diff --git a/Cargo.lock b/Cargo.lock
index 050ba7c88..18d42029f 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -341,6 +341,17 @@ dependencies = [
  "unicode-width",
 ]
 
+[[package]]
+name = "concat-arrays"
+version = "0.1.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1df715824eb382e34b7afb7463b0247bf41538aeba731fba05241ecdb5dc3747"
+dependencies = [
+ "proc-macro2 1.0.27",
+ "quote 1.0.9",
+ "syn 1.0.73",
+]
+
 [[package]]
 name = "convert_case"
 version = "0.4.0"
@@ -1378,6 +1389,7 @@ dependencies = [
  "bstr",
  "byteorder",
  "chrono",
+ "concat-arrays",
  "csv",
  "either",
  "flate2",
@@ -1609,9 +1621,9 @@ dependencies = [
 
 [[package]]
 name = "obkv"
-version = "0.1.1"
+version = "0.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ddd8a5a0aa2f3adafe349259a5b3e21a19c388b792414c1161d60a69c1fa48e8"
+checksum = "f69e48cd7c8e5bb52a1da1287fdbfd877c32673176583ce664cd63b201aba385"
 
 [[package]]
 name = "once_cell"
diff --git a/infos/src/main.rs b/infos/src/main.rs
index 151e8c664..d5d1ad0af 100644
--- a/infos/src/main.rs
+++ b/infos/src/main.rs
@@ -7,7 +7,7 @@ use byte_unit::Byte;
 use heed::EnvOpenOptions;
 use milli::facet::FacetType;
 use milli::index::db_name::*;
-use milli::{Index, TreeLevel};
+use milli::{FieldId, Index, TreeLevel};
 use structopt::StructOpt;
 use Command::*;
 
@@ -322,7 +322,7 @@ fn most_common_words(index: &Index, rtxn: &heed::RoTxn, limit: usize) -> anyhow:
 fn facet_values_iter<'txn, KC: 'txn, DC: 'txn>(
     rtxn: &'txn heed::RoTxn,
     db: heed::Database<KC, DC>,
-    field_id: u8,
+    field_id: FieldId,
 ) -> heed::Result<Box<dyn Iterator<Item = heed::Result<(KC::DItem, DC::DItem)>> + 'txn>>
 where
     KC: heed::BytesDecode<'txn>,
@@ -330,7 +330,7 @@ where
 {
     let iter = db
         .remap_key_type::<heed::types::ByteSlice>()
-        .prefix_iter(&rtxn, &[field_id])?
+        .prefix_iter(&rtxn, &field_id.to_be_bytes())?
         .remap_key_type::<KC>();
 
     Ok(Box::new(iter))
diff --git a/milli/Cargo.toml b/milli/Cargo.toml
index dfa02f89d..6af928041 100644
--- a/milli/Cargo.toml
+++ b/milli/Cargo.toml
@@ -8,6 +8,7 @@ edition = "2018"
 bstr = "0.2.15"
 byteorder = "1.4.2"
 chrono = { version = "0.4.19", features = ["serde"] }
+concat-arrays = "0.1.2"
 csv = "1.1.5"
 either = "1.6.1"
 flate2 = "1.0.20"
@@ -20,7 +21,7 @@ levenshtein_automata = { version = "0.2.0", features = ["fst_automaton"] }
 linked-hash-map = "0.5.4"
 meilisearch-tokenizer = { git = "https://github.com/meilisearch/Tokenizer.git", tag = "v0.2.3" }
 memmap = "0.7.0"
-obkv = "0.1.1"
+obkv = "0.2.0"
 once_cell = "1.5.2"
 ordered-float = "2.1.1"
 rayon = "1.5.0"
diff --git a/milli/src/heed_codec/facet/facet_level_value_f64_codec.rs b/milli/src/heed_codec/facet/facet_level_value_f64_codec.rs
index b23dcb269..1e66427ca 100644
--- a/milli/src/heed_codec/facet/facet_level_value_f64_codec.rs
+++ b/milli/src/heed_codec/facet/facet_level_value_f64_codec.rs
@@ -2,7 +2,7 @@ use std::borrow::Cow;
 use std::convert::TryInto;
 
 use crate::facet::value_encoding::f64_into_bytes;
-use crate::FieldId;
+use crate::{try_split_array_at, FieldId};
 
