diff --git a/src/fields_ids_map.rs b/src/fields_ids_map.rs
index 82d06e818..ce79e6e04 100644
--- a/src/fields_ids_map.rs
+++ b/src/fields_ids_map.rs
@@ -1,11 +1,12 @@
 use std::collections::BTreeMap;
 use serde::{Serialize, Deserialize};
+use crate::FieldId;
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct FieldsIdsMap {
-    names_ids: BTreeMap<String, u8>,
-    ids_names: BTreeMap<u8, String>,
-    next_id: Option<u8>,
+    names_ids: BTreeMap<String, FieldId>,
+    ids_names: BTreeMap<FieldId, String>,
+    next_id: Option<FieldId>,
 }
 
 impl FieldsIdsMap {
@@ -29,7 +30,7 @@ impl FieldsIdsMap {
 
     /// Returns the field id related to a field name, it will create a new field id if the
     /// name is not already known. Returns `None` if the maximum field id as been reached.
-    pub fn insert(&mut self, name: &str) -> Option<u8> {
+    pub fn insert(&mut self, name: &str) -> Option<FieldId> {
         match self.names_ids.get(name) {
             Some(id) => Some(*id),
             None => {
@@ -43,17 +44,17 @@ impl FieldsIdsMap {
     }
 
     /// Get the id of a field based on its name.
-    pub fn id(&self, name: &str) -> Option<u8> {
+    pub fn id(&self, name: &str) -> Option<FieldId> {
         self.names_ids.get(name).copied()
     }
 
     /// Get the name of a field based on its id.
-    pub fn name(&self, id: u8) -> Option<&str> {
+    pub fn name(&self, id: FieldId) -> Option<&str> {
         self.ids_names.get(&id).map(String::as_str)
     }
 
     /// Remove a field name and id based on its name.
-    pub fn remove(&mut self, name: &str) -> Option<u8> {
+    pub fn remove(&mut self, name: &str) -> Option<FieldId> {
         match self.names_ids.remove(name) {
             Some(id) => self.ids_names.remove_entry(&id).map(|(id, _)| id),
             None => None,
@@ -61,7 +62,7 @@ impl FieldsIdsMap {
     }
 
     /// Iterate over the ids and names in the ids order.
-    pub fn iter(&self) -> impl Iterator<Item=(u8, &str)> {
+    pub fn iter(&self) -> impl Iterator<Item=(FieldId, &str)> {
         self.ids_names.iter().map(|(id, name)| (*id, name.as_str()))
     }
 }
diff --git a/src/heed_codec/facet/facet_level_value_f64_codec.rs b/src/heed_codec/facet/facet_level_value_f64_codec.rs
index 1ee8e6bf3..a4642f961 100644
--- a/src/heed_codec/facet/facet_level_value_f64_codec.rs
+++ b/src/heed_codec/facet/facet_level_value_f64_codec.rs
@@ -2,12 +2,13 @@ use std::borrow::Cow;
 use std::convert::TryInto;
 
 use crate::facet::value_encoding::f64_into_bytes;
+use crate::FieldId;
 
 // TODO do not de/serialize right bound when level = 0
 pub struct FacetLevelValueF64Codec;
 
 impl<'a> heed::BytesDecode<'a> for FacetLevelValueF64Codec {
-    type DItem = (u8, u8, f64, f64);
+    type DItem = (FieldId, u8, f64, f64);
 
     fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
         let (field_id, bytes) = bytes.split_first()?;
@@ -27,7 +28,7 @@ impl<'a> heed::BytesDecode<'a> for FacetLevelValueF64Codec {
 }
 
 impl heed::BytesEncode<'_> for FacetLevelValueF64Codec {
-    type EItem = (u8, u8, f64, f64);
+    type EItem = (FieldId, u8, f64, f64);
 
     fn bytes_encode((field_id, level, left, right): &Self::EItem) -> Option<Cow<[u8]>> {
         let mut buffer = [0u8; 32];
diff --git a/src/heed_codec/facet/facet_level_value_i64_codec.rs b/src/heed_codec/facet/facet_level_value_i64_codec.rs
index 7cf9a714b..cc0d3120d 100644
--- a/src/heed_codec/facet/facet_level_value_i64_codec.rs
+++ b/src/heed_codec/facet/facet_level_value_i64_codec.rs
@@ -2,11 +2,12 @@ use std::borrow::Cow;
 use std::convert::TryInto;
 
