diff --git a/src/rank/criterion/exact.rs b/src/rank/criterion/exact.rs
index 041a6ae67..6d33a6f38 100644
--- a/src/rank/criterion/exact.rs
+++ b/src/rank/criterion/exact.rs
@@ -2,9 +2,8 @@ use std::cmp::Ordering;
 use std::ops::Deref;
 
 use rocksdb::DB;
-use group_by::GroupBy;
 
-use crate::rank::{match_query_index, Document};
+use crate::rank::{Document, Matches};
 use crate::rank::criterion::Criterion;
 use crate::database::DatabaseView;
 use crate::Match;
@@ -15,8 +14,8 @@ fn contains_exact(matches: &[Match]) -> bool {
 }
 
 #[inline]
-fn number_exact_matches(matches: &[Match]) -> usize {
-    GroupBy::new(matches, match_query_index).map(contains_exact).count()
+fn number_exact_matches(matches: &Matches) -> usize {
+    matches.query_index_groups().map(contains_exact).count()
 }
 
 #[derive(Debug, Clone, Copy)]
diff --git a/src/rank/criterion/number_of_words.rs b/src/rank/criterion/number_of_words.rs
index 855d997ba..23cf36a2c 100644
--- a/src/rank/criterion/number_of_words.rs
+++ b/src/rank/criterion/number_of_words.rs
@@ -2,16 +2,14 @@ use std::cmp::Ordering;
 use std::ops::Deref;
 
 use rocksdb::DB;
-use group_by::GroupBy;
 
-use crate::rank::{match_query_index, Document};
+use crate::rank::{Document, Matches};
 use crate::rank::criterion::Criterion;
 use crate::database::DatabaseView;
-use crate::Match;
 
 #[inline]
-fn number_of_query_words(matches: &[Match]) -> usize {
-    GroupBy::new(matches, match_query_index).count()
+fn number_of_query_words(matches: &Matches) -> usize {
+    matches.query_index_groups().count()
 }
 
 #[derive(Debug, Clone, Copy)]
diff --git a/src/rank/criterion/sum_of_typos.rs b/src/rank/criterion/sum_of_typos.rs
index 409c37cb4..18a199113 100644
--- a/src/rank/criterion/sum_of_typos.rs
+++ b/src/rank/criterion/sum_of_typos.rs
@@ -3,21 +3,18 @@ use std::ops::Deref;
 
 use rocksdb::DB;
 
-use group_by::GroupBy;
-
-use crate::rank::{match_query_index, Document};
+use crate::rank::{Document, Matches};
 use crate::rank::criterion::Criterion;
 use crate::database::DatabaseView;
-use crate::Match;
 
 #[inline]
-fn sum_matches_typos(matches: &[Match]) -> i8 {
+fn sum_matches_typos(matches: &Matches) -> i8 {
     let mut sum_typos = 0;
     let mut number_words = 0;
 
     // note that GroupBy will never return an empty group
     // so we can do this assumption safely
-    for group in GroupBy::new(matches, match_query_index) {
+    for group in matches.query_index_groups() {
         sum_typos += unsafe { group.get_unchecked(0).distance } as i8;
         number_words += 1;
     }
@@ -44,7 +41,7 @@ where D: Deref<Target=DB>
 mod tests {
     use super::*;
 
-    use crate::{DocumentId, Attribute, WordArea};
+    use crate::{Match, DocumentId, Attribute, WordArea};
 
     // typing: "Geox CEO"
     //
diff --git a/src/rank/criterion/sum_of_words_attribute.rs b/src/rank/criterion/sum_of_words_attribute.rs
index 718ae7447..cfcaaec68 100644
--- a/src/rank/criterion/sum_of_words_attribute.rs
+++ b/src/rank/criterion/sum_of_words_attribute.rs
@@ -2,19 +2,17 @@ use std::cmp::Ordering;
 use std::ops::Deref;
 
 use rocksdb::DB;
-use group_by::GroupBy;
 
+use crate::rank::{Document, Matches};
 use crate::database::DatabaseView;
-use crate::rank::{match_query_index, Document};
 use crate::rank::criterion::Criterion;
-use crate::Match;
 
 #[inline]
-fn sum_matches_attributes(matches: &[Match]) -> u16 {
+fn sum_matches_attributes(matches: &Matches) -> u16 {
     // note that GroupBy will never return an empty group
     // so we can do this assumption safely
-    GroupBy::new(matches, match_query_index).map(|group| unsafe {
-        group.get_unchecked(0).attribute.attribute()
+    matches.query_index_groups().map(|group| {
+        unsafe { group.get_unchecked(0).attribute.attribute() }
     }).sum()
 }
 
diff --git a/src/rank/criterion/sum_of_words_position.rs b/src/rank/criterion/sum_of_words_position.rs
index d0ebaa74f..e1d650534 100644
--- a/src/rank/criterion/sum_of_words_position.rs
+++ b/src/rank/criterion/sum_of_words_position.rs
@@ -2,19 +2,17 @@ use std::cmp::Ordering;
 use std::ops::Deref;
 
 use rocksdb::DB;
-use group_by::GroupBy;
 
-use crate::database::DatabaseView;
-use crate::rank::{match_query_index, Document};
+use crate::rank::{Document, Matches};
 use crate::rank::criterion::Criterion;
-use crate::Match;
+use crate::database::DatabaseView;
 
