Merge #4929

4929: Add facets support to federated r=Kerollmops a=dureuill

# Pull Request

## Related issue 

- Fixes #4932 (sprint issue)
- Fixes  #4913 (user-opened issue)

## What does this PR do?

See [public usage](https://meilisearch.notion.site/v1-11-Federated-search-59b30e03383c40729d7541a3dffb0069)

> [!CAUTION]
> This PR introduces a 🚨**breaking change**🚨: `queries.facets` when `federation` is present and non-`null` is now **an error**

### Implementation standpoint:

- Facet distribution: fix issue where truncated facet distribution would have a wrong order
- facet distribution: implement Display for OrderBy


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

meilisearch-types/src/error.rs

@@ -238,8 +238,14 @@ InvalidIndexLimit                     , InvalidRequest       , BAD_REQUEST ;
 InvalidIndexOffset                    , InvalidRequest       , BAD_REQUEST ;
 InvalidIndexPrimaryKey                , InvalidRequest       , BAD_REQUEST ;
 InvalidIndexUid                       , InvalidRequest       , BAD_REQUEST ;
+InvalidMultiSearchFacets              , InvalidRequest       , BAD_REQUEST ;
+InvalidMultiSearchFacetsByIndex       , InvalidRequest       , BAD_REQUEST ;
+InvalidMultiSearchFacetOrder          , InvalidRequest       , BAD_REQUEST ;
 InvalidMultiSearchFederated           , InvalidRequest       , BAD_REQUEST ;
 InvalidMultiSearchFederationOptions   , InvalidRequest       , BAD_REQUEST ;
+InvalidMultiSearchMaxValuesPerFacet   , InvalidRequest       , BAD_REQUEST ;
+InvalidMultiSearchMergeFacets         , InvalidRequest       , BAD_REQUEST ;
+InvalidMultiSearchQueryFacets         , InvalidRequest       , BAD_REQUEST ;
 InvalidMultiSearchQueryPagination     , InvalidRequest       , BAD_REQUEST ;
 InvalidMultiSearchQueryRankingRules   , InvalidRequest       , BAD_REQUEST ;
 InvalidMultiSearchWeight              , InvalidRequest       , BAD_REQUEST ;

meilisearch-types/src/index_uid.rs

@@ -1,3 +1,4 @@
+use std::borrow::Borrow;
 use std::error::Error;
 use std::fmt;
 use std::str::FromStr;
@@ -8,7 +9,7 @@ use crate::error::{Code, ErrorCode};
 
 /// An index uid is composed of only ascii alphanumeric characters, - and _, between 1 and 400
 /// bytes long
-#[derive(Debug, Clone, PartialEq, Eq, Deserr)]
+#[derive(Debug, Clone, PartialEq, Eq, Deserr, PartialOrd, Ord)]
 #[deserr(try_from(String) = IndexUid::try_from -> IndexUidFormatError)]
 pub struct IndexUid(String);
 
@@ -70,6 +71,12 @@ impl From<IndexUid> for String {
     }
 }
 
+impl Borrow<String> for IndexUid {
+    fn borrow(&self) -> &String {
+        &self.0
+    }
+}
+
 #[derive(Debug)]
 pub struct IndexUidFormatError {
     pub invalid_uid: String,

meilisearch/src/error.rs

@@ -4,6 +4,7 @@ use byte_unit::{Byte, UnitType};
 use meilisearch_types::document_formats::{DocumentFormatError, PayloadType};
 use meilisearch_types::error::{Code, ErrorCode, ResponseError};
 use meilisearch_types::index_uid::{IndexUid, IndexUidFormatError};
+use meilisearch_types::milli::OrderBy;
 use serde_json::Value;
 use tokio::task::JoinError;
 
