Merge #3423

3423: Add min and max facet stats r=dureuill a=dureuill

# Pull Request

## Related issue
Fixes #3426

## What does this PR do?

### User standpoint

- When using a `facets` parameter in search, the facets that have numeric values are displayed in a new section of the response called `facetStats` that contains, per facet, the numeric min and max value of the hits returned by the search.

<details>
<summary>
Sample request/response
</summary>

```json
❯ curl \
  -X POST 'http://localhost:7700/indexes/meteorites/search?facets=mass' \
  -H 'Content-Type: application/json' \
  --data-binary '{ "q": "LL6", "facets":["mass", "recclass"], "limit": 5 }' | jsonxf
{
  "hits": [
    {
      "name": "Niger (LL6)",
      "id": "16975",
      "nametype": "Valid",
      "recclass": "LL6",
      "mass": 3.3,
      "fall": "Fell"
    },
    {
      "name": "Appley Bridge",
      "id": "2318",
      "nametype": "Valid",
      "recclass": "LL6",
      "mass": 15000,
      "fall": "Fell",
      "_geo": {
        "lat": 53.58333,
        "lng": -2.71667
      }
    },
    {
      "name": "Athens",
      "id": "4885",
      "nametype": "Valid",
      "recclass": "LL6",
      "mass": 265,
      "fall": "Fell",
      "_geo": {
        "lat": 34.75,
        "lng": -87.0
      }
    },
    {
      "name": "Bandong",
      "id": "4935",
      "nametype": "Valid",
      "recclass": "LL6",
      "mass": 11500,
      "fall": "Fell",
      "_geo": {
        "lat": -6.91667,
        "lng": 107.6
      }
    },
    {
      "name": "Benguerir",
      "id": "30443",
      "nametype": "Valid",
      "recclass": "LL6",
      "mass": 25000,
      "fall": "Fell",
      "_geo": {
        "lat": 32.25,
        "lng": -8.15
      }
    }
  ],
  "query": "LL6",
  "processingTimeMs": 15,
  "limit": 5,
  "offset": 0,
  "estimatedTotalHits": 42,
  "facetDistribution": {
    "mass": {
      "110000": 1,
      "11500": 1,
      "1161": 1,
      "12000": 1,
      "1215.5": 1,
      "127000": 1,
      "15000": 1,
      "1676": 1,
      "1700": 1,
      "1710.5": 1,
      "18000": 1,
      "19000": 1,
      "220000": 1,
      "2220": 1,
      "22300": 1,
      "25000": 2,
      "265": 1,
      "271000": 1,
      "2840": 1,
      "3.3": 1,
      "3000": 1,
      "303": 1,
      "32000": 1,
      "34000": 1,
      "36.1": 1,
      "45000": 1,
      "460": 1,
      "478": 1,
      "483": 1,
      "5500": 2,
      "600": 1,
      "6000": 1,
      "67.8": 1,
      "678": 1,
      "680.5": 1,
      "6930": 1,
      "8": 1,
      "8300": 1,
      "840": 1,
      "8400": 1
    },
    "recclass": {
      "L/LL6": 3,
      "LL6": 39
    }
  },
  "facetStats": {
    "mass": {
      "min": 3.3,
      "max": 271000.0
    }
  }
}
```

</details>

## PR checklist
Please check if your PR fulfills the following requirements:
- [ ] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
- [ ] Have you read the contributing guidelines?
- [ ] Have you made sure that the title is accurate and descriptive of the changes?

Thank you so much for contributing to Meilisearch!


Co-authored-by: Louis Dureuil <louis@meilisearch.com>

meilisearch/src/search.rs

@@ -108,7 +108,7 @@ pub struct SearchHit {
     pub matches_position: Option<MatchesPosition>,
 }
 
-#[derive(Serialize, Debug, Clone, PartialEq, Eq)]
+#[derive(Serialize, Debug, Clone, PartialEq)]
 #[serde(rename_all = "camelCase")]
 pub struct SearchResult {
     pub hits: Vec<SearchHit>,
@@ -118,6 +118,8 @@ pub struct SearchResult {
     pub hits_info: HitsInfo,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub facet_distribution: Option<BTreeMap<String, BTreeMap<String, u64>>>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub facet_stats: Option<BTreeMap<String, FacetStats>>,
 }
 
