Make the search for facet work

meilisearch/src/routes/indexes/facet_search.rs

@@ -0,0 +1,178 @@
+use std::collections::{BTreeSet, HashSet};
+
+use actix_web::web::Data;
+use actix_web::{web, HttpRequest, HttpResponse};
+use deserr::actix_web::{AwebJson, AwebQueryParameter};
+use index_scheduler::IndexScheduler;
+use log::debug;
+use meilisearch_types::deserr::query_params::Param;
+use meilisearch_types::deserr::{DeserrJsonError, DeserrQueryParamError};
+use meilisearch_types::error::deserr_codes::*;
+use meilisearch_types::error::ResponseError;
+use meilisearch_types::index_uid::IndexUid;
+use meilisearch_types::milli::facet;
+use meilisearch_types::serde_cs::vec::CS;
+use serde_json::Value;
+
+use crate::analytics::{Analytics, SearchAggregator};
+use crate::extractors::authentication::policies::*;
+use crate::extractors::authentication::GuardedData;
+use crate::search::{
+    add_search_rules, perform_facet_search, MatchingStrategy, SearchQuery, DEFAULT_CROP_LENGTH,
+    DEFAULT_CROP_MARKER, DEFAULT_HIGHLIGHT_POST_TAG, DEFAULT_HIGHLIGHT_PRE_TAG,
+    DEFAULT_SEARCH_LIMIT, DEFAULT_SEARCH_OFFSET,
+};
+
+pub fn configure(cfg: &mut web::ServiceConfig) {
+    cfg.service(web::resource("").route(web::post().to(search)));
+}
+
+// #[derive(Debug, deserr::Deserr)]
+// #[deserr(error = DeserrQueryParamError, rename_all = camelCase, deny_unknown_fields)]
+// pub struct FacetSearchQuery {
+//     #[deserr(default, error = DeserrQueryParamError<InvalidSearchQ>)]
+//     facetQuery: Option<String>,
+//     #[deserr(default = Param(DEFAULT_SEARCH_OFFSET()), error = DeserrQueryParamError<InvalidSearchOffset>)]
+//     offset: Param<usize>,
+//     #[deserr(default = Param(DEFAULT_SEARCH_LIMIT()), error = DeserrQueryParamError<InvalidSearchLimit>)]
+//     limit: Param<usize>,
+//     #[deserr(default, error = DeserrQueryParamError<InvalidSearchPage>)]
+//     page: Option<Param<usize>>,
+//     #[deserr(default, error = DeserrQueryParamError<InvalidSearchHitsPerPage>)]
+//     hits_per_page: Option<Param<usize>>,
+//     #[deserr(default, error = DeserrQueryParamError<InvalidSearchAttributesToRetrieve>)]
+//     attributes_to_retrieve: Option<CS<String>>,
+//     #[deserr(default, error = DeserrQueryParamError<InvalidSearchAttributesToCrop>)]
+//     attributes_to_crop: Option<CS<String>>,
+//     #[deserr(default = Param(DEFAULT_CROP_LENGTH()), error = DeserrQueryParamError<InvalidSearchCropLength>)]
+//     crop_length: Param<usize>,
+//     #[deserr(default, error = DeserrQueryParamError<InvalidSearchAttributesToHighlight>)]
+//     attributes_to_highlight: Option<CS<String>>,
+//     #[deserr(default, error = DeserrQueryParamError<InvalidSearchFilter>)]
+//     filter: Option<String>,
+//     #[deserr(default, error = DeserrQueryParamError<InvalidSearchSort>)]
+//     sort: Option<String>,
+//     #[deserr(default, error = DeserrQueryParamError<InvalidSearchShowMatchesPosition>)]
+//     show_matches_position: Param<bool>,
+//     #[deserr(default, error = DeserrQueryParamError<InvalidSearchFacets>)]
+//     facets: Option<CS<String>>,
+//     #[deserr( default = DEFAULT_HIGHLIGHT_PRE_TAG(), error = DeserrQueryParamError<InvalidSearchHighlightPreTag>)]
+//     highlight_pre_tag: String,
+//     #[deserr( default = DEFAULT_HIGHLIGHT_POST_TAG(), error = DeserrQueryParamError<InvalidSearchHighlightPostTag>)]
+//     highlight_post_tag: String,
+//     #[deserr(default = DEFAULT_CROP_MARKER(), error = DeserrQueryParamError<InvalidSearchCropMarker>)]
+//     crop_marker: String,
+//     #[deserr(default, error = DeserrQueryParamError<InvalidSearchMatchingStrategy>)]
+//     matching_strategy: MatchingStrategy,
+// }
+
+// TODO improve the error messages
+#[derive(Debug, Clone, Default, PartialEq, Eq, deserr::Deserr)]
+#[deserr(error = DeserrJsonError, rename_all = camelCase, deny_unknown_fields)]
+pub struct FacetSearchQuery {
+    #[deserr(default, error = DeserrJsonError<InvalidSearchQ>)]
+    pub facet_query: Option<String>,
+    #[deserr(default, error = DeserrJsonError<InvalidSearchQ>)]
+    pub facet_name: String,
+    #[deserr(default, error = DeserrJsonError<InvalidSearchQ>)]
+    pub q: Option<String>,
+    #[deserr(default = DEFAULT_SEARCH_OFFSET(), error = DeserrJsonError<InvalidSearchOffset>)]
+    pub offset: usize,