@@ -27,10 +28,20 @@ pub enum MeilisearchHttpError {
     EmptyFilter,
     #[error("Invalid syntax for the filter parameter: `expected {}, found: {1}`.", .0.join(", "))]
     InvalidExpression(&'static [&'static str], Value),
-    #[error("Using `federationOptions` is not allowed in a non-federated search.\n Hint: remove `federationOptions` from query #{0} or add `federation: {{}}` to the request.")]
+    #[error("Using `federationOptions` is not allowed in a non-federated search.\n - Hint: remove `federationOptions` from query #{0} or add `federation` to the request.")]
     FederationOptionsInNonFederatedRequest(usize),
-    #[error("Inside `.queries[{0}]`: Using pagination options is not allowed in federated queries.\n Hint: remove `{1}` from query #{0} or remove `federation: {{}}` from the request")]
+    #[error("Inside `.queries[{0}]`: Using pagination options is not allowed in federated queries.\n - Hint: remove `{1}` from query #{0} or remove `federation` from the request\n - Hint: pass `federation.limit` and `federation.offset` for pagination in federated search")]
     PaginationInFederatedQuery(usize, &'static str),
+    #[error("Inside `.queries[{0}]`: Using facet options is not allowed in federated queries.\n - Hint: remove `facets` from query #{0} or remove `federation` from the request\n - Hint: pass `federation.facetsByIndex.{1}: {2:?}` for facets in federated search")]
+    FacetsInFederatedQuery(usize, String, Vec<String>),
+    #[error("Inconsistent order for values in facet `{facet}`: index `{previous_uid}` orders {previous_facet_order}, but index `{current_uid}` orders {index_facet_order}.\n - Hint: Remove `federation.mergeFacets` or change `faceting.sortFacetValuesBy` to be consistent in settings.")]
+    InconsistentFacetOrder {
+        facet: String,
+        previous_facet_order: OrderBy,
+        previous_uid: String,
+        index_facet_order: OrderBy,
+        current_uid: String,
+    },
     #[error("A {0} payload is missing.")]
     MissingPayload(PayloadType),
     #[error("Too many search requests running at the same time: {0}. Retry after 10s.")]
@@ -96,6 +107,10 @@ impl ErrorCode for MeilisearchHttpError {
             MeilisearchHttpError::PaginationInFederatedQuery(_, _) => {
                 Code::InvalidMultiSearchQueryPagination
             }
+            MeilisearchHttpError::FacetsInFederatedQuery(..) => Code::InvalidMultiSearchQueryFacets,
+            MeilisearchHttpError::InconsistentFacetOrder { .. } => {
+                Code::InvalidMultiSearchFacetOrder
+            }
         }
     }
 }

meilisearch/src/search/federated.rs

@@ -9,20 +9,24 @@ use std::vec::{IntoIter, Vec};
 
 use actix_http::StatusCode;
 use index_scheduler::{IndexScheduler, RoFeatures};
+use indexmap::IndexMap;
 use meilisearch_types::deserr::DeserrJsonError;
 use meilisearch_types::error::deserr_codes::{
-    InvalidMultiSearchWeight, InvalidSearchLimit, InvalidSearchOffset,
+    InvalidMultiSearchFacetsByIndex, InvalidMultiSearchMaxValuesPerFacet,
+    InvalidMultiSearchMergeFacets, InvalidMultiSearchWeight, InvalidSearchLimit,
+    InvalidSearchOffset,
 };
 use meilisearch_types::error::ResponseError;
+use meilisearch_types::index_uid::IndexUid;
 use meilisearch_types::milli::score_details::{ScoreDetails, ScoreValue};
-use meilisearch_types::milli::{self, DocumentId, TimeBudget};
+use meilisearch_types::milli::{self, DocumentId, OrderBy, TimeBudget};
 use roaring::RoaringBitmap;
 use serde::Serialize;
 