 #[derive(Serialize, Debug, Clone, PartialEq, Eq)]
@@ -129,6 +131,12 @@ pub enum HitsInfo {
     OffsetLimit { limit: usize, offset: usize, estimated_total_hits: usize },
 }
 
+#[derive(Serialize, Debug, Clone, PartialEq)]
+pub struct FacetStats {
+    pub min: f64,
+    pub max: f64,
+}
+
 pub fn perform_search(
     index: &Index,
     query: SearchQuery,
@@ -300,7 +308,7 @@ pub fn perform_search(
         HitsInfo::OffsetLimit { limit: query.limit, offset, estimated_total_hits: number_of_hits }
     };
 
-    let facet_distribution = match query.facets {
+    let (facet_distribution, facet_stats) = match query.facets {
         Some(ref fields) => {
             let mut facet_distribution = index.facets_distribution(&rtxn);
 
@@ -314,18 +322,23 @@ pub fn perform_search(
                 facet_distribution.facets(fields);
             }
             let distribution = facet_distribution.candidates(candidates).execute()?;
-
+            let stats = facet_distribution.compute_stats()?;
-            Some(distribution)
+            (Some(distribution), Some(stats))
         }
-        None => None,
+        None => (None, None),
     };
 
+    let facet_stats = facet_stats.map(|stats| {
+        stats.into_iter().map(|(k, (min, max))| (k, FacetStats { min, max })).collect()
+    });
+
     let result = SearchResult {
         hits: documents,
         hits_info,
         query: query.q.clone().unwrap_or_default(),
         processing_time_ms: before_search.elapsed().as_millis(),
         facet_distribution,
+        facet_stats,
     };
     Ok(result)
 }

milli/src/search/criteria/asc_desc.rs

@@ -1,5 +1,6 @@
 use std::mem::take;
 
+use heed::BytesDecode;
 use itertools::Itertools;
 use log::debug;
 use ordered_float::OrderedFloat;
@@ -7,7 +8,7 @@ use roaring::RoaringBitmap;
 
 use super::{Criterion, CriterionParameters, CriterionResult};
 use crate::facet::FacetType;
-use crate::heed_codec::facet::FacetGroupKeyCodec;
+use crate::heed_codec::facet::{FacetGroupKeyCodec, OrderedF64Codec};
 use crate::heed_codec::ByteSliceRefCodec;
 use crate::search::criteria::{resolve_query_tree, CriteriaBuilder, InitialCandidates};
 use crate::search::facet::{ascending_facet_sort, descending_facet_sort};
@@ -196,6 +197,38 @@ fn facet_ordered_iterative<'t>(
     Ok(Box::new(number_iter.chain(string_iter).map(Ok)) as Box<dyn Iterator<Item = _>>)
 }
 
+fn facet_extreme_value<'t>(
+    mut extreme_it: impl Iterator<Item = heed::Result<(RoaringBitmap, &'t [u8])>> + 't,
+) -> Result<Option<f64>> {
+    let extreme_value =
+        if let Some(extreme_value) = extreme_it.next() { extreme_value } else { return Ok(None) };
+    let (_, extreme_value) = extreme_value?;
+
+    Ok(OrderedF64Codec::bytes_decode(extreme_value))
+}
+
+pub fn facet_min_value<'t>(
+    index: &'t Index,
+    rtxn: &'t heed::RoTxn,
+    field_id: FieldId,
+    candidates: RoaringBitmap,
+) -> Result<Option<f64>> {
+    let db = index.facet_id_f64_docids.remap_key_type::<FacetGroupKeyCodec<ByteSliceRefCodec>>();
+    let it = ascending_facet_sort(rtxn, db, field_id, candidates)?;
+    facet_extreme_value(it)
+}
+
+pub fn facet_max_value<'t>(
+    index: &'t Index,
+    rtxn: &'t heed::RoTxn,
+    field_id: FieldId,
+    candidates: RoaringBitmap,
+) -> Result<Option<f64>> {
+    let db = index.facet_id_f64_docids.remap_key_type::<FacetGroupKeyCodec<ByteSliceRefCodec>>();
+    let it = descending_facet_sort(rtxn, db, field_id, candidates)?;
+    facet_extreme_value(it)
+}
+
 fn facet_ordered_set_based<'t>(
     index: &'t Index,
     rtxn: &'t heed::RoTxn,
@@ -203,23 +236,24 @@ fn facet_ordered_set_based<'t>(
     is_ascending: bool,
     candidates: RoaringBitmap,
 ) -> Result<Box<dyn Iterator<Item = heed::Result<RoaringBitmap>> + 't>> {
-    let make_iter = if is_ascending { ascending_facet_sort } else { descending_facet_sort };
+    let number_db =
+        index.facet_id_f64_docids.remap_key_type::<FacetGroupKeyCodec<ByteSliceRefCodec>>();
+    let string_db =
+        index.facet_id_string_docids.remap_key_type::<FacetGroupKeyCodec<ByteSliceRefCodec>>();
 