 // TODO do not de/serialize right bound when level = 0
 pub struct FacetLevelValueF64Codec;
@@ -11,7 +11,8 @@ impl<'a> heed::BytesDecode<'a> for FacetLevelValueF64Codec {
     type DItem = (FieldId, u8, f64, f64);
 
     fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
-        let (field_id, bytes) = bytes.split_first()?;
+        let (field_id_bytes, bytes) = try_split_array_at(bytes)?;
+        let field_id = u16::from_be_bytes(field_id_bytes);
         let (level, bytes) = bytes.split_first()?;
 
         let (left, right) = if *level != 0 {
@@ -23,7 +24,7 @@ impl<'a> heed::BytesDecode<'a> for FacetLevelValueF64Codec {
             (left, left)
         };
 
-        Some((*field_id, *level, left, right))
+        Some((field_id, *level, left, right))
     }
 }
 
@@ -61,8 +62,8 @@ impl heed::BytesEncode<'_> for FacetLevelValueF64Codec {
             16 // length
         };
 
-        let mut bytes = Vec::with_capacity(len + 2);
-        bytes.push(*field_id);
+        let mut bytes = Vec::with_capacity(len + 3);
+        bytes.extend_from_slice(&field_id.to_be_bytes());
         bytes.push(*level);
         bytes.extend_from_slice(&buffer[..len]);
         Some(Cow::Owned(bytes))
diff --git a/milli/src/heed_codec/facet/facet_value_string_codec.rs b/milli/src/heed_codec/facet/facet_value_string_codec.rs
index 259dab972..54abb7886 100644
--- a/milli/src/heed_codec/facet/facet_value_string_codec.rs
+++ b/milli/src/heed_codec/facet/facet_value_string_codec.rs
@@ -1,14 +1,14 @@
 use std::borrow::Cow;
 use std::str;
 
-use crate::FieldId;
+use crate::{try_split_array_at, FieldId};
 
 pub struct FacetValueStringCodec;
 
 impl FacetValueStringCodec {
     pub fn serialize_into(field_id: FieldId, value: &str, out: &mut Vec<u8>) {
-        out.reserve(value.len() + 1);
-        out.push(field_id);
+        out.reserve(value.len() + 2);
+        out.extend_from_slice(&field_id.to_be_bytes());
         out.extend_from_slice(value.as_bytes());
     }
 }
@@ -17,9 +17,10 @@ impl<'a> heed::BytesDecode<'a> for FacetValueStringCodec {
     type DItem = (FieldId, &'a str);
 
     fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
-        let (field_id, bytes) = bytes.split_first()?;
+        let (field_id_bytes, bytes) = try_split_array_at(bytes)?;
+        let field_id = u16::from_be_bytes(field_id_bytes);
         let value = str::from_utf8(bytes).ok()?;
-        Some((*field_id, value))
+        Some((field_id, value))
     }
 }
 
diff --git a/milli/src/heed_codec/facet/field_doc_id_facet_f64_codec.rs b/milli/src/heed_codec/facet/field_doc_id_facet_f64_codec.rs
index b3c0fa381..22159601c 100644
--- a/milli/src/heed_codec/facet/field_doc_id_facet_f64_codec.rs
+++ b/milli/src/heed_codec/facet/field_doc_id_facet_f64_codec.rs
@@ -2,7 +2,7 @@ use std::borrow::Cow;
 use std::convert::TryInto;
 
 use crate::facet::value_encoding::f64_into_bytes;
-use crate::{DocumentId, FieldId};
+use crate::{try_split_array_at, DocumentId, FieldId};
 
 pub struct FieldDocIdFacetF64Codec;
 
@@ -10,14 +10,15 @@ impl<'a> heed::BytesDecode<'a> for FieldDocIdFacetF64Codec {
     type DItem = (FieldId, DocumentId, f64);
 
     fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
-        let (field_id, bytes) = bytes.split_first()?;
+        let (field_id_bytes, bytes) = try_split_array_at(bytes)?;
+        let field_id = u16::from_be_bytes(field_id_bytes);
 