 use crate::facet::value_encoding::{i64_from_bytes, i64_into_bytes};
+use crate::FieldId;
 
 pub struct FacetLevelValueI64Codec;
 
 impl<'a> heed::BytesDecode<'a> for FacetLevelValueI64Codec {
-    type DItem = (u8, u8, i64, i64);
+    type DItem = (FieldId, u8, i64, i64);
 
     fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
         let (field_id, bytes) = bytes.split_first()?;
@@ -24,7 +25,7 @@ impl<'a> heed::BytesDecode<'a> for FacetLevelValueI64Codec {
 }
 
 impl heed::BytesEncode<'_> for FacetLevelValueI64Codec {
-    type EItem = (u8, u8, i64, i64);
+    type EItem = (FieldId, u8, i64, i64);
 
     fn bytes_encode((field_id, level, left, right): &Self::EItem) -> Option<Cow<[u8]>> {
         let left = i64_into_bytes(*left);
diff --git a/src/heed_codec/facet/facet_value_string_codec.rs b/src/heed_codec/facet/facet_value_string_codec.rs
index faa8b407b..350efc450 100644
--- a/src/heed_codec/facet/facet_value_string_codec.rs
+++ b/src/heed_codec/facet/facet_value_string_codec.rs
@@ -1,10 +1,12 @@
 use std::borrow::Cow;
 use std::str;
 
+use crate::FieldId;
+
 pub struct FacetValueStringCodec;
 
 impl<'a> heed::BytesDecode<'a> for FacetValueStringCodec {
-    type DItem = (u8, &'a str);
+    type DItem = (FieldId, &'a str);
 
     fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
         let (field_id, bytes) = bytes.split_first()?;
@@ -14,7 +16,7 @@ impl<'a> heed::BytesDecode<'a> for FacetValueStringCodec {
 }
 
 impl<'a> heed::BytesEncode<'a> for FacetValueStringCodec {
-    type EItem = (u8, &'a str);
+    type EItem = (FieldId, &'a str);
 
     fn bytes_encode((field_id, value): &Self::EItem) -> Option<Cow<[u8]>> {
         let mut bytes = Vec::with_capacity(value.len() + 1);
diff --git a/src/index.rs b/src/index.rs
index b21c7d39b..b0c2b1a3f 100644
--- a/src/index.rs
+++ b/src/index.rs
@@ -10,7 +10,7 @@ use roaring::RoaringBitmap;
 use crate::facet::FacetType;
 use crate::fields_ids_map::FieldsIdsMap;
 use crate::Search;
-use crate::{BEU32, DocumentId, ExternalDocumentsIds};
+use crate::{BEU32, DocumentId, FieldId, ExternalDocumentsIds};
 use crate::{
     RoaringBitmapCodec, BEU32StrCodec, StrStrU8Codec, ObkvCodec,
     BoRoaringBitmapCodec, CboRoaringBitmapCodec,
@@ -107,8 +107,8 @@ impl Index {
     /* primary key */
 
     /// Writes the documents primary key, this is the field name that is used to store the id.
-    pub fn put_primary_key(&self, wtxn: &mut RwTxn, primary_key: u8) -> heed::Result<()> {
-        self.main.put::<_, Str, OwnedType<u8>>(wtxn, PRIMARY_KEY_KEY, &primary_key)
+    pub fn put_primary_key(&self, wtxn: &mut RwTxn, primary_key: FieldId) -> heed::Result<()> {
+        self.main.put::<_, Str, OwnedType<FieldId>>(wtxn, PRIMARY_KEY_KEY, &primary_key)
     }
 
     /// Deletes the primary key of the documents, this can be done to reset indexes settings.
@@ -117,8 +117,8 @@ impl Index {
     }
 