+    #[deserr(default = DEFAULT_SEARCH_LIMIT(), error = DeserrJsonError<InvalidSearchLimit>)]
+    pub limit: usize,
+    #[deserr(default, error = DeserrJsonError<InvalidSearchPage>)]
+    pub page: Option<usize>,
+    #[deserr(default, error = DeserrJsonError<InvalidSearchHitsPerPage>)]
+    pub hits_per_page: Option<usize>,
+    #[deserr(default, error = DeserrJsonError<InvalidSearchAttributesToRetrieve>)]
+    pub attributes_to_retrieve: Option<BTreeSet<String>>,
+    #[deserr(default, error = DeserrJsonError<InvalidSearchAttributesToCrop>)]
+    pub attributes_to_crop: Option<Vec<String>>,
+    #[deserr(default, error = DeserrJsonError<InvalidSearchCropLength>, default = DEFAULT_CROP_LENGTH())]
+    pub crop_length: usize,
+    #[deserr(default, error = DeserrJsonError<InvalidSearchAttributesToHighlight>)]
+    pub attributes_to_highlight: Option<HashSet<String>>,
+    #[deserr(default, error = DeserrJsonError<InvalidSearchShowMatchesPosition>, default)]
+    pub show_matches_position: bool,
+    #[deserr(default, error = DeserrJsonError<InvalidSearchFilter>)]
+    pub filter: Option<Value>,
+    #[deserr(default, error = DeserrJsonError<InvalidSearchSort>)]
+    pub sort: Option<Vec<String>>,
+    #[deserr(default, error = DeserrJsonError<InvalidSearchFacets>)]
+    pub facets: Option<Vec<String>>,
+    #[deserr(default, error = DeserrJsonError<InvalidSearchHighlightPreTag>, default = DEFAULT_HIGHLIGHT_PRE_TAG())]
+    pub highlight_pre_tag: String,
+    #[deserr(default, error = DeserrJsonError<InvalidSearchHighlightPostTag>, default = DEFAULT_HIGHLIGHT_POST_TAG())]
+    pub highlight_post_tag: String,
+    #[deserr(default, error = DeserrJsonError<InvalidSearchCropMarker>, default = DEFAULT_CROP_MARKER())]
+    pub crop_marker: String,
+    #[deserr(default, error = DeserrJsonError<InvalidSearchMatchingStrategy>, default)]
+    pub matching_strategy: MatchingStrategy,
+}
+
+pub async fn search(
+    index_scheduler: GuardedData<ActionPolicy<{ actions::SEARCH }>, Data<IndexScheduler>>,
+    index_uid: web::Path<String>,
+    params: AwebJson<FacetSearchQuery, DeserrJsonError>,
+    req: HttpRequest,
+    analytics: web::Data<dyn Analytics>,
+) -> Result<HttpResponse, ResponseError> {
+    let index_uid = IndexUid::try_from(index_uid.into_inner())?;
+
+    let query = params.into_inner();
+    debug!("facet search called with params: {:?}", query);
+
+    let facet_query = query.facet_query.clone();
+    let facet_name = query.facet_name.clone();
+    let mut search_query = SearchQuery::from(query);
+
+    // Tenant token search_rules.
+    if let Some(search_rules) = index_scheduler.filters().get_index_search_rules(&index_uid) {
+        add_search_rules(&mut search_query, search_rules);
+    }
+
+    // TODO log stuff
+    // let mut aggregate = SearchAggregator::from_query(&query, &req);
+
+    let index = index_scheduler.index(&index_uid)?;
+    let search_result = tokio::task::spawn_blocking(move || {
+        perform_facet_search(&index, search_query, facet_query, facet_name)
+    })
+    .await?;
+
+    // TODO log stuff
+    // if let Ok(ref search_result) = search_result {
+    //     aggregate.succeed(search_result);
+    // }
+    // TODO analytics
+    // analytics.post_search(aggregate);
+
+    let search_result = search_result?;
+
+    debug!("returns: {:?}", search_result);
+    Ok(HttpResponse::Ok().json(search_result))
+}
+
+impl From<FacetSearchQuery> for SearchQuery {
+    fn from(value: FacetSearchQuery) -> Self {
+        SearchQuery {
+            q: value.q,
+            offset: value.offset,
+            limit: value.limit,
+            page: value.page,
+            hits_per_page: value.hits_per_page,
+            attributes_to_retrieve: value.attributes_to_retrieve,
+            attributes_to_crop: value.attributes_to_crop,
+            crop_length: value.crop_length,
+            attributes_to_highlight: value.attributes_to_highlight,
+            show_matches_position: value.show_matches_position,
+            filter: value.filter,
+            sort: value.sort,
+            facets: value.facets,
+            highlight_pre_tag: value.highlight_pre_tag,
+            highlight_post_tag: value.highlight_post_tag,
+            crop_marker: value.crop_marker,
+            matching_strategy: value.matching_strategy,
+        }
+    }
+}