 use super::ranking_rules::{self, RankingRules};
 use super::{
-    prepare_search, AttributesFormat, HitMaker, HitsInfo, RetrieveVectors, SearchHit, SearchKind,
-    SearchQuery, SearchQueryWithIndex,
+    compute_facet_distribution_stats, prepare_search, AttributesFormat, ComputedFacets, FacetStats,
+    HitMaker, HitsInfo, RetrieveVectors, SearchHit, SearchKind, SearchQuery, SearchQueryWithIndex,
 };
 use crate::error::MeilisearchHttpError;
 use crate::routes::indexes::search::search_kind;
@@ -73,6 +77,17 @@ pub struct Federation {
     pub limit: usize,
     #[deserr(default = super::DEFAULT_SEARCH_OFFSET(), error = DeserrJsonError<InvalidSearchOffset>)]
     pub offset: usize,
+    #[deserr(default, error = DeserrJsonError<InvalidMultiSearchFacetsByIndex>)]
+    pub facets_by_index: BTreeMap<IndexUid, Option<Vec<String>>>,
+    #[deserr(default, error = DeserrJsonError<InvalidMultiSearchMergeFacets>)]
+    pub merge_facets: Option<MergeFacets>,
+}
+
+#[derive(Copy, Clone, Debug, deserr::Deserr, Default)]
+#[deserr(error = DeserrJsonError<InvalidMultiSearchMergeFacets>, rename_all = camelCase, deny_unknown_fields)]
+pub struct MergeFacets {
+    #[deserr(default, error = DeserrJsonError<InvalidMultiSearchMaxValuesPerFacet>)]
+    pub max_values_per_facet: Option<usize>,
 }
 
 #[derive(Debug, deserr::Deserr)]
@@ -82,7 +97,7 @@ pub struct FederatedSearch {
     #[deserr(default)]
     pub federation: Option<Federation>,
 }
-#[derive(Serialize, Clone, PartialEq)]
+#[derive(Serialize, Clone)]
 #[serde(rename_all = "camelCase")]
 pub struct FederatedSearchResult {
     pub hits: Vec<SearchHit>,
@@ -93,6 +108,13 @@ pub struct FederatedSearchResult {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub semantic_hit_count: Option<u32>,
 
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub facet_distribution: Option<BTreeMap<String, IndexMap<String, u64>>>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub facet_stats: Option<BTreeMap<String, FacetStats>>,
+    #[serde(skip_serializing_if = "FederatedFacets::is_empty")]
+    pub facets_by_index: FederatedFacets,
+
     // These fields are only used for analytics purposes
     #[serde(skip)]
     pub degraded: bool,
@@ -109,6 +131,9 @@ impl fmt::Debug for FederatedSearchResult {
             semantic_hit_count,
             degraded,
             used_negative_operator,
+            facet_distribution,
+            facet_stats,
+            facets_by_index,
         } = self;
 
         let mut debug = f.debug_struct("SearchResult");
@@ -122,9 +147,18 @@ impl fmt::Debug for FederatedSearchResult {
         if *degraded {
             debug.field("degraded", degraded);
         }
+        if let Some(facet_distribution) = facet_distribution {
+            debug.field("facet_distribution", &facet_distribution);
+        }
+        if let Some(facet_stats) = facet_stats {
+            debug.field("facet_stats", &facet_stats);
+        }
         if let Some(semantic_hit_count) = semantic_hit_count {
             debug.field("semantic_hit_count", &semantic_hit_count);
         }
+        if !facets_by_index.is_empty() {
+            debug.field("facets_by_index", &facets_by_index);
+        }
 
