diff --git a/src/postings/mod.rs b/src/postings/mod.rs
index efcdc811a..c4d0162b7 100644
--- a/src/postings/mod.rs
+++ b/src/postings/mod.rs
@@ -31,7 +31,7 @@ pub use self::vec_postings::VecPostings;
 
 pub use self::segment_postings::{BlockSegmentPostings, SegmentPostings};
 pub use self::intersection::Intersection;
-pub use self::union::UnionDocSet;
+pub use self::union::Union;
 
 pub use common::HasLen;
 
diff --git a/src/postings/union.rs b/src/postings/union.rs
index 9efdcfcfb..575a0f69b 100644
--- a/src/postings/union.rs
+++ b/src/postings/union.rs
@@ -3,22 +3,25 @@ use postings::SkipResult;
 use common::TinySet;
 use std::cmp::Ordering;
 use DocId;
+use query::score_combiner::{DoNothingCombiner, ScoreCombiner};
 
 
-const HORIZON_NUM_TINYBITSETS: usize = 2048;
+const HORIZON_NUM_TINYBITSETS: usize = 32;
 const HORIZON: u32 = 64u32 * HORIZON_NUM_TINYBITSETS as u32;
 
 /// Creates a `DocSet` that iterator through the intersection of two `DocSet`s.
-pub struct UnionDocSet<TDocSet: DocSet> {
+pub struct Union<TDocSet: DocSet, TScoreCombiner=DoNothingCombiner>
+    where TDocSet: DocSet,  TScoreCombiner: ScoreCombiner {
     docsets: Vec<TDocSet>,
     bitsets: Box<[TinySet; HORIZON_NUM_TINYBITSETS]>,
+    scores: Box<[TScoreCombiner; HORIZON as usize]>,
     cursor: usize,
     offset: DocId,
     doc: DocId,
 }
 
-impl<TDocSet: DocSet> From<Vec<TDocSet>> for UnionDocSet<TDocSet> {
-    fn from(docsets: Vec<TDocSet>) -> UnionDocSet<TDocSet> {
+impl<TDocSet: DocSet, TScoreCombiner: ScoreCombiner> From<Vec<TDocSet>> for Union<TDocSet, TScoreCombiner> {
+    fn from(docsets: Vec<TDocSet>) -> Union<TDocSet> {
         let non_empty_docsets: Vec<TDocSet> =
             docsets
                 .into_iter()
@@ -30,9 +33,10 @@ impl<TDocSet: DocSet> From<Vec<TDocSet>> for UnionDocSet<TDocSet> {
                     }
                 })
                 .collect();
-        UnionDocSet {
+        Union {
             docsets: non_empty_docsets,
             bitsets: Box::new([TinySet::empty(); HORIZON_NUM_TINYBITSETS]),
+            scores: Box::new([TScoreCombiner::default(); HORIZON]),
             cursor: HORIZON_NUM_TINYBITSETS,
             offset: 0,
             doc: 0
@@ -61,7 +65,7 @@ fn refill<TDocSet: DocSet>(docsets: &mut Vec<TDocSet>, bitsets: &mut [TinySet; H
         });
 }
 
-impl<TDocSet: DocSet> UnionDocSet<TDocSet> {
+impl<TDocSet: DocSet, TScoreCombiner: ScoreCombiner> Union<TDocSet, TScoreCombiner> {
     fn refill(&mut self) -> bool {
         if let Some(min_doc) = self.docsets
             .iter_mut()
@@ -90,7 +94,7 @@ impl<TDocSet: DocSet> UnionDocSet<TDocSet> {
     }
 }
 
-impl<TDocSet: DocSet> DocSet for UnionDocSet<TDocSet> {
+impl<TDocSet: DocSet, TScoreCombiner: ScoreCombiner> DocSet for Union<TDocSet, TScoreCombiner> {
 
     fn advance(&mut self) -> bool {
         if self.advance_buffered() {
@@ -190,7 +194,7 @@ impl<TDocSet: DocSet> DocSet for UnionDocSet<TDocSet> {
 #[cfg(test)]
 mod tests {
 