meilisearch/src/routes/indexes/mod.rs

@@ -24,6 +24,7 @@ use crate::extractors::authentication::{AuthenticationError, GuardedData};
 use crate::extractors::sequential_extractor::SeqHandler;
 
 pub mod documents;
+pub mod facet_search;
 pub mod search;
 pub mod settings;
 
@@ -44,6 +45,7 @@ pub fn configure(cfg: &mut web::ServiceConfig) {
             .service(web::resource("/stats").route(web::get().to(SeqHandler(get_index_stats))))
             .service(web::scope("/documents").configure(documents::configure))
             .service(web::scope("/search").configure(search::configure))
+            .service(web::scope("/facet-search").configure(facet_search::configure))
             .service(web::scope("/settings").configure(settings::configure)),
     );
 }

meilisearch/src/search.rs

@@ -3,12 +3,15 @@ use std::collections::{BTreeMap, BTreeSet, HashSet};
 use std::str::FromStr;
 use std::time::Instant;
 
+use actix_http::header::q;
 use deserr::Deserr;
 use either::Either;
 use meilisearch_auth::IndexSearchRules;
 use meilisearch_types::deserr::DeserrJsonError;
 use meilisearch_types::error::deserr_codes::*;
+use meilisearch_types::heed::RoTxn;
 use meilisearch_types::index_uid::IndexUid;
+use meilisearch_types::milli::{facet, FacetSearchResult, SearchForFacetValue};
 use meilisearch_types::settings::DEFAULT_PAGINATION_MAX_TOTAL_HITS;
 use meilisearch_types::{milli, Document};
 use milli::tokenizer::TokenizerBuilder;
@@ -21,6 +24,7 @@ use serde::Serialize;
 use serde_json::{json, Value};
 
 use crate::error::MeilisearchHttpError;
+use crate::routes::indexes::facet_search::FacetSearchQuery;
 
 type MatchesPosition = BTreeMap<String, Vec<MatchBounds>>;
 
@@ -170,7 +174,7 @@ impl SearchQueryWithIndex {
     }
 }
 
-#[derive(Debug, Clone, PartialEq, Eq, Deserr)]
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Deserr)]
 #[deserr(rename_all = camelCase)]
 pub enum MatchingStrategy {
     /// Remove query words from last to first
@@ -261,13 +265,11 @@ pub fn add_search_rules(query: &mut SearchQuery, rules: IndexSearchRules) {
     }
 }
 
