Index: src/java/org/apache/lucene/search/CachingWrapperFilter.java
===================================================================
--- src/java/org/apache/lucene/search/CachingWrapperFilter.java	(revision 814707)
+++ src/java/org/apache/lucene/search/CachingWrapperFilter.java	(working copy)
@@ -19,6 +19,7 @@
 
 import org.apache.lucene.index.IndexReader;
 import org.apache.lucene.util.DocIdBitSet;
+import org.apache.lucene.util.OpenBitSet;
 import java.util.BitSet;
 import java.util.WeakHashMap;
 import java.util.Map;
@@ -30,6 +31,7 @@
  */
 public class CachingWrapperFilter extends Filter {
   protected Filter filter;
+  protected boolean cacheAsOpenBitSet = true;
 
   /**
    * A transient Filter cache.
@@ -42,6 +44,20 @@
   public CachingWrapperFilter(Filter filter) {
     this.filter = filter;
   }
+  
+  /**
+   * You can specify if the wrapper should cache all DocIdSet that are not implemented by {@link OpenBitSet}
+   * or {@link DocIdBitSet} as such ones. This defaults to <code>true</code>.
+   * Set this to <code>false</code> if you have a very special DocIdSet implementation that is easily cacheable.
+   */
+  public void setCacheAsOpenBitSet(boolean value) {
+    cacheAsOpenBitSet = value;
+  }
+ 
+  /** @see #setCacheAsOpenBitSet */
+  public boolean getCacheAsOpenBitSet() {
+    return cacheAsOpenBitSet;
+  }
 
   /**
    * @deprecated Use {@link #getDocIdSet(IndexReader)} instead.
@@ -75,10 +91,22 @@
 
   /** Provide the DocIdSet to be cached, using the DocIdSet provided
    *  by the wrapped Filter.
-   *  This implementation returns the given DocIdSet.
+   *  This implementation returns the given {@link DocIdSet}, if {@link #getCacheAsOpenBitSet}
+   *  returns <code>false</code>. The default is to wrap all {@link DocIdSet}s, that are not implemented
+   *  by {@link OpenBitSet} or {@link DocIdBitSet}.
    */
-  protected DocIdSet docIdSetToCache(DocIdSet docIdSet, IndexReader reader) {
-    return docIdSet;
+  protected DocIdSet docIdSetToCache(DocIdSet docIdSet, IndexReader reader) throws IOException {
+    if (cacheAsOpenBitSet && !(docIdSet instanceof OpenBitSet || docIdSet instanceof DocIdBitSet)) {
+      final OpenBitSet bits = new OpenBitSet(reader.maxDoc());
+      final DocIdSetIterator it = docIdSet.iterator();
+      int doc;
+      while ((doc = it.nextDoc()) != DocIdSetIterator.NO_MORE_DOCS) {
+        bits.fastSet(doc);
+      }
+      return bits;
+    } else {
+      return docIdSet;
+    }
   }
   
   public DocIdSet getDocIdSet(IndexReader reader) throws IOException {