-    use super::UnionDocSet;
+    use super::Union;
     use postings::{VecPostings, DocSet};
     use tests;
     use test::Bencher;
@@ -214,7 +218,7 @@ mod tests {
             .collect();
         let mut union_expected = VecPostings::from(union_vals);
 
-        let mut union = UnionDocSet::from(
+        let mut union = Union::from(
             vals.into_iter()
                 .map(VecPostings::from)
                 .collect::<Vec<VecPostings>>()
@@ -260,7 +264,7 @@ mod tests {
             }
         }
         let docset_factory = || {
-            let res: Box<DocSet> = box UnionDocSet::from(
+            let res: Box<DocSet> = box Union::from(
                 docs_list
                     .iter()
                     .map(|docs| docs.clone())
@@ -298,7 +302,7 @@ mod tests {
 
     #[test]
     fn test_union_skip_corner_case3() {
-        let mut docset = UnionDocSet::from(vec![
+        let mut docset = Union::from(vec![
                 VecPostings::from(vec![0u32, 5u32]),
                 VecPostings::from(vec![1u32, 4u32]),
         ]);
@@ -338,7 +342,7 @@ mod tests {
             tests::sample_with_seed(100_000, 0.2, 1),
         ];
         bench.iter(|| {
-            let mut v = UnionDocSet::from(union_docset.iter()
+            let mut v = Union::from(union_docset.iter()
                 .map(|doc_ids| VecPostings::from(doc_ids.clone()))
                 .collect::<Vec<VecPostings>>());
             while v.advance() {};
@@ -352,7 +356,7 @@ mod tests {
             tests::sample_with_seed(100_000, 0.001, 2)
         ];
         bench.iter(|| {
-            let mut v = UnionDocSet::from(union_docset.iter()
+            let mut v = Union::from(union_docset.iter()
                 .map(|doc_ids| VecPostings::from(doc_ids.clone()))
                 .collect::<Vec<VecPostings>>());
             while v.advance() {};
diff --git a/src/query/boolean_query/boolean_scorer.rs b/src/query/boolean_query/boolean_scorer.rs
index 779c908a6..0b28c58e0 100644
--- a/src/query/boolean_query/boolean_scorer.rs
+++ b/src/query/boolean_query/boolean_scorer.rs
@@ -4,7 +4,7 @@ use std::collections::BinaryHeap;
 use std::cmp::Ordering;
 use postings::DocSet;
 use query::OccurFilter;
-use query::boolean_query::ScoreCombiner;
+use query::score_combiner::{ScoreCombiner, SumWithCoordsCombiner};
 
 /// Each `HeapItem` represents the head of
 /// one of scorer being merged.
diff --git a/src/query/boolean_query/boolean_weight.rs b/src/query/boolean_query/boolean_weight.rs
index fef4b870e..e6e9cfd0a 100644
--- a/src/query/boolean_query/boolean_weight.rs
+++ b/src/query/boolean_query/boolean_weight.rs
@@ -1,6 +1,6 @@
 use query::Weight;
 use core::SegmentReader;
-use postings::{Intersection, UnionDocSet};
+use postings::{Intersection, Union};
 use std::collections::HashMap;
 use query::EmptyScorer;
 use query::Scorer;
@@ -22,7 +22,7 @@ fn scorer_union<'a>(docsets: Vec<Box<Scorer + 'a>>) -> Box<Scorer + 'a> {
             .unwrap() //< we checked the size beforehands
     } else {
         // TODO have a UnionScorer instead.
-        box ConstScorer::new(UnionDocSet::from(docsets))
+        box ConstScorer::new(Union::from(docsets))
     }
 }
 
diff --git a/src/query/boolean_query/mod.rs b/src/query/boolean_query/mod.rs
index e01e77e52..524da536f 100644
--- a/src/query/boolean_query/mod.rs
+++ b/src/query/boolean_query/mod.rs
@@ -1,11 +1,9 @@
 mod boolean_query;
 mod boolean_scorer;
 mod boolean_weight;
-mod score_combiner;
 
 pub use self::boolean_query::BooleanQuery;
 pub use self::boolean_scorer::BooleanScorer;
-pub use self::score_combiner::ScoreCombiner;
 