-pub fn perform_search(
-    index: &Index,
-    query: SearchQuery,
-) -> Result<SearchResult, MeilisearchHttpError> {
-    let before_search = Instant::now();
-    let rtxn = index.read_txn()?;
-
+fn prepare_search<'t>(
+    index: &'t Index,
+    rtxn: &'t RoTxn,
+    query: &'t SearchQuery,
+) -> Result<(milli::Search<'t>, bool, usize, usize), MeilisearchHttpError> {
     let mut search = index.search(&rtxn);
 
     if let Some(ref query) = query.q {
@@ -320,6 +322,19 @@ pub fn perform_search(
         search.sort_criteria(sort);
     }
 
+    Ok((search, is_finite_pagination, max_total_hits, offset))
+}
+
+pub fn perform_search(
+    index: &Index,
+    query: SearchQuery,
+) -> Result<SearchResult, MeilisearchHttpError> {
+    let before_search = Instant::now();
+    let rtxn = index.read_txn()?;
+
+    let (search, is_finite_pagination, max_total_hits, offset) =
+        prepare_search(index, &rtxn, &query)?;
+
     let milli::SearchResult { documents_ids, matching_words, candidates, .. } = search.execute()?;
 
     let fields_ids_map = index.fields_ids_map(&rtxn).unwrap();
@@ -473,6 +488,24 @@ pub fn perform_search(
     Ok(result)
 }
 
+pub fn perform_facet_search(
+    index: &Index,
+    search_query: SearchQuery,
+    mut facet_query: Option<String>,
+    facet_name: String,
+) -> Result<Vec<FacetSearchResult>, MeilisearchHttpError> {
+    let before_search = Instant::now();
+    let rtxn = index.read_txn()?;
+
+    let (search, _, _, _) = prepare_search(index, &rtxn, &search_query)?;
+    let mut facet_search = SearchForFacetValue::new(facet_name, search);
+    if let Some(facet_query) = facet_query.take() {
+        facet_search.query(facet_query);
+    }
+
+    facet_search.execute().map_err(Into::into)
+}
+
 fn insert_geo_distance(sorts: &[String], document: &mut Document) {
     lazy_static::lazy_static! {
         static ref GEO_REGEX: Regex =

milli/src/heed_codec/fst_set_codec.rs

@@ -0,0 +1,23 @@
+use fst::Set;
+use std::borrow::Cow;
+
+use heed::{BytesDecode, BytesEncode};
+
+/// A codec for values of type `Set<&[u8]>`.
+pub struct FstSetCodec;
+
+impl<'a> BytesEncode<'a> for FstSetCodec {
+    type EItem = Set<Vec<u8>>;
+
+    fn bytes_encode(item: &'a Self::EItem) -> Option<Cow<'a, [u8]>> {
+        Some(Cow::Borrowed(item.as_fst().as_bytes()))
+    }
+}
+
+impl<'a> BytesDecode<'a> for FstSetCodec {
+    type DItem = Set<&'a [u8]>;
+
+    fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
+        Some(Set::new(bytes).ok()?)
+    }
+}

milli/src/heed_codec/mod.rs

@@ -2,6 +2,7 @@ mod beu32_str_codec;
 mod byte_slice_ref;
 pub mod facet;
 mod field_id_word_count_codec;
+mod fst_set_codec;
 mod obkv_codec;
 mod roaring_bitmap;
 mod roaring_bitmap_length;
@@ -15,6 +16,7 @@ pub use str_ref::StrRefCodec;
 
 pub use self::beu32_str_codec::BEU32StrCodec;
 pub use self::field_id_word_count_codec::FieldIdWordCountCodec;
+pub use self::fst_set_codec::FstSetCodec;
 pub use self::obkv_codec::ObkvCodec;
 pub use self::roaring_bitmap::{BoRoaringBitmapCodec, CboRoaringBitmapCodec, RoaringBitmapCodec};
 pub use self::roaring_bitmap_length::{