         debug.finish()
     }
@@ -313,16 +347,104 @@ struct SearchHitByIndex {
 }
 
 struct SearchResultByIndex {
+    index: String,
     hits: Vec<SearchHitByIndex>,
-    candidates: RoaringBitmap,
+    estimated_total_hits: usize,
     degraded: bool,
     used_negative_operator: bool,
+    facets: Option<ComputedFacets>,
+}
+
+#[derive(Debug, Clone, Default, Serialize)]
+pub struct FederatedFacets(pub BTreeMap<String, ComputedFacets>);
+
+impl FederatedFacets {
+    pub fn insert(&mut self, index: String, facets: Option<ComputedFacets>) {
+        if let Some(facets) = facets {
+            self.0.insert(index, facets);
+        }
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    pub fn merge(
+        self,
+        MergeFacets { max_values_per_facet }: MergeFacets,
+        facet_order: BTreeMap<String, (String, OrderBy)>,
+    ) -> Option<ComputedFacets> {
+        if self.is_empty() {
+            return None;
+        }
+
+        let mut distribution: BTreeMap<String, _> = Default::default();
+        let mut stats: BTreeMap<String, FacetStats> = Default::default();
+
+        for facets_by_index in self.0.into_values() {
+            for (facet, index_distribution) in facets_by_index.distribution {
+                match distribution.entry(facet) {
+                    std::collections::btree_map::Entry::Vacant(entry) => {
+                        entry.insert(index_distribution);
+                    }
+                    std::collections::btree_map::Entry::Occupied(mut entry) => {
+                        let distribution = entry.get_mut();
+
+                        for (value, index_count) in index_distribution {
+                            distribution
+                                .entry(value)
+                                .and_modify(|count| *count += index_count)
+                                .or_insert(index_count);
+                        }
+                    }
+                }
+            }
+
+            for (facet, index_stats) in facets_by_index.stats {
+                match stats.entry(facet) {
+                    std::collections::btree_map::Entry::Vacant(entry) => {
+                        entry.insert(index_stats);
+                    }
+                    std::collections::btree_map::Entry::Occupied(mut entry) => {
+                        let stats = entry.get_mut();
+
+                        stats.min = f64::min(stats.min, index_stats.min);
+                        stats.max = f64::max(stats.max, index_stats.max);
+                    }
+                }
+            }
+        }
+
+        // fixup order
+        for (facet, values) in &mut distribution {
+            let order_by = facet_order.get(facet).map(|(_, order)| *order).unwrap_or_default();
+
+            match order_by {
+                OrderBy::Lexicographic => {
+                    values.sort_unstable_by(|left, _, right, _| left.cmp(right))
+                }
+                OrderBy::Count => {
+                    values.sort_unstable_by(|_, left, _, right| {
+                        left.cmp(right)
+                            // biggest first
+                            .reverse()
+                    })
+                }
+            }
+
+            if let Some(max_values_per_facet) = max_values_per_facet {
+                values.truncate(max_values_per_facet)
+            };
+        }
+
+        Some(ComputedFacets { distribution, stats })
+    }
 }
 
 pub fn perform_federated_search(
     index_scheduler: &IndexScheduler,
     queries: Vec<SearchQueryWithIndex>,
-    federation: Federation,
+    mut federation: Federation,
     features: RoFeatures,
 ) -> Result<FederatedSearchResult, ResponseError> {
     let before_search = std::time::Instant::now();
@@ -342,6 +464,16 @@ pub fn perform_federated_search(
             .into());
         }
 
+        if let Some(facets) = federated_query.has_facets() {
+            let facets = facets.to_owned();
+            return Err(MeilisearchHttpError::FacetsInFederatedQuery(
+                query_index,
+                federated_query.index_uid.into_inner(),
+                facets,
+            )
+            .into());
+        }
+
         let (index_uid, query, federation_options) = federated_query.into_index_query_federation();
 
         queries_by_index.entry(index_uid.into_inner()).or_default().push(QueryByIndex {
@@ -353,13 +485,24 @@ pub fn perform_federated_search(
 
     // 2. perform queries, merge and make hits index by index
     let required_hit_count = federation.limit + federation.offset;
+
     // In step (2), semantic_hit_count will be set to Some(0) if any search kind uses semantic
     // Then in step (3), we'll update its value if there is any semantic search
     let mut semantic_hit_count = None;
     let mut results_by_index = Vec::with_capacity(queries_by_index.len());
     let mut previous_query_data: Option<(RankingRules, usize, String)> = None;
 