-        let (document_id_bytes, bytes) = bytes.split_at(4);
-        let document_id = document_id_bytes.try_into().map(u32::from_be_bytes).ok()?;
+        let (document_id_bytes, bytes) = try_split_array_at(bytes)?;
+        let document_id = u32::from_be_bytes(document_id_bytes);
 
         let value = bytes[8..16].try_into().map(f64::from_be_bytes).ok()?;
 
-        Some((*field_id, document_id, value))
+        Some((field_id, document_id, value))
     }
 }
 
@@ -25,8 +26,8 @@ impl<'a> heed::BytesEncode<'a> for FieldDocIdFacetF64Codec {
     type EItem = (FieldId, DocumentId, f64);
 
     fn bytes_encode((field_id, document_id, value): &Self::EItem) -> Option<Cow<[u8]>> {
-        let mut bytes = Vec::with_capacity(1 + 4 + 8 + 8);
-        bytes.push(*field_id);
+        let mut bytes = Vec::with_capacity(2 + 4 + 8 + 8);
+        bytes.extend_from_slice(&field_id.to_be_bytes());
         bytes.extend_from_slice(&document_id.to_be_bytes());
         let value_bytes = f64_into_bytes(*value)?;
         bytes.extend_from_slice(&value_bytes);
diff --git a/milli/src/heed_codec/facet/field_doc_id_facet_string_codec.rs b/milli/src/heed_codec/facet/field_doc_id_facet_string_codec.rs
index fd3f1143d..36408f578 100644
--- a/milli/src/heed_codec/facet/field_doc_id_facet_string_codec.rs
+++ b/milli/src/heed_codec/facet/field_doc_id_facet_string_codec.rs
@@ -1,8 +1,7 @@
 use std::borrow::Cow;
-use std::convert::TryInto;
 use std::str;
 
-use crate::{DocumentId, FieldId};
+use crate::{try_split_array_at, DocumentId, FieldId};
 
 pub struct FieldDocIdFacetStringCodec;
 
@@ -13,8 +12,8 @@ impl FieldDocIdFacetStringCodec {
         value: &str,
         out: &mut Vec<u8>,
     ) {
-        out.reserve(1 + 4 + value.len());
-        out.push(field_id);
+        out.reserve(2 + 4 + value.len());
+        out.extend_from_slice(&field_id.to_be_bytes());
         out.extend_from_slice(&document_id.to_be_bytes());
         out.extend_from_slice(value.as_bytes());
     }
@@ -24,11 +23,14 @@ impl<'a> heed::BytesDecode<'a> for FieldDocIdFacetStringCodec {
     type DItem = (FieldId, DocumentId, &'a str);
 
     fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
-        let (field_id, bytes) = bytes.split_first()?;
-        let (document_id_bytes, bytes) = bytes.split_at(4);
-        let document_id = document_id_bytes.try_into().map(u32::from_be_bytes).ok()?;
+        let (field_id_bytes, bytes) = try_split_array_at(bytes)?;
+        let field_id = u16::from_be_bytes(field_id_bytes);
+
+        let (document_id_bytes, bytes) = try_split_array_at(bytes)?;
+        let document_id = u32::from_be_bytes(document_id_bytes);
+
         let value = str::from_utf8(bytes).ok()?;
-        Some((*field_id, document_id, value))
+        Some((field_id, document_id, value))
     }
 }
 
diff --git a/milli/src/heed_codec/field_id_word_count_codec.rs b/milli/src/heed_codec/field_id_word_count_codec.rs
index 64f0e1db6..aca7a80c4 100644
--- a/milli/src/heed_codec/field_id_word_count_codec.rs
+++ b/milli/src/heed_codec/field_id_word_count_codec.rs
@@ -1,7 +1,6 @@
 use std::borrow::Cow;
-use std::convert::TryInto;
 
-use crate::FieldId;
+use crate::{try_split_array_at, FieldId};
 
 pub struct FieldIdWordCountCodec;
 