     /// Returns the documents primary key, `None` if it hasn't been defined.
-    pub fn primary_key(&self, rtxn: &RoTxn) -> heed::Result<Option<u8>> {
-        self.main.get::<_, Str, OwnedType<u8>>(rtxn, PRIMARY_KEY_KEY)
+    pub fn primary_key(&self, rtxn: &RoTxn) -> heed::Result<Option<FieldId>> {
+        self.main.get::<_, Str, OwnedType<FieldId>>(rtxn, PRIMARY_KEY_KEY)
     }
 
     /* external documents ids */
@@ -172,7 +172,7 @@ impl Index {
 
     /// Writes the fields ids that must be displayed in the defined order.
     /// There must be not be any duplicate field id.
-    pub fn put_displayed_fields(&self, wtxn: &mut RwTxn, fields: &[u8]) -> heed::Result<()> {
+    pub fn put_displayed_fields(&self, wtxn: &mut RwTxn, fields: &[FieldId]) -> heed::Result<()> {
         self.main.put::<_, Str, ByteSlice>(wtxn, DISPLAYED_FIELDS_KEY, fields)
     }
 
@@ -184,14 +184,14 @@ impl Index {
 
     /// Returns the displayed fields ids in the order they must be returned. If it returns
     /// `None` it means that all the attributes are displayed in the order of the `FieldsIdsMap`.
-    pub fn displayed_fields<'t>(&self, rtxn: &'t RoTxn) -> heed::Result<Option<&'t [u8]>> {
+    pub fn displayed_fields<'t>(&self, rtxn: &'t RoTxn) -> heed::Result<Option<&'t [FieldId]>> {
         self.main.get::<_, Str, ByteSlice>(rtxn, DISPLAYED_FIELDS_KEY)
     }
 
     /* searchable fields */
 
     /// Writes the searchable fields, when this list is specified, only these are indexed.
-    pub fn put_searchable_fields(&self, wtxn: &mut RwTxn, fields: &[u8]) -> heed::Result<()> {
+    pub fn put_searchable_fields(&self, wtxn: &mut RwTxn, fields: &[FieldId]) -> heed::Result<()> {
         assert!(fields.windows(2).all(|win| win[0] < win[1])); // is sorted
         self.main.put::<_, Str, ByteSlice>(wtxn, SEARCHABLE_FIELDS_KEY, fields)
     }
@@ -203,7 +203,7 @@ impl Index {
 
     /// Returns the searchable fields ids, those are the fields that are indexed,
     /// if the searchable fields aren't there it means that **all** the fields are indexed.
-    pub fn searchable_fields<'t>(&self, rtxn: &'t RoTxn) -> heed::Result<Option<&'t [u8]>> {
+    pub fn searchable_fields<'t>(&self, rtxn: &'t RoTxn) -> heed::Result<Option<&'t [FieldId]>> {
         self.main.get::<_, Str, ByteSlice>(rtxn, SEARCHABLE_FIELDS_KEY)
     }
 
@@ -211,7 +211,7 @@ impl Index {
 
     /// Writes the facet fields ids associated with their facet type or `None` if
     /// the facet type is currently unknown.
-    pub fn put_faceted_fields(&self, wtxn: &mut RwTxn, fields_types: &HashMap<u8, FacetType>) -> heed::Result<()> {
+    pub fn put_faceted_fields(&self, wtxn: &mut RwTxn, fields_types: &HashMap<FieldId, FacetType>) -> heed::Result<()> {
         self.main.put::<_, Str, SerdeJson<_>>(wtxn, FACETED_FIELDS_KEY, fields_types)
     }
 
@@ -221,14 +221,14 @@ impl Index {
     }
 
     /// Returns the facet fields ids associated with their facet type.
-    pub fn faceted_fields(&self, wtxn: &RoTxn) -> heed::Result<HashMap<u8, FacetType>> {
+    pub fn faceted_fields(&self, wtxn: &RoTxn) -> heed::Result<HashMap<FieldId, FacetType>> {
         Ok(self.main.get::<_, Str, SerdeJson<_>>(wtxn, FACETED_FIELDS_KEY)?.unwrap_or_default())
     }
 