milli/src/index.rs

@@ -19,7 +19,7 @@ use crate::heed_codec::facet::{
     FacetGroupKeyCodec, FacetGroupValueCodec, FieldDocIdFacetF64Codec, FieldDocIdFacetStringCodec,
     FieldIdCodec, OrderedF64Codec,
 };
-use crate::heed_codec::{ScriptLanguageCodec, StrRefCodec};
+use crate::heed_codec::{FstSetCodec, ScriptLanguageCodec, StrRefCodec};
 use crate::{
     default_criteria, BEU32StrCodec, BoRoaringBitmapCodec, CboRoaringBitmapCodec, Criterion,
     DocumentId, ExternalDocumentsIds, FacetDistribution, FieldDistribution, FieldId,
@@ -81,6 +81,7 @@ pub mod db_name {
     pub const FACET_ID_F64_DOCIDS: &str = "facet-id-f64-docids";
     pub const FACET_ID_EXISTS_DOCIDS: &str = "facet-id-exists-docids";
     pub const FACET_ID_STRING_DOCIDS: &str = "facet-id-string-docids";
+    pub const FACET_ID_STRING_FST: &str = "facet-id-string-fst";
     pub const FIELD_ID_DOCID_FACET_F64S: &str = "field-id-docid-facet-f64s";
     pub const FIELD_ID_DOCID_FACET_STRINGS: &str = "field-id-docid-facet-strings";
     pub const DOCUMENTS: &str = "documents";
@@ -134,6 +135,8 @@ pub struct Index {
     pub facet_id_f64_docids: Database<FacetGroupKeyCodec<OrderedF64Codec>, FacetGroupValueCodec>,
     /// Maps the facet field id and ranges of strings with the docids that corresponds to them.
     pub facet_id_string_docids: Database<FacetGroupKeyCodec<StrRefCodec>, FacetGroupValueCodec>,
+    /// Maps the facet field id of the string facets with an FST containing all the facets values.
+    pub facet_id_string_fst: Database<OwnedType<BEU16>, FstSetCodec>,
 
     /// Maps the document id, the facet field id and the numbers.
     pub field_id_docid_facet_f64s: Database<FieldDocIdFacetF64Codec, Unit>,
@@ -153,7 +156,7 @@ impl Index {
     ) -> Result<Index> {
         use db_name::*;
 
-        options.max_dbs(19);
+        options.max_dbs(20);
         unsafe { options.flag(Flags::MdbAlwaysFreePages) };
 
         let env = options.open(path)?;
@@ -174,6 +177,7 @@ impl Index {
         let word_prefix_position_docids = env.create_database(Some(WORD_PREFIX_POSITION_DOCIDS))?;
         let facet_id_f64_docids = env.create_database(Some(FACET_ID_F64_DOCIDS))?;
         let facet_id_string_docids = env.create_database(Some(FACET_ID_STRING_DOCIDS))?;
+        let facet_id_string_fst = env.create_database(Some(FACET_ID_STRING_FST))?;
         let facet_id_exists_docids = env.create_database(Some(FACET_ID_EXISTS_DOCIDS))?;
 
         let field_id_docid_facet_f64s = env.create_database(Some(FIELD_ID_DOCID_FACET_F64S))?;
@@ -200,6 +204,7 @@ impl Index {
             field_id_word_count_docids,
             facet_id_f64_docids,
             facet_id_string_docids,
+            facet_id_string_fst,
             facet_id_exists_docids,
             field_id_docid_facet_f64s,
             field_id_docid_facet_strings,

milli/src/lib.rs

@@ -43,9 +43,9 @@ pub use self::heed_codec::{
 };
 pub use self::index::Index;
 pub use self::search::{
-    CriterionImplementationStrategy, FacetDistribution, Filter, FormatOptions, MatchBounds,
-    MatcherBuilder, MatchingWord, MatchingWords, Search, SearchResult, TermsMatchingStrategy,
-    DEFAULT_VALUES_PER_FACET,
+    CriterionImplementationStrategy, FacetDistribution, FacetSearchResult, Filter, FormatOptions,
+    MatchBounds, MatcherBuilder, MatchingWord, MatchingWords, Search, SearchForFacetValue,
+    SearchResult, TermsMatchingStrategy, DEFAULT_VALUES_PER_FACET,
 };
 
 pub type Result<T> = std::result::Result<T, error::Error>;

milli/src/search/mod.rs

@@ -22,15 +22,20 @@ pub use self::matches::{
 };
 use self::query_tree::QueryTreeBuilder;
 use crate::error::UserError;
+use crate::heed_codec::facet::{FacetGroupKey, FacetGroupValue};
+use crate::index::db_name::EXACT_WORD_DOCIDS;
 use crate::search::criteria::r#final::{Final, FinalResult};
 use crate::search::criteria::InitialCandidates;
-use crate::{AscDesc, Criterion, DocumentId, Index, Member, Result};
+use crate::{fields_ids_map, AscDesc, Criterion, DocumentId, Index, Member, Result, BEU16};
 