+    // remember the order and name of first index for each facet when merging with index settings
+    // to detect if the order is inconsistent for a facet.
+    let mut facet_order: Option<BTreeMap<String, (String, OrderBy)>> = match federation.merge_facets
+    {
+        Some(MergeFacets { .. }) => Some(Default::default()),
+        _ => None,
+    };
+
     for (index_uid, queries) in queries_by_index {
+        let first_query_index = queries.first().map(|query| query.query_index);
+
         let index = match index_scheduler.index(&index_uid) {
             Ok(index) => index,
             Err(err) => {
@@ -367,9 +510,8 @@ pub fn perform_federated_search(
                 // Patch the HTTP status code to 400 as it defaults to 404 for `index_not_found`, but
                 // here the resource not found is not part of the URL.
                 err.code = StatusCode::BAD_REQUEST;
-                if let Some(query) = queries.first() {
-                    err.message =
-                        format!("Inside `.queries[{}]`: {}", query.query_index, err.message);
+                if let Some(query_index) = first_query_index {
+                    err.message = format!("Inside `.queries[{}]`: {}", query_index, err.message);
                 }
                 return Err(err);
             }
@@ -394,6 +536,23 @@ pub fn perform_federated_search(
         let mut used_negative_operator = false;
         let mut candidates = RoaringBitmap::new();
 
+        let facets_by_index = federation.facets_by_index.remove(&index_uid).flatten();
+
+        // TODO: recover the max size + facets_by_index as return value of this function so as not to ask it for all queries
+        if let Err(mut error) =
+            check_facet_order(&mut facet_order, &index_uid, &facets_by_index, &index, &rtxn)
+        {
+            error.message = format!(
+                "Inside `.federation.facetsByIndex.{index_uid}`: {error}{}",
+                if let Some(query_index) = first_query_index {
+                    format!("\n - Note: index `{index_uid}` used in `.queries[{query_index}]`")
+                } else {
+                    Default::default()
+                }
+            );
+            return Err(error);
+        }
+
         // 2.1. Compute all candidates for each query in the index
         let mut results_by_query = Vec::with_capacity(queries.len());
 
@@ -562,34 +721,116 @@ pub fn perform_federated_search(
                 .collect();
 
         let merged_result = merged_result?;
+
+        let estimated_total_hits = candidates.len() as usize;
+
+        let facets = facets_by_index
+            .map(|facets_by_index| {
+                compute_facet_distribution_stats(
+                    &facets_by_index,
+                    &index,
+                    &rtxn,
+                    candidates,
+                    super::Route::MultiSearch,
+                )
+            })
+            .transpose()
+            .map_err(|mut error| {
+                error.message = format!(
+                    "Inside `.federation.facetsByIndex.{index_uid}`: {}{}",
+                    error.message,
+                    if let Some(query_index) = first_query_index {
+                        format!("\n - Note: index `{index_uid}` used in `.queries[{query_index}]`")
+                    } else {
+                        Default::default()
+                    }
+                );
+                error
+            })?;
+
         results_by_index.push(SearchResultByIndex {
+            index: index_uid,
             hits: merged_result,
-            candidates,
+            estimated_total_hits,
             degraded,
             used_negative_operator,
+            facets,
         });
     }
 
+    // bonus step, make sure to return an error if an index wants a non-faceted field, even if no query actually uses that index.
+    for (index_uid, facets) in federation.facets_by_index {
+        let index = match index_scheduler.index(&index_uid) {
+            Ok(index) => index,
+            Err(err) => {
+                let mut err = ResponseError::from(err);
+                // Patch the HTTP status code to 400 as it defaults to 404 for `index_not_found`, but
+                // here the resource not found is not part of the URL.
+                err.code = StatusCode::BAD_REQUEST;
+                err.message = format!(
+                    "Inside `.federation.facetsByIndex.{index_uid}`: {}\n - Note: index `{index_uid}` is not used in queries",
+                    err.message
+                );
+                return Err(err);
+            }
+        };
+
+        // Important: this is the only transaction we'll use for this index during this federated search
+        let rtxn = index.read_txn()?;
+
+        if let Err(mut error) =
+            check_facet_order(&mut facet_order, &index_uid, &facets, &index, &rtxn)
+        {
+            error.message = format!(
+                "Inside `.federation.facetsByIndex.{index_uid}`: {error}\n - Note: index `{index_uid}` is not used in queries",
+            );
+            return Err(error);
+        }
+
+        if let Some(facets) = facets {
+            if let Err(mut error) = compute_facet_distribution_stats(
+                &facets,
+                &index,
+                &rtxn,
+                Default::default(),
+                super::Route::MultiSearch,
+            ) {
+                error.message =
+                    format!("Inside `.federation.facetsByIndex.{index_uid}`: {}\n - Note: index `{index_uid}` is not used in queries", error.message);
+                return Err(error);
+            }
+        }
+    }
+
     // 3. merge hits and metadata across indexes
     // 3.1 merge metadata
-    let (estimated_total_hits, degraded, used_negative_operator) = {
+    let (estimated_total_hits, degraded, used_negative_operator, facets) = {
         let mut estimated_total_hits = 0;
         let mut degraded = false;
         let mut used_negative_operator = false;
 