@@ -9,7 +8,9 @@ impl<'a> heed::BytesDecode<'a> for FieldIdWordCountCodec {
     type DItem = (FieldId, u8);
 
     fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
-        let [field_id, word_count]: [u8; 2] = bytes.try_into().ok()?;
+        let (field_id_bytes, bytes) = try_split_array_at(bytes)?;
+        let field_id = u16::from_be_bytes(field_id_bytes);
+        let ([word_count], _nothing) = try_split_array_at(bytes)?;
         Some((field_id, word_count))
     }
 }
@@ -18,6 +19,9 @@ impl<'a> heed::BytesEncode<'a> for FieldIdWordCountCodec {
     type EItem = (FieldId, u8);
 
     fn bytes_encode((field_id, word_count): &Self::EItem) -> Option<Cow<[u8]>> {
-        Some(Cow::Owned(vec![*field_id, *word_count]))
+        let mut bytes = Vec::with_capacity(2 + 1);
+        bytes.extend_from_slice(&field_id.to_be_bytes());
+        bytes.push(*word_count);
+        Some(Cow::Owned(bytes))
     }
 }
diff --git a/milli/src/heed_codec/obkv_codec.rs b/milli/src/heed_codec/obkv_codec.rs
index b7414b693..6dad771a8 100644
--- a/milli/src/heed_codec/obkv_codec.rs
+++ b/milli/src/heed_codec/obkv_codec.rs
@@ -1,19 +1,19 @@
 use std::borrow::Cow;
 
-use obkv::{KvReader, KvWriter};
+use obkv::{KvReaderU16, KvWriterU16};
 
 pub struct ObkvCodec;
 
 impl<'a> heed::BytesDecode<'a> for ObkvCodec {
-    type DItem = KvReader<'a>;
+    type DItem = KvReaderU16<'a>;
 
     fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
-        Some(KvReader::new(bytes))
+        Some(KvReaderU16::new(bytes))
     }
 }
 
 impl heed::BytesEncode<'_> for ObkvCodec {
-    type EItem = KvWriter<Vec<u8>>;
+    type EItem = KvWriterU16<Vec<u8>>;
 
     fn bytes_encode(item: &Self::EItem) -> Option<Cow<[u8]>> {
         item.clone().into_inner().map(Cow::Owned).ok()
diff --git a/milli/src/index.rs b/milli/src/index.rs
index 247e67d52..099a5891d 100644
--- a/milli/src/index.rs
+++ b/milli/src/index.rs
@@ -523,10 +523,11 @@ impl Index {
         field_id: FieldId,
         docids: &RoaringBitmap,
     ) -> heed::Result<()> {
-        let mut buffer = [0u8; main_key::STRING_FACETED_DOCUMENTS_IDS_PREFIX.len() + 1];
+        let mut buffer = [0u8; main_key::STRING_FACETED_DOCUMENTS_IDS_PREFIX.len() + 2];
         buffer[..main_key::STRING_FACETED_DOCUMENTS_IDS_PREFIX.len()]
             .copy_from_slice(main_key::STRING_FACETED_DOCUMENTS_IDS_PREFIX.as_bytes());
-        *buffer.last_mut().unwrap() = field_id;
+        buffer[main_key::STRING_FACETED_DOCUMENTS_IDS_PREFIX.len()..]
+            .copy_from_slice(&field_id.to_be_bytes());
         self.main.put::<_, ByteSlice, RoaringBitmapCodec>(wtxn, &buffer, docids)
     }
 