 // Building these factories is not free.
 static LEVDIST0: Lazy<LevBuilder> = Lazy::new(|| LevBuilder::new(0, true));
 static LEVDIST1: Lazy<LevBuilder> = Lazy::new(|| LevBuilder::new(1, true));
 static LEVDIST2: Lazy<LevBuilder> = Lazy::new(|| LevBuilder::new(2, true));
 
+/// The maximum number of facets returned by the facet search route.
+const MAX_NUMBER_OF_FACETS: usize = 1000;
+
 mod criteria;
 mod distinct;
 pub mod facet;
@@ -446,6 +451,114 @@ pub fn build_dfa(word: &str, typos: u8, is_prefix: bool) -> DFA {
     }
 }
 
+pub struct SearchForFacetValue<'a> {
+    query: Option<String>,
+    facet: String,
+    search_query: Search<'a>,
+}
+
+impl<'a> SearchForFacetValue<'a> {
+    pub fn new(facet: String, search_query: Search<'a>) -> SearchForFacetValue<'a> {
+        SearchForFacetValue { query: None, facet, search_query }
+    }
+
+    pub fn query(&mut self, query: impl Into<String>) -> &mut Self {
+        self.query = Some(query.into());
+        self
+    }
+
+    pub fn execute(&self) -> Result<Vec<FacetSearchResult>> {
+        let index = self.search_query.index;
+        let rtxn = self.search_query.rtxn;
+
+        let filterable_fields = index.filterable_fields(rtxn)?;
+        if !filterable_fields.contains(&self.facet) {
+            // TODO create a new type of error
+            return Err(UserError::InvalidSortableAttribute {
+                field: self.facet.clone(),
+                valid_fields: filterable_fields.into_iter().collect(),
+            })?;
+        }
+
+        let fields_ids_map = index.fields_ids_map(rtxn)?;
+        let (field_id, fst) = match fields_ids_map.id(&self.facet) {
+            Some(fid) => {
+                match self.search_query.index.facet_id_string_fst.get(rtxn, &BEU16::new(fid))? {
+                    Some(fst) => (fid, fst),
+                    None => todo!("return an error, is the user trying to search in numbers?"),
+                }
+            }
+            None => todo!("return an internal error bug"),
+        };
+
+        let search_candidates = self.search_query.execute()?.candidates;
+
+        match self.query.as_ref() {
+            Some(query) => {
+                let is_prefix = true;
+                let starts = StartsWith(Str::new(get_first(query)));
+                let first = Intersection(build_dfa(query, 1, is_prefix), Complement(&starts));
+                let second_dfa = build_dfa(query, 2, is_prefix);
+                let second = Intersection(&second_dfa, &starts);
+                let automaton = Union(first, &second);
+
+                let mut stream = fst.search(automaton).into_stream();
+                let mut result = vec![];
+                let mut length = 0;
+                while let Some(facet_value) = stream.next() {
+                    let value = std::str::from_utf8(facet_value)?;
+                    let key = FacetGroupKey { field_id, level: 0, left_bound: value };
+                    let docids = match index.facet_id_string_docids.get(rtxn, &key)? {
+                        Some(FacetGroupValue { bitmap, .. }) => bitmap,
+                        None => todo!("return an internal error"),
+                    };
+                    let count = search_candidates.intersection_len(&docids);
+                    if count != 0 {
+                        result.push(FacetSearchResult { value: value.to_string(), count });
+                        length += 1;
+                    }
+                    if length >= MAX_NUMBER_OF_FACETS {
+                        break;
+                    }
+                }
+
+                Ok(result)
+            }
+            None => {
+                let mut stream = fst.stream();
+                let mut result = vec![];
+                let mut length = 0;
+                while let Some(facet_value) = stream.next() {
+                    let value = std::str::from_utf8(facet_value)?;
+                    let key = FacetGroupKey { field_id, level: 0, left_bound: value };
+                    let docids = match index.facet_id_string_docids.get(rtxn, &key)? {
+                        Some(FacetGroupValue { bitmap, .. }) => bitmap,
+                        None => todo!("return an internal error"),
+                    };
+                    let count = search_candidates.intersection_len(&docids);
+                    if count != 0 {
+                        result.push(FacetSearchResult { value: value.to_string(), count });
+                        length += 1;
+                    }
+                    if length >= MAX_NUMBER_OF_FACETS {
+                        break;
+                    }
+                }
+
+                Ok(result)
+            }
+        }
+    }
+}
+
+#[derive(Debug, serde::Serialize)]
+pub struct FacetSearchResult {
+    /// The original facet value
+    pub value: String,
+    /// The number of documents associated to this facet
+    pub count: u64,
+}
+
 #[cfg(test)]
 mod test {
     use super::*;