-    let number_iter = make_iter(
+    let (number_iter, string_iter) = if is_ascending {
-        rtxn,
+        let number_iter = ascending_facet_sort(rtxn, number_db, field_id, candidates.clone())?;
-        index.facet_id_f64_docids.remap_key_type::<FacetGroupKeyCodec<ByteSliceRefCodec>>(),
+        let string_iter = ascending_facet_sort(rtxn, string_db, field_id, candidates)?;
-        field_id,
-        candidates.clone(),
-    )?;
 
-    let string_iter = make_iter(
+        (itertools::Either::Left(number_iter), itertools::Either::Left(string_iter))
-        rtxn,
+    } else {
-        index.facet_id_string_docids.remap_key_type::<FacetGroupKeyCodec<ByteSliceRefCodec>>(),
+        let number_iter = descending_facet_sort(rtxn, number_db, field_id, candidates.clone())?;
-        field_id,
+        let string_iter = descending_facet_sort(rtxn, string_db, field_id, candidates)?;
-        candidates,
-    )?;
 
-    Ok(Box::new(number_iter.chain(string_iter)))
+        (itertools::Either::Right(number_iter), itertools::Either::Right(string_iter))
+    };
+
+    Ok(Box::new(number_iter.chain(string_iter).map(|res| res.map(|(doc_ids, _)| doc_ids))))
 }
 
 /// Returns an iterator over groups of the given candidates in ascending or descending order.

milli/src/search/criteria/mod.rs

@@ -21,6 +21,7 @@ use crate::update::{MAX_LENGTH_FOR_PREFIX_PROXIMITY_DB, MAX_PROXIMITY_FOR_PREFIX
 use crate::{AscDesc as AscDescName, DocumentId, FieldId, Index, Member, Result};
 
 mod asc_desc;
+pub use asc_desc::{facet_max_value, facet_min_value};
 mod attribute;
 mod exactness;
 pub mod r#final;

milli/src/search/facet/facet_distribution.rs

@@ -278,6 +278,65 @@ impl<'a> FacetDistribution<'a> {
         }
     }
 
+    pub fn compute_stats(&self) -> Result<BTreeMap<String, (f64, f64)>> {
+        let fields_ids_map = self.index.fields_ids_map(self.rtxn)?;
+        let filterable_fields = self.index.filterable_fields(self.rtxn)?;
+        let candidates = if let Some(candidates) = self.candidates.clone() {
+            candidates
+        } else {
+            return Ok(Default::default());
+        };
+
+        let fields = match &self.facets {
+            Some(facets) => {
+                let invalid_fields: HashSet<_> = facets
+                    .iter()
+                    .filter(|facet| !crate::is_faceted(facet, &filterable_fields))
+                    .collect();
+                if !invalid_fields.is_empty() {
+                    return Err(UserError::InvalidFacetsDistribution {
+                        invalid_facets_name: invalid_fields.into_iter().cloned().collect(),
+                        valid_facets_name: filterable_fields.into_iter().collect(),
+                    }
+                    .into());
+                } else {
+                    facets.clone()
+                }
+            }
+            None => filterable_fields,
+        };
+
+        let mut distribution = BTreeMap::new();
+        for (fid, name) in fields_ids_map.iter() {
+            if crate::is_faceted(name, &fields) {
+                let min_value = if let Some(min_value) = crate::search::criteria::facet_min_value(
+                    self.index,
+                    self.rtxn,
+                    fid,
+                    candidates.clone(),
+                )? {
+                    min_value
+                } else {
+                    continue;
+                };
+                let max_value = if let Some(max_value) = crate::search::criteria::facet_max_value(
+                    self.index,
+                    self.rtxn,
+                    fid,
+                    candidates.clone(),
+                )? {
+                    max_value
+                } else {
+                    continue;
+                };
+
+                distribution.insert(name.to_string(), (min_value, max_value));
+            }
+        }
+
+        Ok(distribution)
+    }
+
     pub fn execute(&self) -> Result<BTreeMap<String, BTreeMap<String, u64>>> {
         let fields_ids_map = self.index.fields_ids_map(self.rtxn)?;
         let filterable_fields = self.index.filterable_fields(self.rtxn)?;
@@ -537,4 +596,216 @@ mod tests {
 