@@ -536,10 +537,11 @@ impl Index {
         rtxn: &RoTxn,
         field_id: FieldId,
     ) -> heed::Result<RoaringBitmap> {
-        let mut buffer = [0u8; main_key::STRING_FACETED_DOCUMENTS_IDS_PREFIX.len() + 1];
+        let mut buffer = [0u8; main_key::STRING_FACETED_DOCUMENTS_IDS_PREFIX.len() + 2];
         buffer[..main_key::STRING_FACETED_DOCUMENTS_IDS_PREFIX.len()]
             .copy_from_slice(main_key::STRING_FACETED_DOCUMENTS_IDS_PREFIX.as_bytes());
-        *buffer.last_mut().unwrap() = field_id;
+        buffer[main_key::STRING_FACETED_DOCUMENTS_IDS_PREFIX.len()..]
+            .copy_from_slice(&field_id.to_be_bytes());
         match self.main.get::<_, ByteSlice, RoaringBitmapCodec>(rtxn, &buffer)? {
             Some(docids) => Ok(docids),
             None => Ok(RoaringBitmap::new()),
@@ -553,10 +555,11 @@ impl Index {
         field_id: FieldId,
         docids: &RoaringBitmap,
     ) -> heed::Result<()> {
-        let mut buffer = [0u8; main_key::NUMBER_FACETED_DOCUMENTS_IDS_PREFIX.len() + 1];
+        let mut buffer = [0u8; main_key::NUMBER_FACETED_DOCUMENTS_IDS_PREFIX.len() + 2];
         buffer[..main_key::NUMBER_FACETED_DOCUMENTS_IDS_PREFIX.len()]
             .copy_from_slice(main_key::NUMBER_FACETED_DOCUMENTS_IDS_PREFIX.as_bytes());
-        *buffer.last_mut().unwrap() = field_id;
+        buffer[main_key::NUMBER_FACETED_DOCUMENTS_IDS_PREFIX.len()..]
+            .copy_from_slice(&field_id.to_be_bytes());
         self.main.put::<_, ByteSlice, RoaringBitmapCodec>(wtxn, &buffer, docids)
     }
 
@@ -569,7 +572,8 @@ impl Index {
         let mut buffer = [0u8; main_key::NUMBER_FACETED_DOCUMENTS_IDS_PREFIX.len() + 1];
         buffer[..main_key::NUMBER_FACETED_DOCUMENTS_IDS_PREFIX.len()]
             .copy_from_slice(main_key::NUMBER_FACETED_DOCUMENTS_IDS_PREFIX.as_bytes());
-        *buffer.last_mut().unwrap() = field_id;
+        buffer[main_key::NUMBER_FACETED_DOCUMENTS_IDS_PREFIX.len()..]
+            .copy_from_slice(&field_id.to_be_bytes());
         match self.main.get::<_, ByteSlice, RoaringBitmapCodec>(rtxn, &buffer)? {
             Some(docids) => Ok(docids),
             None => Ok(RoaringBitmap::new()),
@@ -723,7 +727,7 @@ impl Index {
         &self,
         rtxn: &'t RoTxn,
         ids: impl IntoIterator<Item = DocumentId>,
-    ) -> Result<Vec<(DocumentId, obkv::KvReader<'t>)>> {
+    ) -> Result<Vec<(DocumentId, obkv::KvReaderU16<'t>)>> {
         let mut documents = Vec::new();
 
         for id in ids {
@@ -741,7 +745,7 @@ impl Index {
     pub fn all_documents<'t>(
         &self,
         rtxn: &'t RoTxn,
-    ) -> Result<impl Iterator<Item = heed::Result<(DocumentId, obkv::KvReader<'t>)>>> {
+    ) -> Result<impl Iterator<Item = heed::Result<(DocumentId, obkv::KvReaderU16<'t>)>>> {
         Ok(self
             .documents
             .iter(rtxn)?
diff --git a/milli/src/lib.rs b/milli/src/lib.rs
index ec9bc32c6..f3bababf6 100644
--- a/milli/src/lib.rs
+++ b/milli/src/lib.rs
@@ -15,6 +15,7 @@ pub mod update;
 
 use std::borrow::Cow;
 use std::collections::{BTreeMap, HashMap};
+use std::convert::{TryFrom, TryInto};
 use std::hash::BuildHasherDefault;
 use std::result::Result as StdResult;
 
@@ -48,7 +49,7 @@ pub type BEU32 = heed::zerocopy::U32<heed::byteorder::BE>;
 pub type BEU64 = heed::zerocopy::U64<heed::byteorder::BE>;
 pub type Attribute = u32;
 pub type DocumentId = u32;
-pub type FieldId = u8;
+pub type FieldId = u16;
 pub type Position = u32;
 pub type FieldDistribution = BTreeMap<String, u64>;
 