+        let mut facets: FederatedFacets = FederatedFacets::default();
+
         for SearchResultByIndex {
+            index,
             hits: _,
-            candidates,
+            estimated_total_hits: estimated_total_hits_by_index,
+            facets: facets_by_index,
             degraded: degraded_by_index,
             used_negative_operator: used_negative_operator_by_index,
-        } in &results_by_index
+        } in &mut results_by_index
         {
-            estimated_total_hits += candidates.len() as usize;
+            estimated_total_hits += *estimated_total_hits_by_index;
             degraded |= *degraded_by_index;
             used_negative_operator |= *used_negative_operator_by_index;
+
+            let facets_by_index = std::mem::take(facets_by_index);
+            let index = std::mem::take(index);
+
+            facets.insert(index, facets_by_index);
         }
 
-        (estimated_total_hits, degraded, used_negative_operator)
+        (estimated_total_hits, degraded, used_negative_operator, facets)
     };
 
     // 3.2 merge hits
@@ -606,6 +847,20 @@ pub fn perform_federated_search(
         .map(|hit| hit.hit)
         .collect();
 
+    let (facet_distribution, facet_stats, facets_by_index) =
+        match federation.merge_facets.zip(facet_order) {
+            Some((merge_facets, facet_order)) => {
+                let facets = facets.merge(merge_facets, facet_order);
+
+                let (facet_distribution, facet_stats) = facets
+                    .map(|ComputedFacets { distribution, stats }| (distribution, stats))
+                    .unzip();
+
+                (facet_distribution, facet_stats, FederatedFacets::default())
+            }
+            None => (None, None, facets),
+        };
+
     let search_result = FederatedSearchResult {
         hits: merged_hits,
         processing_time_ms: before_search.elapsed().as_millis(),
@@ -617,7 +872,39 @@ pub fn perform_federated_search(
         semantic_hit_count,
         degraded,
         used_negative_operator,
+        facet_distribution,
+        facet_stats,
+        facets_by_index,
     };
 
     Ok(search_result)
 }
+
+fn check_facet_order(
+    facet_order: &mut Option<BTreeMap<String, (String, OrderBy)>>,
+    current_index: &str,
+    facets_by_index: &Option<Vec<String>>,
+    index: &milli::Index,
+    rtxn: &milli::heed::RoTxn<'_>,
+) -> Result<(), ResponseError> {
+    if let (Some(facet_order), Some(facets_by_index)) = (facet_order, facets_by_index) {
+        let index_facet_order = index.sort_facet_values_by(rtxn)?;
+        for facet in facets_by_index {
+            let index_facet_order = index_facet_order.get(facet);
+            let (previous_index, previous_facet_order) = facet_order
+                .entry(facet.to_owned())
+                .or_insert_with(|| (current_index.to_owned(), index_facet_order));
+            if previous_facet_order != &index_facet_order {
+                return Err(MeilisearchHttpError::InconsistentFacetOrder {
+                    facet: facet.clone(),
+                    previous_facet_order: *previous_facet_order,
+                    previous_uid: previous_index.clone(),
+                    current_uid: current_index.to_owned(),
+                    index_facet_order,
+                }
+                .into());
+            }
+        }
+    };
+    Ok(())
+}

meilisearch/src/search/mod.rs

@@ -441,9 +441,6 @@ pub struct SearchQueryWithIndex {
 }
 
 impl SearchQueryWithIndex {
-    pub fn has_federation_options(&self) -> bool {
-        self.federation_options.is_some()
-    }
     pub fn has_pagination(&self) -> Option<&'static str> {
         if self.offset.is_some() {
             Some("offset")
@@ -458,6 +455,10 @@ impl SearchQueryWithIndex {
         }
     }
 