 #[cfg(test)]
 mod tests {
diff --git a/src/query/boolean_query/score_combiner.rs b/src/query/boolean_query/score_combiner.rs
deleted file mode 100644
index f6d9c8944..000000000
--- a/src/query/boolean_query/score_combiner.rs
+++ /dev/null
@@ -1,45 +0,0 @@
-use Score;
-
-pub struct ScoreCombiner {
-    coords: Vec<Score>,
-    num_fields: usize,
-    score: Score,
-}
-
-impl ScoreCombiner {
-    pub fn update(&mut self, score: Score) {
-        self.score += score;
-        self.num_fields += 1;
-    }
-
-    pub fn clear(&mut self) {
-        self.score = 0f32;
-        self.num_fields = 0;
-    }
-
-    /// Compute the coord term
-    fn coord(&self) -> f32 {
-        self.coords[self.num_fields]
-    }
-
-    pub fn score(&self) -> Score {
-        self.score * self.coord()
-    }
-
-    pub fn default_for_num_scorers(num_scorers: usize) -> ScoreCombiner {
-        let query_coords: Vec<Score> = (0..num_scorers + 1)
-            .map(|i| (i as Score) / (num_scorers as Score))
-            .collect();
-        ScoreCombiner::from(query_coords)
-    }
-}
-
-impl From<Vec<Score>> for ScoreCombiner {
-    fn from(coords: Vec<Score>) -> ScoreCombiner {
-        ScoreCombiner {
-            coords,
-            num_fields: 0,
-            score: 0f32,
-        }
-    }
-}
diff --git a/src/query/mod.rs b/src/query/mod.rs
index 85353af90..f77f48972 100644
--- a/src/query/mod.rs
+++ b/src/query/mod.rs
@@ -17,6 +17,7 @@ mod range_query;
 mod exclude;
 mod reqopt_scorer;
 
+pub mod score_combiner;
 pub use self::reqopt_scorer::RequiredOptionalScorer;
 pub use self::exclude::Exclude;
 pub use self::bitset::BitSetDocSet;
diff --git a/src/query/score_combiner.rs b/src/query/score_combiner.rs
new file mode 100644
index 000000000..b20ea851a
--- /dev/null
+++ b/src/query/score_combiner.rs
@@ -0,0 +1,67 @@
+use Score;
+
+pub trait ScoreCombiner: Default + Copy {
+    fn update(&mut self, score: Score);
+    fn clear(&mut self);
+    fn score(&self) -> Score;
+}
+
+
+pub struct DoNothingCombiner;
+impl ScoreCombiner for DoNothingCombiner {
+    fn update(&mut self, score: Score) {}
+
+    fn clear(&mut self) {}
+
+    fn score(&self) -> Score {
+        1f32
+    }
+}
+
+pub struct SumWithCoordsCombiner {
+    coords: Vec<Score>,
+    num_fields: usize,
+    score: Score,
+}
+
+impl ScoreCombiner for SumWithCoordsCombiner {
+    fn update(&mut self, score: Score) {
+        self.score += score;
+        self.num_fields += 1;
+    }
+
+    fn clear(&mut self) {
+        self.score = 0f32;
+        self.num_fields = 0;
+    }
+
+    fn score(&self) -> Score {
+        self.score * self.coord()
+    }
+
+}
+
+impl SumWithCoordsCombiner {
+    /// Compute the coord term
+    fn coord(&self) -> f32 {
+        self.coords[self.num_fields]
+    }
+
+
+    pub fn default_for_num_scorers(num_scorers: usize) -> Self {
+        let query_coords: Vec<Score> = (0..num_scorers + 1)
+            .map(|i| (i as Score) / (num_scorers as Score))
+            .collect();
+        ScoreCombiner::from(query_coords)
+    }
+}
+
+impl From<Vec<Score>> for ScoreCombiner {
+    fn from(coords: Vec<Score>) -> SumWithCoordsCombiner {
+        SumWithCoordsCombiner {
+            coords,
+            num_fields: 0,
+            score: 0f32,
+        }
+    }
+}