 #[inline]
-fn sum_matches_attribute_index(matches: &[Match]) -> u32 {
+fn sum_matches_attribute_index(matches: &Matches) -> u32 {
     // note that GroupBy will never return an empty group
     // so we can do this assumption safely
-    GroupBy::new(matches, match_query_index).map(|group| unsafe {
-        group.get_unchecked(0).attribute.word_index()
+    matches.query_index_groups().map(|group| {
+        unsafe { group.get_unchecked(0).attribute.word_index() }
     }).sum()
 }
 
diff --git a/src/rank/criterion/words_proximity.rs b/src/rank/criterion/words_proximity.rs
index fc80dfaec..41f4b49b8 100644
--- a/src/rank/criterion/words_proximity.rs
+++ b/src/rank/criterion/words_proximity.rs
@@ -2,9 +2,8 @@ use std::cmp::{self, Ordering};
 use std::ops::Deref;
 
 use rocksdb::DB;
-use group_by::GroupBy;
 
-use crate::rank::{match_query_index, Document};
+use crate::rank::{Document, Matches};
 use crate::rank::criterion::Criterion;
 use crate::database::DatabaseView;
 use crate::Match;
@@ -34,9 +33,9 @@ fn min_proximity(lhs: &[Match], rhs: &[Match]) -> u32 {
     min_prox
 }
 
-fn matches_proximity(matches: &[Match]) -> u32 {
+fn matches_proximity(matches: &Matches) -> u32 {
     let mut proximity = 0;
-    let mut iter = GroupBy::new(matches, match_query_index);
+    let mut iter = matches.query_index_groups();
 
     // iterate over groups by windows of size 2
     let mut last = iter.next();
@@ -91,6 +90,7 @@ mod tests {
         //   soup -> of = 8
         // + of -> the  = 1
         // + the -> day = 8 (not 1)
+        let matches = Matches::from_unsorted_matches(matches.to_vec());
         assert_eq!(matches_proximity(matches), 17);
     }
 
diff --git a/src/rank/mod.rs b/src/rank/mod.rs
index 4d1b6b1ea..0d3f538d0 100644
--- a/src/rank/mod.rs
+++ b/src/rank/mod.rs
@@ -2,6 +2,11 @@ pub mod criterion;
 mod query_builder;
 mod distinct_map;
 
+use std::slice::Windows;
+
+use sdset::SetBuf;
+use group_by::GroupBy;
+
 use crate::{Match, DocumentId};
 
 pub use self::query_builder::{FilterFunc, QueryBuilder, DistinctQueryBuilder};
@@ -14,20 +19,70 @@ fn match_query_index(a: &Match, b: &Match) -> bool {
 #[derive(Debug, Clone)]
 pub struct Document {
     pub id: DocumentId,
-    pub matches: Vec<Match>,
+    pub matches: Matches,
 }
 
 impl Document {
     pub fn new(doc: DocumentId, match_: Match) -> Self {
-        unsafe { Self::from_sorted_matches(doc, vec![match_]) }
+        let matches = SetBuf::new_unchecked(vec![match_]);
+        Self::from_matches(doc, matches)
     }
 
-    pub fn from_matches(doc: DocumentId, mut matches: Vec<Match>) -> Self {
-        matches.sort_unstable();
-        unsafe { Self::from_sorted_matches(doc, matches) }
-    }
+    pub fn from_matches(id: DocumentId, matches: SetBuf<Match>) -> Self {
+        let mut last = 0;
+        let mut slices = vec![0];
+        for group in GroupBy::new(&matches, match_query_index) {
+            let index = last + group.len();
+            slices.push(index);
+            last = index;
+        }
 
-    pub unsafe fn from_sorted_matches(id: DocumentId, matches: Vec<Match>) -> Self {
+        let matches = Matches { matches, slices };
         Self { id, matches }
     }
+
+    pub fn from_unsorted_matches(doc: DocumentId, mut matches: Vec<Match>) -> Self {
+        matches.sort_unstable();
+        let matches = SetBuf::new_unchecked(matches);
+        Self::from_matches(doc, matches)
+    }
 }
+
+#[derive(Debug, Clone)]
+pub struct Matches {
+    matches: SetBuf<Match>,
+    slices: Vec<usize>,
+}
+
+impl Matches {
+    pub fn query_index_groups(&self) -> QueryIndexGroups {
+        QueryIndexGroups {
+            matches: &self.matches,
+            windows: self.slices.windows(2),
+        }
+    }
+}
+
+pub struct QueryIndexGroups<'a, 'b> {
+    matches: &'a [Match],
+    windows: Windows<'b, usize>,
+}
+
+impl<'a, 'b> Iterator for QueryIndexGroups<'a, 'b> {
+    type Item = &'a [Match];
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.windows.next().map(|range| {
+            match *range {
+                [left, right] => &self.matches[left..right],
+                _             => unreachable!()
+            }
+        })
+    }
+}
+
+// impl ExactSizeIterator for QueryIndexGroups<'_, '_> {
+//     fn len(&self) -> usize {
+//         self.windows.len() // FIXME (+1) ?
+//     }
+// }
diff --git a/src/rank/query_builder.rs b/src/rank/query_builder.rs
index 4df983f8a..9a17e0473 100644
--- a/src/rank/query_builder.rs
+++ b/src/rank/query_builder.rs
@@ -116,7 +116,7 @@ where D: Deref<Target=DB>,
             }
         }
 
-        matches.into_iter().map(|(id, matches)| Document::from_matches(id, matches)).collect()
+        matches.into_iter().map(|(id, m)| Document::from_unsorted_matches(id, m)).collect()
     }
 }