@@ -58,7 +59,7 @@ type MergeFn<E> = for<'a> fn(&[u8], &[Cow<'a, [u8]>]) -> StdResult<Vec<u8>, E>;
 pub fn obkv_to_json(
     displayed_fields: &[FieldId],
     fields_ids_map: &FieldsIdsMap,
-    obkv: obkv::KvReader,
+    obkv: obkv::KvReaderU16,
 ) -> Result<Map<String, Value>> {
     displayed_fields
         .iter()
@@ -123,6 +124,26 @@ pub fn json_to_string(value: &Value) -> Option<String> {
     }
 }
 
+/// Divides one slice into two at an index, returns `None` if mid is out of bounds.
+fn try_split_at<T>(slice: &[T], mid: usize) -> Option<(&[T], &[T])> {
+    if mid <= slice.len() {
+        Some(slice.split_at(mid))
+    } else {
+        None
+    }
+}
+
+/// Divides one slice into an array and the tail at an index,
+/// returns `None` if `N` is out of bounds.
+fn try_split_array_at<T, const N: usize>(slice: &[T]) -> Option<([T; N], &[T])>
+where
+    [T; N]: for<'a> TryFrom<&'a [T]>,
+{
+    let (head, tail) = try_split_at(slice, N)?;
+    let head = head.try_into().ok()?;
+    Some((head, tail))
+}
+
 #[cfg(test)]
 mod tests {
     use serde_json::json;
diff --git a/milli/src/search/facet/facet_distribution.rs b/milli/src/search/facet/facet_distribution.rs
index 3f55006f2..b0b22ac49 100644
--- a/milli/src/search/facet/facet_distribution.rs
+++ b/milli/src/search/facet/facet_distribution.rs
@@ -78,7 +78,7 @@ impl<'a> FacetDistribution<'a> {
             K: fmt::Display,
             KC: BytesDecode<'t, DItem = (FieldId, DocumentId, K)>,
         {
-            let mut key_buffer = vec![field_id];
+            let mut key_buffer: Vec<_> = field_id.to_be_bytes().iter().copied().collect();
 
             for docid in candidates.into_iter().take(CANDIDATES_THRESHOLD as usize) {
                 key_buffer.truncate(1);
@@ -157,7 +157,7 @@ impl<'a> FacetDistribution<'a> {
             .index
             .facet_id_string_docids
             .remap_key_type::<ByteSlice>()
-            .prefix_iter(self.rtxn, &[field_id])?
+            .prefix_iter(self.rtxn, &field_id.to_be_bytes())?
             .remap_key_type::<FacetValueStringCodec>();
 
         for result in iter {
diff --git a/milli/src/search/facet/mod.rs b/milli/src/search/facet/mod.rs
index 4e900bff4..9774bdd52 100644
--- a/milli/src/search/facet/mod.rs
+++ b/milli/src/search/facet/mod.rs
@@ -187,7 +187,7 @@ impl<'t> FacetIter<'t> {
     ) -> heed::Result<Option<u8>> {
         let level = db
             .remap_types::<ByteSlice, DecodeIgnore>()
-            .prefix_iter(rtxn, &[fid][..])?
+            .prefix_iter(rtxn, &fid.to_be_bytes())?
             .remap_key_type::<FacetLevelValueF64Codec>()
             .last()
             .transpose()?
diff --git a/milli/src/update/delete_documents.rs b/milli/src/update/delete_documents.rs
index 313f8a909..222f3b2d3 100644
--- a/milli/src/update/delete_documents.rs
+++ b/milli/src/update/delete_documents.rs
@@ -430,8 +430,10 @@ where
     C: heed::BytesDecode<'a, DItem = K> + heed::BytesEncode<'a, EItem = K>,
     F: Fn(K) -> DocumentId,
 {
-    let mut iter =
-        db.remap_key_type::<ByteSlice>().prefix_iter_mut(wtxn, &[field_id])?.remap_key_type::<C>();
+    let mut iter = db
+        .remap_key_type::<ByteSlice>()
+        .prefix_iter_mut(wtxn, &field_id.to_be_bytes())?
+        .remap_key_type::<C>();
 