         milli_snap!(format!("{map:?}"), "candidates_0_5_000", @"825f23a4090d05756f46176987b7d992");
     }
+
+    #[test]
+    fn facet_stats() {
+        let mut index = TempIndex::new_with_map_size(4096 * 10_000);
+        index.index_documents_config.autogenerate_docids = true;
+
+        index
+            .update_settings(|settings| settings.set_filterable_fields(hashset! { S("colour") }))
+            .unwrap();
+
+        let facet_values = (0..1000).into_iter().collect::<Vec<_>>();
+
+        let mut documents = vec![];
+        for i in 0..1000 {
+            let document = serde_json::json!({
+                "colour": facet_values[i % 1000],
+            })
+            .as_object()
+            .unwrap()
+            .clone();
+            documents.push(document);
+        }
+
+        let documents = documents_batch_reader_from_objects(documents);
+
+        index.add_documents(documents).unwrap();
+
+        let txn = index.read_txn().unwrap();
+
+        let map = FacetDistribution::new(&txn, &index)
+            .facets(std::iter::once("colour"))
+            .compute_stats()
+            .unwrap();
+
+        milli_snap!(format!("{map:?}"), "no_candidates", @"{}");
+
+        let map = FacetDistribution::new(&txn, &index)
+            .facets(std::iter::once("colour"))
+            .candidates((0..1000).into_iter().collect())
+            .compute_stats()
+            .unwrap();
+
+        milli_snap!(format!("{map:?}"), "candidates_0_1000", @r###"{"colour": (0.0, 999.0)}"###);
+
+        let map = FacetDistribution::new(&txn, &index)
+            .facets(std::iter::once("colour"))
+            .candidates((217..777).into_iter().collect())
+            .compute_stats()
+            .unwrap();
+
+        milli_snap!(format!("{map:?}"), "candidates_217_777", @r###"{"colour": (217.0, 776.0)}"###);
+    }
+
+    #[test]
+    fn facet_stats_array() {
+        let mut index = TempIndex::new_with_map_size(4096 * 10_000);
+        index.index_documents_config.autogenerate_docids = true;
+
+        index
+            .update_settings(|settings| settings.set_filterable_fields(hashset! { S("colour") }))
+            .unwrap();
+
+        let facet_values = (0..1000).into_iter().collect::<Vec<_>>();
+
+        let mut documents = vec![];
+        for i in 0..1000 {
+            let document = serde_json::json!({
+                "colour": [facet_values[i % 1000], facet_values[i % 1000] + 1000],
+            })
+            .as_object()
+            .unwrap()
+            .clone();
+            documents.push(document);
+        }
+
+        let documents = documents_batch_reader_from_objects(documents);
+
+        index.add_documents(documents).unwrap();
+
+        let txn = index.read_txn().unwrap();
+
+        let map = FacetDistribution::new(&txn, &index)
+            .facets(std::iter::once("colour"))
+            .compute_stats()
+            .unwrap();
+
+        milli_snap!(format!("{map:?}"), "no_candidates", @"{}");
+
+        let map = FacetDistribution::new(&txn, &index)
+            .facets(std::iter::once("colour"))
+            .candidates((0..1000).into_iter().collect())
+            .compute_stats()
+            .unwrap();
+
+        milli_snap!(format!("{map:?}"), "candidates_0_1000", @r###"{"colour": (0.0, 1999.0)}"###);
+
+        let map = FacetDistribution::new(&txn, &index)
+            .facets(std::iter::once("colour"))
+            .candidates((217..777).into_iter().collect())
+            .compute_stats()
+            .unwrap();
+
+        milli_snap!(format!("{map:?}"), "candidates_217_777", @r###"{"colour": (217.0, 1776.0)}"###);
+    }
+
+    #[test]
+    fn facet_stats_mixed_array() {