     /* faceted documents ids */
 
     /// Writes the documents ids that are faceted under this field id.
-    pub fn put_faceted_documents_ids(&self, wtxn: &mut RwTxn, field_id: u8, docids: &RoaringBitmap) -> heed::Result<()> {
+    pub fn put_faceted_documents_ids(&self, wtxn: &mut RwTxn, field_id: FieldId, docids: &RoaringBitmap) -> heed::Result<()> {
         let mut buffer = [0u8; FACETED_DOCUMENTS_IDS_PREFIX.len() + 1];
         buffer[..FACETED_DOCUMENTS_IDS_PREFIX.len()].clone_from_slice(FACETED_DOCUMENTS_IDS_PREFIX.as_bytes());
         *buffer.last_mut().unwrap() = field_id;
@@ -236,7 +236,7 @@ impl Index {
     }
 
     /// Retrieve all the documents ids that faceted under this field id.
-    pub fn faceted_documents_ids(&self, rtxn: &RoTxn, field_id: u8) -> heed::Result<RoaringBitmap> {
+    pub fn faceted_documents_ids(&self, rtxn: &RoTxn, field_id: FieldId) -> heed::Result<RoaringBitmap> {
         let mut buffer = [0u8; FACETED_DOCUMENTS_IDS_PREFIX.len() + 1];
         buffer[..FACETED_DOCUMENTS_IDS_PREFIX.len()].clone_from_slice(FACETED_DOCUMENTS_IDS_PREFIX.as_bytes());
         *buffer.last_mut().unwrap() = field_id;
diff --git a/src/lib.rs b/src/lib.rs
index 93f9cc0df..9fa19c68c 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -40,15 +40,16 @@ pub type SmallVec16<T> = smallvec::SmallVec<[T; 16]>;
 pub type SmallVec8<T> = smallvec::SmallVec<[T; 8]>;
 pub type BEU32 = heed::zerocopy::U32<heed::byteorder::BE>;
 pub type BEU64 = heed::zerocopy::U64<heed::byteorder::BE>;
-pub type DocumentId = u32;
 pub type Attribute = u32;
+pub type DocumentId = u32;
+pub type FieldId = u8;
 pub type Position = u32;
 
 type MergeFn = for<'a> fn(&[u8], &[Cow<'a, [u8]>]) -> anyhow::Result<Vec<u8>>;
 
 /// Transform a raw obkv store into a JSON Object.
 pub fn obkv_to_json(
-    displayed_fields: &[u8],
+    displayed_fields: &[FieldId],
     fields_ids_map: &FieldsIdsMap,
     obkv: obkv::KvReader,
 ) -> anyhow::Result<Map<String, Value>>
diff --git a/src/search/facet/mod.rs b/src/search/facet/mod.rs
index c47f290e0..283c95e0d 100644
--- a/src/search/facet/mod.rs
+++ b/src/search/facet/mod.rs
@@ -15,7 +15,7 @@ use roaring::RoaringBitmap;
 use crate::facet::FacetType;
 use crate::heed_codec::facet::FacetValueStringCodec;
 use crate::heed_codec::facet::{FacetLevelValueI64Codec, FacetLevelValueF64Codec};
-use crate::{Index, FieldsIdsMap, CboRoaringBitmapCodec};
+use crate::{Index, FieldId, FieldsIdsMap, CboRoaringBitmapCodec};
 
 use self::FacetCondition::*;
 use self::FacetNumberOperator::*;
@@ -75,18 +75,18 @@ impl FacetStringOperator {
 
 #[derive(Debug, Clone, PartialEq)]
 pub enum FacetCondition {
-    OperatorI64(u8, FacetNumberOperator<i64>),
-    OperatorF64(u8, FacetNumberOperator<f64>),
-    OperatorString(u8, FacetStringOperator),
+    OperatorI64(FieldId, FacetNumberOperator<i64>),
+    OperatorF64(FieldId, FacetNumberOperator<f64>),
+    OperatorString(FieldId, FacetStringOperator),
     Or(Box<Self>, Box<Self>),
     And(Box<Self>, Box<Self>),
 }
 
 fn get_field_id_facet_type<'a>(
     fields_ids_map: &FieldsIdsMap,
-    faceted_fields: &HashMap<u8, FacetType>,
+    faceted_fields: &HashMap<FieldId, FacetType>,
     items: &mut Pairs<'a, Rule>,
-) -> Result<(u8, FacetType), PestError<Rule>>
+) -> Result<(FieldId, FacetType), PestError<Rule>>
 {
     // lexing ensures that we at least have a key
     let key = items.next().unwrap();
@@ -154,7 +154,7 @@ impl FacetCondition {
 
