Index: IndexWriter.java
===================================================================
--- IndexWriter.java	(revision 429606)
+++ IndexWriter.java	(working copy)
@@ -110,6 +110,7 @@
   private SegmentInfos segmentInfos = new SegmentInfos(); // the segments
   private final Directory ramDirectory = new RAMDirectory(); // for temp segs
 
+  private int singleDocSegmentsCount = 0; // for speeding decision on merge candidates 
   private Lock writeLock;
 
   private int termIndexInterval = DEFAULT_TERM_INDEX_INTERVAL;
@@ -509,6 +510,7 @@
     dw.addDocument(segmentName, doc);
     synchronized (this) {
       segmentInfos.addElement(new SegmentInfo(segmentName, 1, ramDirectory));
+      singleDocSegmentsCount++;
       maybeMergeSegments();
     }
   }
@@ -694,8 +696,8 @@
     long targetMergeDocs = minMergeDocs;
     while (targetMergeDocs <= maxMergeDocs) {
       // find segments smaller than current target size
-      int minSegment = segmentInfos.size();
-      int mergeDocs = 0;
+      int minSegment = segmentInfos.size() - singleDocSegmentsCount; // top 1-doc segments are taken for sure
+      int mergeDocs = singleDocSegmentsCount;
       while (--minSegment >= 0) {
         SegmentInfo si = segmentInfos.info(minSegment);
         if (si.docCount >= targetMergeDocs)
@@ -703,10 +705,12 @@
         mergeDocs += si.docCount;
       }
 
-      if (mergeDocs >= targetMergeDocs)		  // found a merge to do
+      if (mergeDocs >= targetMergeDocs)	{	  // found a merge to do
         mergeSegments(minSegment+1);
-      else
+        singleDocSegmentsCount = 0;  
+      } else {
         break;
+      }
 
       targetMergeDocs *= mergeFactor;		  // increase target size
     }