diff --git a/src/bin/indexer.rs b/src/bin/indexer.rs
index 81bd5e9fa..f1acfbf15 100644
--- a/src/bin/indexer.rs
+++ b/src/bin/indexer.rs
@@ -4,8 +4,9 @@ use std::fs::File;
 use std::io::{self, Read, Write};
 use std::iter::FromIterator;
 use std::path::PathBuf;
-use std::{iter, thread};
+use std::sync::mpsc::sync_channel;
 use std::time::Instant;
+use std::{iter, thread};
 
 use anyhow::{Context, bail};
 use bstr::ByteSlice as _;
@@ -778,7 +779,9 @@ fn main() -> anyhow::Result<()> {
         documents_readers.push(readers.documents);
     });
 
-    let merge_readers = |readers, merge| {
+    // This is the function that merge the readers
+    // by using the given merge function.
+    let merge_readers = move |readers, merge| {
         let mut writer = tempfile().map(|f| {
             create_writer(chunk_compression_type, chunk_compression_level, f)
         })?;
@@ -787,39 +790,69 @@ fn main() -> anyhow::Result<()> {
         writer_into_reader(writer)
     };
 
-    debug!("Merging the main, word docids and words pairs proximity docids in parallel...");
-    let (main, (word_docids, words_pairs_proximities_docids)) = rayon::join(move || {
-        merge_readers(main_readers, main_merge)
-    }, || rayon::join(|| {
-        merge_readers(word_docids_readers, word_docids_merge)
-    }, || {
-        merge_readers(words_pairs_proximities_docids_readers, words_pairs_proximities_docids_merge)
-    }));
+    // The enum and the channel which is used to transfert
+    // the readers merges potentially done on another thread.
+    enum DatabaseType { Main, WordDocids, WordsPairsProximitiesDocids };
+    let (sender, receiver) = sync_channel(3);
+    let main_sender = sender.clone();
+    let word_docids_sender = sender.clone();
+    let words_pairs_proximities_docids_sender = sender;
 
-    let main = main?;
-    let word_docids = word_docids?;
-    let words_pairs_proximities_docids = words_pairs_proximities_docids?;
+    debug!("Merging the main, word docids and words pairs proximity docids in parallel...");
+    rayon::spawn(move || {
+        let result = merge_readers(main_readers, main_merge);
+        main_sender.send((DatabaseType::Main, result)).unwrap();
+    });
+    rayon::spawn(move || {
+        let result = merge_readers(word_docids_readers, word_docids_merge);
+        word_docids_sender.send((DatabaseType::WordDocids, result)).unwrap();
+    });
+    rayon::spawn(move || {
+        let result = merge_readers(
+            words_pairs_proximities_docids_readers,
+            words_pairs_proximities_docids_merge,
+        );
+        let message = (DatabaseType::WordsPairsProximitiesDocids, result);
+        words_pairs_proximities_docids_sender.send(message).unwrap();
+    });
 
     let mut wtxn = env.write_txn()?;
 
-    debug!("Writing the main elements into LMDB on disk...");
-    write_into_lmdb_database(&mut wtxn, index.main, main)?;
-
-    debug!("Writing the words docids into LMDB on disk...");
-    let db = *index.word_docids.as_polymorph();
-    write_into_lmdb_database(&mut wtxn, db, word_docids)?;
-
     debug!("Writing the docid word positions into LMDB on disk...");
-    let db = *index.docid_word_positions.as_polymorph();
-    merge_into_lmdb_database(&mut wtxn, db, docid_word_positions_readers, docid_word_positions_merge)?;
-
-    debug!("Writing the words pairs proximities docids into LMDB on disk...");
-    let db = *index.word_pair_proximity_docids.as_polymorph();
-    write_into_lmdb_database(&mut wtxn, db, words_pairs_proximities_docids)?;
+    merge_into_lmdb_database(
+        &mut wtxn,
+        *index.docid_word_positions.as_polymorph(),
+        docid_word_positions_readers,
+        docid_word_positions_merge,
+    )?;
 
     debug!("Writing the documents into LMDB on disk...");
-    let db = *index.documents.as_polymorph();
-    merge_into_lmdb_database(&mut wtxn, db, documents_readers, documents_merge)?;
+    merge_into_lmdb_database(
+        &mut wtxn,
+        *index.documents.as_polymorph(),
+        documents_readers,
+        documents_merge,
+    )?;
+
+    for (db_type, result) in receiver {
+        let content = result?;
+        match db_type {
+            DatabaseType::Main => {
+                debug!("Writing the main elements into LMDB on disk...");
+                write_into_lmdb_database(&mut wtxn, index.main, content)?;
+            },
+            DatabaseType::WordDocids => {
+                debug!("Writing the words docids into LMDB on disk...");
+                let db = *index.word_docids.as_polymorph();
+                write_into_lmdb_database(&mut wtxn, db, content)?;
+            },
+            DatabaseType::WordsPairsProximitiesDocids => {
+                debug!("Writing the words pairs proximities docids into LMDB on disk...");
+                let db = *index.word_pair_proximity_docids.as_polymorph();
+                write_into_lmdb_database(&mut wtxn, db, content)?;
+            },
+        }
+    }
 
     debug!("Retrieving the number of documents...");
     let count = index.number_of_documents(&wtxn)?;