     fn from_pairs(
         fim: &FieldsIdsMap,
-        ff: &HashMap<u8, FacetType>,
+        ff: &HashMap<FieldId, FacetType>,
         expression: Pairs<Rule>,
     ) -> anyhow::Result<Self>
     {
@@ -201,7 +201,7 @@ impl FacetCondition {
 
     fn between(
         fields_ids_map: &FieldsIdsMap,
-        faceted_fields: &HashMap<u8, FacetType>,
+        faceted_fields: &HashMap<FieldId, FacetType>,
         item: Pair<Rule>,
     ) -> anyhow::Result<FacetCondition>
     {
@@ -234,7 +234,7 @@ impl FacetCondition {
 
     fn equal(
         fields_ids_map: &FieldsIdsMap,
-        faceted_fields: &HashMap<u8, FacetType>,
+        faceted_fields: &HashMap<FieldId, FacetType>,
         item: Pair<Rule>,
     ) -> anyhow::Result<FacetCondition>
     {
@@ -250,7 +250,7 @@ impl FacetCondition {
 
     fn greater_than(
         fields_ids_map: &FieldsIdsMap,
-        faceted_fields: &HashMap<u8, FacetType>,
+        faceted_fields: &HashMap<FieldId, FacetType>,
         item: Pair<Rule>,
     ) -> anyhow::Result<FacetCondition>
     {
@@ -274,7 +274,7 @@ impl FacetCondition {
 
     fn greater_than_or_equal(
         fields_ids_map: &FieldsIdsMap,
-        faceted_fields: &HashMap<u8, FacetType>,
+        faceted_fields: &HashMap<FieldId, FacetType>,
         item: Pair<Rule>,
     ) -> anyhow::Result<FacetCondition>
     {
@@ -298,7 +298,7 @@ impl FacetCondition {
 
     fn lower_than(
         fields_ids_map: &FieldsIdsMap,
-        faceted_fields: &HashMap<u8, FacetType>,
+        faceted_fields: &HashMap<FieldId, FacetType>,
         item: Pair<Rule>,
     ) -> anyhow::Result<FacetCondition>
     {
@@ -322,7 +322,7 @@ impl FacetCondition {
 
     fn lower_than_or_equal(
         fields_ids_map: &FieldsIdsMap,
-        faceted_fields: &HashMap<u8, FacetType>,
+        faceted_fields: &HashMap<FieldId, FacetType>,
         item: Pair<Rule>,
     ) -> anyhow::Result<FacetCondition>
     {
@@ -351,7 +351,7 @@ impl FacetCondition {
     fn explore_facet_levels<'t, T: 't, KC>(
         rtxn: &'t heed::RoTxn,
         db: heed::Database<ByteSlice, CboRoaringBitmapCodec>,
-        field_id: u8,
+        field_id: FieldId,
         level: u8,
         left: Bound<T>,
         right: Bound<T>,
@@ -447,7 +447,7 @@ impl FacetCondition {
         rtxn: &'t heed::RoTxn,
         index: &Index,
         db: heed::Database<ByteSlice, CboRoaringBitmapCodec>,
-        field_id: u8,
+        field_id: FieldId,
         operator: FacetNumberOperator<T>,
     ) -> anyhow::Result<RoaringBitmap>
     where
@@ -493,7 +493,7 @@ impl FacetCondition {
         rtxn: &heed::RoTxn,
         index: &Index,
         db: heed::Database<FacetValueStringCodec, CboRoaringBitmapCodec>,
-        field_id: u8,
+        field_id: FieldId,
         operator: &FacetStringOperator,
     ) -> anyhow::Result<RoaringBitmap>
     {
diff --git a/src/update/index_documents/store.rs b/src/update/index_documents/store.rs
index 57f99c908..c56b8b09e 100644
--- a/src/update/index_documents/store.rs
+++ b/src/update/index_documents/store.rs
@@ -22,7 +22,7 @@ use crate::heed_codec::{BoRoaringBitmapCodec, CboRoaringBitmapCodec};
 use crate::heed_codec::facet::{FacetValueStringCodec, FacetLevelValueF64Codec, FacetLevelValueI64Codec};
 use crate::tokenizer::{simple_tokenizer, only_token};
 use crate::update::UpdateIndexingStep;
-use crate::{json_to_string, SmallVec8, SmallVec32, SmallString32, Position, DocumentId};
+use crate::{json_to_string, SmallVec8, SmallVec32, SmallString32, Position, DocumentId, FieldId};
 