+        let mut index = TempIndex::new_with_map_size(4096 * 10_000);
+        index.index_documents_config.autogenerate_docids = true;
+
+        index
+            .update_settings(|settings| settings.set_filterable_fields(hashset! { S("colour") }))
+            .unwrap();
+
+        let facet_values = (0..1000).into_iter().collect::<Vec<_>>();
+
+        let mut documents = vec![];
+        for i in 0..1000 {
+            let document = serde_json::json!({
+                "colour": [facet_values[i % 1000], format!("{}", facet_values[i % 1000] + 1000)],
+            })
+            .as_object()
+            .unwrap()
+            .clone();
+            documents.push(document);
+        }
+
+        let documents = documents_batch_reader_from_objects(documents);
+
+        index.add_documents(documents).unwrap();
+
+        let txn = index.read_txn().unwrap();
+
+        let map = FacetDistribution::new(&txn, &index)
+            .facets(std::iter::once("colour"))
+            .compute_stats()
+            .unwrap();
+
+        milli_snap!(format!("{map:?}"), "no_candidates", @"{}");
+
+        let map = FacetDistribution::new(&txn, &index)
+            .facets(std::iter::once("colour"))
+            .candidates((0..1000).into_iter().collect())
+            .compute_stats()
+            .unwrap();
+
+        milli_snap!(format!("{map:?}"), "candidates_0_1000", @r###"{"colour": (0.0, 999.0)}"###);
+
+        let map = FacetDistribution::new(&txn, &index)
+            .facets(std::iter::once("colour"))
+            .candidates((217..777).into_iter().collect())
+            .compute_stats()
+            .unwrap();
+
+        milli_snap!(format!("{map:?}"), "candidates_217_777", @r###"{"colour": (217.0, 776.0)}"###);
+    }
+
+    #[test]
+    fn facet_mixed_values() {
+        let mut index = TempIndex::new_with_map_size(4096 * 10_000);
+        index.index_documents_config.autogenerate_docids = true;
+
+        index
+            .update_settings(|settings| settings.set_filterable_fields(hashset! { S("colour") }))
+            .unwrap();
+
+        let facet_values = (0..1000).into_iter().collect::<Vec<_>>();
+
+        let mut documents = vec![];
+        for i in 0..1000 {
+            let document = if i % 2 == 0 {
+                serde_json::json!({
+                    "colour": [facet_values[i % 1000], facet_values[i % 1000] + 1000],
+                })
+            } else {
+                serde_json::json!({
+                    "colour": format!("{}", facet_values[i % 1000] + 10000),
+                })
+            };
+            let document = document.as_object().unwrap().clone();
+            documents.push(document);
+        }
+
+        let documents = documents_batch_reader_from_objects(documents);
+
+        index.add_documents(documents).unwrap();
+
+        let txn = index.read_txn().unwrap();
+
+        let map = FacetDistribution::new(&txn, &index)
+            .facets(std::iter::once("colour"))
+            .compute_stats()
+            .unwrap();
+
+        milli_snap!(format!("{map:?}"), "no_candidates", @"{}");
+
+        let map = FacetDistribution::new(&txn, &index)
+            .facets(std::iter::once("colour"))
+            .candidates((0..1000).into_iter().collect())
+            .compute_stats()
+            .unwrap();
+
+        milli_snap!(format!("{map:?}"), "candidates_0_1000", @r###"{"colour": (0.0, 1998.0)}"###);
+
+        let map = FacetDistribution::new(&txn, &index)
+            .facets(std::iter::once("colour"))
+            .candidates((217..777).into_iter().collect())
+            .compute_stats()
+            .unwrap();
+
+        milli_snap!(format!("{map:?}"), "candidates_217_777", @r###"{"colour": (218.0, 1776.0)}"###);
+    }
 }