+    pub fn has_facets(&self) -> Option<&[String]> {
+        self.facets.as_deref().filter(|v| !v.is_empty())
+    }
+
     pub fn into_index_query_federation(self) -> (IndexUid, SearchQuery, Option<FederationOptions>) {
         let SearchQueryWithIndex {
             index_uid,
@@ -987,39 +988,13 @@ pub fn perform_search(
         HitsInfo::OffsetLimit { limit, offset, estimated_total_hits: number_of_hits }
     };
 
-    let (facet_distribution, facet_stats) = match facets {
-        Some(ref fields) => {
-            let mut facet_distribution = index.facets_distribution(&rtxn);
-
-            let max_values_by_facet = index
-                .max_values_per_facet(&rtxn)
-                .map_err(milli::Error::from)?
-                .map(|x| x as usize)
-                .unwrap_or(DEFAULT_VALUES_PER_FACET);
-            facet_distribution.max_values_per_facet(max_values_by_facet);
-
-            let sort_facet_values_by =
-                index.sort_facet_values_by(&rtxn).map_err(milli::Error::from)?;
-
-            if fields.iter().all(|f| f != "*") {
-                let fields: Vec<_> =
-                    fields.iter().map(|n| (n, sort_facet_values_by.get(n))).collect();
-                facet_distribution.facets(fields);
-            }
-
-            let distribution = facet_distribution
-                .candidates(candidates)
-                .default_order_by(sort_facet_values_by.get("*"))
-                .execute()?;
-            let stats = facet_distribution.compute_stats()?;
-            (Some(distribution), Some(stats))
-        }
-        None => (None, None),
-    };
-
-    let facet_stats = facet_stats.map(|stats| {
-        stats.into_iter().map(|(k, (min, max))| (k, FacetStats { min, max })).collect()
-    });
+    let (facet_distribution, facet_stats) = facets
+        .map(move |facets| {
+            compute_facet_distribution_stats(&facets, index, &rtxn, candidates, Route::Search)
+        })
+        .transpose()?
+        .map(|ComputedFacets { distribution, stats }| (distribution, stats))
+        .unzip();
 
     let result = SearchResult {
         hits: documents,
@@ -1035,6 +1010,61 @@ pub fn perform_search(
     Ok(result)
 }
 
+#[derive(Debug, Clone, Default, Serialize)]
+pub struct ComputedFacets {
+    pub distribution: BTreeMap<String, IndexMap<String, u64>>,
+    pub stats: BTreeMap<String, FacetStats>,
+}
+
+enum Route {
+    Search,
+    MultiSearch,
+}
+
+fn compute_facet_distribution_stats<S: AsRef<str>>(
+    facets: &[S],
+    index: &Index,
+    rtxn: &RoTxn,
+    candidates: roaring::RoaringBitmap,
+    route: Route,
+) -> Result<ComputedFacets, ResponseError> {
+    let mut facet_distribution = index.facets_distribution(rtxn);
+
+    let max_values_by_facet = index
+        .max_values_per_facet(rtxn)
+        .map_err(milli::Error::from)?
+        .map(|x| x as usize)
+        .unwrap_or(DEFAULT_VALUES_PER_FACET);
+
+    facet_distribution.max_values_per_facet(max_values_by_facet);
+
+    let sort_facet_values_by = index.sort_facet_values_by(rtxn).map_err(milli::Error::from)?;
+
+    // add specific facet if there is no placeholder
+    if facets.iter().all(|f| f.as_ref() != "*") {
+        let fields: Vec<_> =
+            facets.iter().map(|n| (n, sort_facet_values_by.get(n.as_ref()))).collect();
+        facet_distribution.facets(fields);
+    }
+
+    let distribution = facet_distribution
+        .candidates(candidates)
+        .default_order_by(sort_facet_values_by.get("*"))
+        .execute()
+        .map_err(|error| match (error, route) {
+            (
+                error @ milli::Error::UserError(milli::UserError::InvalidFacetsDistribution {
+                    ..
+                }),
+                Route::MultiSearch,
+            ) => ResponseError::from_msg(error.to_string(), Code::InvalidMultiSearchFacets),
+            (error, _) => error.into(),
+        })?;
+    let stats = facet_distribution.compute_stats()?;
+    let stats = stats.into_iter().map(|(k, (min, max))| (k, FacetStats { min, max })).collect();
+    Ok(ComputedFacets { distribution, stats })
+}
+
 pub fn search_from_kind(
     search_kind: SearchKind,
     search: milli::Search<'_>,