milli/src/update/clear_documents.rs

@@ -33,6 +33,7 @@ impl<'t, 'u, 'i> ClearDocuments<'t, 'u, 'i> {
             script_language_docids,
             facet_id_f64_docids,
             facet_id_string_docids,
+            facet_id_string_fst: _,
             facet_id_exists_docids,
             field_id_docid_facet_f64s,
             field_id_docid_facet_strings,

milli/src/update/delete_documents.rs

@@ -241,6 +241,7 @@ impl<'t, 'u, 'i> DeleteDocuments<'t, 'u, 'i> {
             word_prefix_position_docids,
             facet_id_f64_docids: _,
             facet_id_string_docids: _,
+            facet_id_string_fst: _,
             field_id_docid_facet_f64s: _,
             field_id_docid_facet_strings: _,
             script_language_docids,

milli/src/update/facet/mod.rs

@@ -78,15 +78,16 @@ pub const FACET_MIN_LEVEL_SIZE: u8 = 5;
 
 use std::fs::File;
 
+use heed::types::DecodeIgnore;
 use log::debug;
 use time::OffsetDateTime;
 
 use self::incremental::FacetsUpdateIncremental;
 use super::FacetsUpdateBulk;
 use crate::facet::FacetType;
-use crate::heed_codec::facet::{FacetGroupKeyCodec, FacetGroupValueCodec};
+use crate::heed_codec::facet::{FacetGroupKey, FacetGroupKeyCodec, FacetGroupValueCodec};
 use crate::heed_codec::ByteSliceRefCodec;
-use crate::{Index, Result};
+use crate::{Index, Result, BEU16};
 
 pub mod bulk;
 pub mod delete;
@@ -157,6 +158,43 @@ impl<'i> FacetsUpdate<'i> {
             );
             incremental_update.execute(wtxn)?;
         }
+
+        // We compute one FST by string facet
+        let mut text_fsts = vec![];
+        let mut current_fst: Option<(u16, fst::SetBuilder<Vec<u8>>)> = None;
+        let database = self.index.facet_id_string_docids.remap_data_type::<DecodeIgnore>();
+        for result in database.iter(&wtxn)? {
+            let (facet_group_key, _) = result?;
+            if let FacetGroupKey { field_id, level: 0, left_bound } = facet_group_key {
+                current_fst = match current_fst.take() {
+                    Some((fid, fst_builder)) if fid != field_id => {
+                        let fst = fst_builder.into_set();
+                        text_fsts.push((field_id, fst));
+                        Some((field_id, fst::SetBuilder::memory()))
+                    }
+                    Some((field_id, fst_builder)) => Some((field_id, fst_builder)),
+                    None => Some((field_id, fst::SetBuilder::memory())),
+                };
+
+                if let Some((_, fst_builder)) = current_fst.as_mut() {
+                    fst_builder.insert(left_bound)?;
+                }
+            }
+        }
+
+        if let Some((field_id, fst_builder)) = current_fst {
+            let fst = fst_builder.into_set();
+            text_fsts.push((field_id, fst));
+        }
+
+        // We remove all of the previous FSTs that were in this database
+        self.index.facet_id_string_fst.clear(wtxn)?;
+
+        // We write those FSTs in LMDB now
+        for (field_id, fst) in text_fsts {
+            self.index.facet_id_string_fst.put(wtxn, &BEU16::new(field_id), &fst)?;
+        }
+
         Ok(())
     }
 }

milli/src/update/facets.rs

@@ -1 +0,0 @@
-