 use super::{MergeFn, create_writer, create_sorter, writer_into_reader};
 use super::merge_function::{
@@ -47,8 +47,8 @@ pub struct Readers {
 
 pub struct Store {
     // Indexing parameters
-    searchable_fields: HashSet<u8>,
-    faceted_fields: HashMap<u8, FacetType>,
+    searchable_fields: HashSet<FieldId>,
+    faceted_fields: HashMap<FieldId, FacetType>,
     // Caches
     word_docids: LinkedHashMap<SmallVec32<u8>, RoaringBitmap>,
     word_docids_limit: usize,
@@ -72,8 +72,8 @@ pub struct Store {
 
 impl Store {
     pub fn new(
-        searchable_fields: HashSet<u8>,
-        faceted_fields: HashMap<u8, FacetType>,
+        searchable_fields: HashSet<FieldId>,
+        faceted_fields: HashMap<FieldId, FacetType>,
         linked_hash_map_size: Option<usize>,
         max_nb_chunks: Option<usize>,
         max_memory: Option<usize>,
@@ -176,7 +176,7 @@ impl Store {
     // Save the documents ids under the facet field id and value we have seen it.
     fn insert_facet_values_docid(
         &mut self,
-        field_id: u8,
+        field_id: FieldId,
         field_value: FacetValue,
         id: DocumentId,
     ) -> anyhow::Result<()>
@@ -243,7 +243,7 @@ impl Store {
         &mut self,
         document_id: DocumentId,
         words_positions: &mut HashMap<String, SmallVec32<Position>>,
-        facet_values: &mut HashMap<u8, SmallVec8<FacetValue>>,
+        facet_values: &mut HashMap<FieldId, SmallVec8<FacetValue>>,
         record: &[u8],
     ) -> anyhow::Result<()>
     {
diff --git a/src/update/index_documents/transform.rs b/src/update/index_documents/transform.rs
index a42da45f1..f44593c05 100644
--- a/src/update/index_documents/transform.rs
+++ b/src/update/index_documents/transform.rs
@@ -10,13 +10,13 @@ use log::info;
 use roaring::RoaringBitmap;
 use serde_json::{Map, Value};
 
-use crate::{BEU32, MergeFn, Index, FieldsIdsMap, ExternalDocumentsIds};
+use crate::{BEU32, MergeFn, Index, FieldId, FieldsIdsMap, ExternalDocumentsIds};
 use crate::update::{AvailableDocumentsIds, UpdateIndexingStep};
 use super::merge_function::merge_two_obkvs;
 use super::{create_writer, create_sorter, IndexDocumentsMethod};
 
 pub struct TransformOutput {
-    pub primary_key: u8,
+    pub primary_key: FieldId,
     pub fields_ids_map: FieldsIdsMap,
     pub external_documents_ids: ExternalDocumentsIds<'static>,
     pub new_documents_ids: RoaringBitmap,
@@ -365,7 +365,7 @@ impl Transform<'_, '_> {
     fn output_from_sorter<F>(
         self,
         sorter: grenad::Sorter<MergeFn>,
-        primary_key: u8,
+        primary_key: FieldId,
         fields_ids_map: FieldsIdsMap,
         approximate_number_of_documents: usize,
         mut external_documents_ids: ExternalDocumentsIds<'_>,
@@ -477,7 +477,7 @@ impl Transform<'_, '_> {
     // TODO this can be done in parallel by using the rayon `ThreadPool`.
     pub fn remap_index_documents(
         self,
-        primary_key: u8,
+        primary_key: FieldId,
         fields_ids_map: FieldsIdsMap,
     ) -> anyhow::Result<TransformOutput>
     {