     while let Some(result) = iter.next() {
         let (key, ()) = result?;
diff --git a/milli/src/update/facets.rs b/milli/src/update/facets.rs
index 0e2cad69d..5fabbc504 100644
--- a/milli/src/update/facets.rs
+++ b/milli/src/update/facets.rs
@@ -15,7 +15,7 @@ use crate::heed_codec::CboRoaringBitmapCodec;
 use crate::update::index_documents::{
     create_writer, write_into_lmdb_database, writer_into_reader, WriteMethod,
 };
-use crate::{Index, Result};
+use crate::{FieldId, Index, Result};
 
 pub struct Facets<'t, 'u, 'i> {
     wtxn: &'t mut heed::RwTxn<'i, 'u>,
@@ -119,7 +119,7 @@ impl<'t, 'u, 'i> Facets<'t, 'u, 'i> {
 fn clear_field_number_levels<'t>(
     wtxn: &'t mut heed::RwTxn,
     db: heed::Database<FacetLevelValueF64Codec, CboRoaringBitmapCodec>,
-    field_id: u8,
+    field_id: FieldId,
 ) -> heed::Result<()> {
     let left = (field_id, 1, f64::MIN, f64::MIN);
     let right = (field_id, u8::MAX, f64::MAX, f64::MAX);
@@ -135,11 +135,11 @@ fn compute_facet_number_levels<'t>(
     shrink_size: Option<u64>,
     level_group_size: NonZeroUsize,
     min_level_size: NonZeroUsize,
-    field_id: u8,
+    field_id: FieldId,
 ) -> Result<Reader<FileFuse>> {
     let first_level_size = db
         .remap_key_type::<ByteSlice>()
-        .prefix_iter(rtxn, &[field_id])?
+        .prefix_iter(rtxn, &field_id.to_be_bytes())?
         .remap_types::<DecodeIgnore, DecodeIgnore>()
         .fold(Ok(0usize), |count, result| result.and(count).map(|c| c + 1))?;
 
@@ -196,11 +196,11 @@ fn compute_facet_number_levels<'t>(
 fn compute_faceted_documents_ids(
     rtxn: &heed::RoTxn,
     db: heed::Database<ByteSlice, CboRoaringBitmapCodec>,
-    field_id: u8,
+    field_id: FieldId,
 ) -> Result<RoaringBitmap> {
     let mut documents_ids = RoaringBitmap::new();
 
-    for result in db.prefix_iter(rtxn, &[field_id])? {
+    for result in db.prefix_iter(rtxn, &field_id.to_be_bytes())? {
         let (_key, docids) = result?;
         documents_ids |= docids;
     }
@@ -210,7 +210,7 @@ fn compute_faceted_documents_ids(
 
 fn write_number_entry(
     writer: &mut Writer<File>,
-    field_id: u8,
+    field_id: FieldId,
     level: u8,
     left: f64,
     right: f64,
diff --git a/milli/src/update/index_documents/merge_function.rs b/milli/src/update/index_documents/merge_function.rs
index 17283b232..8613a8824 100644
--- a/milli/src/update/index_documents/merge_function.rs
+++ b/milli/src/update/index_documents/merge_function.rs
@@ -40,7 +40,7 @@ pub fn keep_first(_key: &[u8], values: &[Cow<[u8]>]) -> Result<Vec<u8>> {
     Ok(values.first().unwrap().to_vec())
 }
 
-pub fn merge_two_obkvs(base: obkv::KvReader, update: obkv::KvReader, buffer: &mut Vec<u8>) {
+pub fn merge_two_obkvs(base: obkv::KvReaderU16, update: obkv::KvReaderU16, buffer: &mut Vec<u8>) {
     use itertools::merge_join_by;
     use itertools::EitherOrBoth::{Both, Left, Right};
 
diff --git a/milli/src/update/index_documents/mod.rs b/milli/src/update/index_documents/mod.rs
index 3876d5dc1..9ac05fe1a 100644
--- a/milli/src/update/index_documents/mod.rs
+++ b/milli/src/update/index_documents/mod.rs
@@ -842,10 +842,12 @@ impl<'t, 'u, 'i, 'a> IndexDocuments<'t, 'u, 'i, 'a> {
 mod tests {
     use std::io::Cursor;
 