milli/src/search/facet/facet_distribution.rs

@@ -1,4 +1,5 @@
 use std::collections::{BTreeMap, HashMap, HashSet};
+use std::fmt::Display;
 use std::ops::ControlFlow;
 use std::{fmt, mem};
 
@@ -37,6 +38,15 @@ pub enum OrderBy {
     Count,
 }
 
+impl Display for OrderBy {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            OrderBy::Lexicographic => f.write_str("alphabetically"),
+            OrderBy::Count => f.write_str("by count"),
+        }
+    }
+}
+
 pub struct FacetDistribution<'a> {
     facets: Option<HashMap<String, OrderBy>>,
     candidates: Option<RoaringBitmap>,
@@ -100,7 +110,6 @@ impl<'a> FacetDistribution<'a> {
                 let mut lexicographic_distribution = BTreeMap::new();
                 let mut key_buffer: Vec<_> = field_id.to_be_bytes().to_vec();
 
-                let distribution_prelength = distribution.len();
                 let db = self.index.field_id_docid_facet_f64s;
                 for docid in candidates {
                     key_buffer.truncate(mem::size_of::<FieldId>());
@@ -113,23 +122,21 @@ impl<'a> FacetDistribution<'a> {
                     for result in iter {
                         let ((_, _, value), ()) = result?;
                         *lexicographic_distribution.entry(value.to_string()).or_insert(0) += 1;
-
-                        if lexicographic_distribution.len() - distribution_prelength
-                            == self.max_values_per_facet
-                        {
-                            break;
-                        }
                     }
                 }
 
-                distribution.extend(lexicographic_distribution);
+                distribution.extend(
+                    lexicographic_distribution
+                        .into_iter()
+                        .take(self.max_values_per_facet.saturating_sub(distribution.len())),
+                );
             }
             FacetType::String => {
                 let mut normalized_distribution = BTreeMap::new();
                 let mut key_buffer: Vec<_> = field_id.to_be_bytes().to_vec();
 
                 let db = self.index.field_id_docid_facet_strings;
-                'outer: for docid in candidates {
+                for docid in candidates {
                     key_buffer.truncate(mem::size_of::<FieldId>());
                     key_buffer.extend_from_slice(&docid.to_be_bytes());
                     let iter = db
@@ -144,14 +151,14 @@ impl<'a> FacetDistribution<'a> {
                             .or_insert_with(|| (original_value, 0));
                         *count += 1;
 
-                        if normalized_distribution.len() == self.max_values_per_facet {
-                            break 'outer;
-                        }
+                        // we'd like to break here if we have enough facet values, but we are collecting them by increasing docid,
+                        // so higher ranked facets could be in later docids
                     }
                 }
 
                 let iter = normalized_distribution
                     .into_iter()
+                    .take(self.max_values_per_facet.saturating_sub(distribution.len()))
                     .map(|(_normalized, (original, count))| (original.to_string(), count));
                 distribution.extend(iter);
             }
@@ -467,7 +474,7 @@ mod tests {
             .execute()
             .unwrap();
 
-        milli_snap!(format!("{map:?}"), @r###"{"colour": {"Blue": 1}}"###);
+        milli_snap!(format!("{map:?}"), @r###"{"colour": {"Blue": 2}}"###);
 
         let map = FacetDistribution::new(&txn, &index)
             .facets(iter::once(("colour", OrderBy::Count)))