milli/src/search/facet/facet_sort_ascending.rs

@@ -34,15 +34,20 @@ pub fn ascending_facet_sort<'t>(
     db: heed::Database<FacetGroupKeyCodec<ByteSliceRefCodec>, FacetGroupValueCodec>,
     field_id: u16,
     candidates: RoaringBitmap,
-) -> Result<Box<dyn Iterator<Item = Result<RoaringBitmap>> + 't>> {
+) -> Result<impl Iterator<Item = Result<(RoaringBitmap, &'t [u8])>> + 't> {
     let highest_level = get_highest_level(rtxn, db, field_id)?;
     if let Some(first_bound) = get_first_facet_value::<ByteSliceRefCodec>(rtxn, db, field_id)? {
         let first_key = FacetGroupKey { field_id, level: highest_level, left_bound: first_bound };
         let iter = db.range(rtxn, &(first_key..)).unwrap().take(usize::MAX);
 
-        Ok(Box::new(AscendingFacetSort { rtxn, db, field_id, stack: vec![(candidates, iter)] }))
+        Ok(itertools::Either::Left(AscendingFacetSort {
+            rtxn,
+            db,
+            field_id,
+            stack: vec![(candidates, iter)],
+        }))
     } else {
-        Ok(Box::new(std::iter::empty()))
+        Ok(itertools::Either::Right(std::iter::empty()))
     }
 }
 
@@ -60,7 +65,7 @@ struct AscendingFacetSort<'t, 'e> {
 }
 
 impl<'t, 'e> Iterator for AscendingFacetSort<'t, 'e> {
-    type Item = Result<RoaringBitmap>;
+    type Item = Result<(RoaringBitmap, &'t [u8])>;
 
     fn next(&mut self) -> Option<Self::Item> {
         'outer: loop {
@@ -90,7 +95,8 @@ impl<'t, 'e> Iterator for AscendingFacetSort<'t, 'e> {
                     *documents_ids -= &bitmap;
 
                     if level == 0 {
-                        return Some(Ok(bitmap));
+                        // Since the level is 0, the left_bound is the exact value.
+                        return Some(Ok((bitmap, left_bound)));
                     }
                     let starting_key_below =
                         FacetGroupKey { field_id: self.field_id, level: level - 1, left_bound };
@@ -130,7 +136,7 @@ mod tests {
             let mut results = String::new();
             let iter = ascending_facet_sort(&txn, index.content, 0, candidates).unwrap();
             for el in iter {
-                let docids = el.unwrap();
+                let (docids, _) = el.unwrap();
                 results.push_str(&display_bitmap(&docids));
                 results.push('\n');
             }
@@ -152,7 +158,7 @@ mod tests {
             let mut results = String::new();
             let iter = ascending_facet_sort(&txn, index.content, 0, candidates.clone()).unwrap();
             for el in iter {
-                let docids = el.unwrap();
+                let (docids, _) = el.unwrap();
                 results.push_str(&display_bitmap(&docids));
                 results.push('\n');
             }
@@ -161,7 +167,7 @@ mod tests {
             let mut results = String::new();
             let iter = ascending_facet_sort(&txn, index.content, 1, candidates).unwrap();
             for el in iter {
-                let docids = el.unwrap();
+                let (docids, _) = el.unwrap();
                 results.push_str(&display_bitmap(&docids));
                 results.push('\n');
             }
@@ -183,7 +189,7 @@ mod tests {
             let mut results = String::new();
             let iter = ascending_facet_sort(&txn, index.content, 0, candidates.clone()).unwrap();
             for el in iter {
-                let docids = el.unwrap();
+                let (docids, _) = el.unwrap();
                 results.push_str(&display_bitmap(&docids));
                 results.push('\n');
             }
@@ -192,7 +198,7 @@ mod tests {
             let mut results = String::new();
             let iter = ascending_facet_sort(&txn, index.content, 1, candidates).unwrap();
             for el in iter {
-                let docids = el.unwrap();
+                let (docids, _) = el.unwrap();
                 results.push_str(&display_bitmap(&docids));
                 results.push('\n');
             }
@@ -214,7 +220,7 @@ mod tests {
             let mut results = String::new();
             let iter = ascending_facet_sort(&txn, index.content, 3, candidates.clone()).unwrap();
             for el in iter {
-                let docids = el.unwrap();
+                let (docids, _) = el.unwrap();
                 results.push_str(&display_bitmap(&docids));
                 results.push('\n');
             }