+    use big_s::S;
     use heed::EnvOpenOptions;
 
     use super::*;
     use crate::update::DeleteDocuments;
+    use crate::HashMap;
 
     #[test]
     fn simple_document_replacement() {
@@ -1352,4 +1354,30 @@ mod tests {
 
         wtxn.commit().unwrap();
     }
+
+    #[test]
+    fn index_more_than_256_fields() {
+        let path = tempfile::tempdir().unwrap();
+        let mut options = EnvOpenOptions::new();
+        options.map_size(10 * 1024 * 1024); // 10 MB
+        let index = Index::new(options, &path).unwrap();
+
+        let mut wtxn = index.write_txn().unwrap();
+
+        let mut big_object = HashMap::new();
+        big_object.insert(S("id"), "wow");
+        for i in 0..1000 {
+            let key = i.to_string();
+            big_object.insert(key, "I am a text!");
+        }
+
+        let content = vec![big_object];
+        let content = serde_json::to_string(&content).unwrap();
+
+        let mut builder = IndexDocuments::new(&mut wtxn, &index, 0);
+        builder.update_format(UpdateFormat::Json);
+        builder.execute(Cursor::new(content), |_, _| ()).unwrap();
+
+        wtxn.commit().unwrap();
+    }
 }
diff --git a/milli/src/update/index_documents/store.rs b/milli/src/update/index_documents/store.rs
index 9ac97c255..ebf365f44 100644
--- a/milli/src/update/index_documents/store.rs
+++ b/milli/src/update/index_documents/store.rs
@@ -7,6 +7,7 @@ use std::time::Instant;
 use std::{cmp, iter};
 
 use bstr::ByteSlice as _;
+use concat_arrays::concat_arrays;
 use fst::Set;
 use grenad::{CompressionType, FileFuse, Reader, Sorter, Writer};
 use heed::BytesEncode;
@@ -776,7 +777,8 @@ impl<'s, A: AsRef<[u8]>> Store<'s, A> {
         for ((fid, count), docids) in self.field_id_word_count_docids {
             docids_buffer.clear();
             CboRoaringBitmapCodec::serialize_into(&docids, &mut docids_buffer);
-            self.field_id_word_count_docids_sorter.insert([fid, count], &docids_buffer)?;
+            let key: [u8; 3] = concat_arrays!(fid.to_be_bytes(), [count]);
+            self.field_id_word_count_docids_sorter.insert(key, &docids_buffer)?;
         }
 
         let fst = builder.into_set();
diff --git a/milli/src/update/index_documents/transform.rs b/milli/src/update/index_documents/transform.rs
index 24ab276d0..b273460d1 100644
--- a/milli/src/update/index_documents/transform.rs
+++ b/milli/src/update/index_documents/transform.rs
@@ -626,7 +626,7 @@ mod test {
                 Some("tata".to_string()),
                 false,
             );
-            assert_eq!(result.unwrap(), (0u8, "toto".to_string()));
+            assert_eq!(result.unwrap(), (0, "toto".to_string()));
             assert_eq!(fields_map.len(), 1);
         }
 
@@ -635,7 +635,7 @@ mod test {
             let mut fields_map = FieldsIdsMap::new();
             let result =
                 compute_primary_key_pair(None, &mut fields_map, Some("tata".to_string()), false);
-            assert_eq!(result.unwrap(), (0u8, "tata".to_string()));
+            assert_eq!(result.unwrap(), (0, "tata".to_string()));
             assert_eq!(fields_map.len(), 1);
         }
 
@@ -643,7 +643,7 @@ mod test {
         fn should_return_default_if_both_are_none() {
             let mut fields_map = FieldsIdsMap::new();
             let result = compute_primary_key_pair(None, &mut fields_map, None, true);
-            assert_eq!(result.unwrap(), (0u8, "id".to_string()));
+            assert_eq!(result.unwrap(), (0, "id".to_string()));
             assert_eq!(fields_map.len(), 1);
         }