diff --git a/src/indexing/transform.rs b/src/indexing/transform.rs
index 9e5b1ae6c..901b12dbd 100644
--- a/src/indexing/transform.rs
+++ b/src/indexing/transform.rs
@@ -4,11 +4,12 @@ use std::fs::File;
 use std::io::{Read, Seek, SeekFrom};
 
 use anyhow::Context;
-use crate::{FieldsIdsMap, AvailableDocumentsIds};
 use fst::{IntoStreamer, Streamer};
 use grenad::CompressionType;
 use roaring::RoaringBitmap;
 
+use crate::FieldsIdsMap;
+use crate::update::AvailableDocumentsIds;
 use super::{create_writer, create_sorter};
 
 pub struct TransformOutput {
@@ -31,6 +32,22 @@ pub struct Transform<A> {
     pub max_memory: Option<usize>,
 }
 
+fn merge_two_obkv(base: obkv::KvReader, update: obkv::KvReader, buffer: &mut Vec<u8>) {
+    use itertools::merge_join_by;
+    use itertools::EitherOrBoth::{Both, Left, Right};
+
+    buffer.clear();
+
+    let mut writer = obkv::KvWriter::new(buffer);
+    for eob in merge_join_by(base.iter(), update.iter(), |(b, _), (u, _)| b.cmp(u)) {
+        match eob {
+            Both(_, (k, v)) | Left((k, v)) | Right((k, v)) => writer.insert(k, v).unwrap(),
+        }
+    }
+
+    writer.finish().unwrap();
+}
+
 impl<A: AsRef<[u8]>> Transform<A> {
     /// Extract the users ids, deduplicate and compute the new internal documents ids
     /// and fields ids, writing all the documents under their internal ids into a final file.
@@ -101,15 +118,26 @@ impl<A: AsRef<[u8]>> Transform<A> {
         // While we write into final file we get or generate the internal documents ids.
         let mut documents_count = 0;
         let mut iter = sorter.into_iter()?;
-        while let Some((user_id, obkv)) = iter.next()? {
+        while let Some((user_id, update_obkv)) = iter.next()? {
 
-            let docid = match self.users_ids_documents_ids.get(user_id) {
+            let (docid, obkv) = match self.users_ids_documents_ids.get(user_id) {
                 Some(docid) => {
                     // If we find the user id in the current users ids documents ids map
                     // we use it and insert it in the list of replaced documents.
                     let docid = u32::try_from(docid).expect("valid document id");
                     replaced_documents_ids.insert(docid);
-                    docid
+
+                    // Depending on the update indexing method we will merge
+                    // the document update with the current document or not.
+                    let must_merge_documents = false;
+                    if must_merge_documents {
+                        let base_obkv = todo!();
+                        let update_obkv = obkv::KvReader::new(update_obkv);
+                        merge_two_obkv(base_obkv, update_obkv, &mut obkv_buffer);
+                        (docid, obkv_buffer.as_slice())
+                    } else {
+                        (docid, update_obkv)
+                    }
                 },
                 None => {
                     // If this user id is new we add it to the users ids documents ids map
@@ -118,7 +146,7 @@ impl<A: AsRef<[u8]>> Transform<A> {
                         .context("no more available documents ids")?;
                     new_users_ids_documents_ids_builder.insert(user_id, new_docid as u64)?;
                     new_documents_ids.insert(new_docid);
-                    new_docid
+                    (new_docid, update_obkv)
                 },
             };