milli/src/search/facet/facet_sort_descending.rs

@@ -17,21 +17,21 @@ pub fn descending_facet_sort<'t>(
     db: heed::Database<FacetGroupKeyCodec<ByteSliceRefCodec>, FacetGroupValueCodec>,
     field_id: u16,
     candidates: RoaringBitmap,
-) -> Result<Box<dyn Iterator<Item = Result<RoaringBitmap>> + 't>> {
+) -> Result<impl Iterator<Item = Result<(RoaringBitmap, &'t [u8])>> + 't> {
     let highest_level = get_highest_level(rtxn, db, field_id)?;
     if let Some(first_bound) = get_first_facet_value::<ByteSliceRefCodec>(rtxn, db, field_id)? {
         let first_key = FacetGroupKey { field_id, level: highest_level, left_bound: first_bound };
         let last_bound = get_last_facet_value::<ByteSliceRefCodec>(rtxn, db, field_id)?.unwrap();
         let last_key = FacetGroupKey { field_id, level: highest_level, left_bound: last_bound };
         let iter = db.rev_range(rtxn, &(first_key..=last_key))?.take(usize::MAX);
-        Ok(Box::new(DescendingFacetSort {
+        Ok(itertools::Either::Left(DescendingFacetSort {
             rtxn,
             db,
             field_id,
             stack: vec![(candidates, iter, Bound::Included(last_bound))],
         }))
     } else {
-        Ok(Box::new(std::iter::empty()))
+        Ok(itertools::Either::Right(std::iter::empty()))
     }
 }
 
@@ -50,7 +50,7 @@ struct DescendingFacetSort<'t> {
 }
 
 impl<'t> Iterator for DescendingFacetSort<'t> {
-    type Item = Result<RoaringBitmap>;
+    type Item = Result<(RoaringBitmap, &'t [u8])>;
 
     fn next(&mut self) -> Option<Self::Item> {
         'outer: loop {
@@ -77,7 +77,8 @@ impl<'t> Iterator for DescendingFacetSort<'t> {
                     *documents_ids -= &bitmap;
 
                     if level == 0 {
-                        return Some(Ok(bitmap));
+                        // Since we're at the level 0 the left_bound is the exact value.
+                        return Some(Ok((bitmap, left_bound)));
                     }
                     let starting_key_below =
                         FacetGroupKey { field_id, level: level - 1, left_bound };
@@ -146,7 +147,7 @@ mod tests {
             let db = index.content.remap_key_type::<FacetGroupKeyCodec<ByteSliceRefCodec>>();
             let iter = descending_facet_sort(&txn, db, 0, candidates).unwrap();
             for el in iter {
-                let docids = el.unwrap();
+                let (docids, _) = el.unwrap();
                 results.push_str(&display_bitmap(&docids));
                 results.push('\n');
             }
@@ -169,7 +170,7 @@ mod tests {
             let db = index.content.remap_key_type::<FacetGroupKeyCodec<ByteSliceRefCodec>>();
             let iter = descending_facet_sort(&txn, db, 0, candidates.clone()).unwrap();
             for el in iter {
-                let docids = el.unwrap();
+                let (docids, _) = el.unwrap();
                 results.push_str(&display_bitmap(&docids));
                 results.push('\n');
             }
@@ -179,7 +180,7 @@ mod tests {
 
             let iter = descending_facet_sort(&txn, db, 1, candidates).unwrap();
             for el in iter {
-                let docids = el.unwrap();
+                let (docids, _) = el.unwrap();
                 results.push_str(&display_bitmap(&docids));
                 results.push('\n');
             }
@@ -200,7 +201,7 @@ mod tests {
             let mut results = String::new();
             let iter = descending_facet_sort(&txn, index.content, 0, candidates.clone()).unwrap();
             for el in iter {
-                let docids = el.unwrap();
+                let (docids, _) = el.unwrap();
                 results.push_str(&display_bitmap(&docids));
                 results.push('\n');
             }
@@ -209,7 +210,7 @@ mod tests {
             let mut results = String::new();
             let iter = descending_facet_sort(&txn, index.content, 1, candidates).unwrap();
             for el in iter {
-                let docids = el.unwrap();
+                let (docids, _) = el.unwrap();
                 results.push_str(&display_bitmap(&docids));
                 results.push('\n');
             }
@@ -231,7 +232,7 @@ mod tests {
             let mut results = String::new();
             let iter = descending_facet_sort(&txn, index.content, 3, candidates.clone()).unwrap();
             for el in iter {
-                let docids = el.unwrap();
+                let (docids, _) = el.unwrap();
                 results.push_str(&display_bitmap(&docids));
                 results.push('\n');
             }