[0].terms();
+    } else {
+      return new MultiTermEnum(this, subReaders, starts, null);
+    }
   }
 
   @Override
   public TermEnum terms(Term term) throws IOException {
     ensureOpen();
-    return new MultiTermEnum(this, subReaders, starts, term);
+    if (subReaders.length == 1) {
+      // Optimize single segment case:
+      return subReaders[0].terms(term);
+    } else {
+      return new MultiTermEnum(this, subReaders, starts, term);
+    }
   }
 
   @Override
@@ -360,13 +370,34 @@
   @Override
   public TermDocs termDocs() throws IOException {
     ensureOpen();
-    return new MultiTermDocs(this, subReaders, starts);
+    if (subReaders.length == 1) {
+      // Optimize single segment case:
+      return subReaders[0].termDocs();
+    } else {
+      return new MultiTermDocs(this, subReaders, starts);
+    }
   }
 
   @Override
+  public TermDocs termDocs(Term term) throws IOException {
+    ensureOpen();
+    if (subReaders.length == 1) {
+      // Optimize single segment case:
+      return subReaders[0].termDocs(term);
+    } else {
+      return super.termDocs(term);
+    }
+  }
+
+  @Override
   public TermPositions termPositions() throws IOException {
     ensureOpen();
-    return new MultiTermPositions(this, subReaders, starts);
+    if (subReaders.length == 1) {
+      // Optimize single segment case:
+      return subReaders[0].termPositions();
+    } else {
+      return new MultiTermPositions(this, subReaders, starts);
+    }
   }
 
   @Override
Index: src/java/org/apache/lucene/index/DirectoryReader.java
===================================================================
--- src/java/org/apache/lucene/index/DirectoryReader.java	(revision 888322)
+++ src/java/org/apache/lucene/index/DirectoryReader.java	(working copy)
@@ -627,13 +627,23 @@
   @Override
   public TermEnum terms() throws IOException {
     ensureOpen();
-    return new MultiTermEnum(this, subReaders, starts, null);
+    if (subReaders.length == 1) {
+      // Optimize single segment case:
+      return subReaders[0].terms();
+    } else {
+      return new MultiTermEnum(this, subReaders, starts, null);
+    }
   }
 
   @Override
   public TermEnum terms(Term term) throws IOException {
     ensureOpen();
-    return new MultiTermEnum(this, subReaders, starts, term);
+    if (subReaders.length == 1) {
+      // Optimize single segment case:
+      return subReaders[0].terms(term);
+    } else {
+      return new MultiTermEnum(this, subReaders, starts, term);
+    }
   }
 
   @Override
@@ -648,13 +658,34 @@
   @Override
   public TermDocs termDocs() throws IOException {
     ensureOpen();
-    return new MultiTermDocs(this, subReaders, starts);
+    if (subReaders.length == 1) {
+      // Optimize single segment case:
+      return subReaders[0].termDocs();
+    } else {
+      return new MultiTermDocs(this, subReaders, starts);
+    }
   }
 
   @Override
+  public TermDocs termDocs(Term term) throws IOException {
+    ensureOpen();
+    if (subReaders.length == 1) {
+      // Optimize single segment case:
+      return subReaders[0].termDocs(term);
+    } else {
+      return super.termDocs(term);
+    }
+  }
+
+  @Override
   public TermPositions termPositions() throws IOException {
     ensureOpen();
-    return new MultiTermPositions(this, subReaders, starts);
+    if (subReaders.length == 1) {
+      // Optimize single segment case:
+      return subReaders[0].termPositions();
+    } else {
+      return new MultiTermPositions(this, subReaders, starts);
+    }
   }
 
   /**