diff --git a/milli/src/update/index_documents/mod.rs b/milli/src/update/index_documents/mod.rs
index 91d108c72..0e6e59e10 100644
--- a/milli/src/update/index_documents/mod.rs
+++ b/milli/src/update/index_documents/mod.rs
@@ -4,11 +4,13 @@ mod transform;
 mod typed_chunk;
 
 use std::collections::HashSet;
-use std::io::{Read, Seek};
+use std::io::{Cursor, Read, Seek};
 use std::iter::FromIterator;
 use std::num::{NonZeroU32, NonZeroUsize};
 
 use crossbeam_channel::{Receiver, Sender};
+use heed::types::Str;
+use heed::Database;
 use log::debug;
 use roaring::RoaringBitmap;
 use serde::{Deserialize, Serialize};
@@ -20,7 +22,7 @@ pub use self::helpers::{
     fst_stream_into_vec, merge_cbo_roaring_bitmaps, merge_roaring_bitmaps,
     sorter_into_lmdb_database, write_into_lmdb_database, writer_into_reader, ClonableMmap, MergeFn,
 };
-use self::helpers::{grenad_obkv_into_chunks, merge_nothing, GrenadParameters};
+use self::helpers::{grenad_obkv_into_chunks, GrenadParameters};
 pub use self::transform::{Transform, TransformOutput};
 use crate::documents::DocumentBatchReader;
 pub use crate::update::index_documents::helpers::CursorClonableMmap;
@@ -28,7 +30,7 @@ use crate::update::{
     self, Facets, IndexerConfig, UpdateIndexingStep, WordPrefixDocids,
     WordPrefixPairProximityDocids, WordPrefixPositionDocids, WordsPrefixesFst,
 };
-use crate::{Index, Result};
+use crate::{Index, Result, RoaringBitmapCodec};
 
 static MERGED_DATABASE_COUNT: usize = 7;
 static PREFIX_DATABASE_COUNT: usize = 5;
@@ -433,25 +435,25 @@ where
         });
 
         if let Some(word_docids) = word_docids {
-            let mut word_docids_builder = grenad::MergerBuilder::new(merge_nothing as MergeFn);
-            word_docids_builder.push(word_docids.into_cursor()?);
-            if let Some(exact_word_docids) = exact_word_docids {
-                word_docids_builder.push(exact_word_docids.into_cursor()?);
-            }
-
-            let word_docids_iter = word_docids_builder.build().into_stream_merger_iter()?;
-            // Run the word prefix docids update operation.
-            let mut builder = WordPrefixDocids::new(
+            execute_word_prefix_docids(
                 self.wtxn,
+                word_docids,
                 self.index.word_docids.clone(),
                 self.index.word_prefix_docids.clone(),
-            );
-            builder.chunk_compression_type = self.indexer_config.chunk_compression_type;
-            builder.chunk_compression_level = self.indexer_config.chunk_compression_level;
-            builder.max_nb_chunks = self.indexer_config.max_nb_chunks;
-            builder.max_memory = self.indexer_config.max_memory;
-            builder.execute(
-                word_docids_iter,
+                &self.indexer_config,
+                &new_prefix_fst_words,
+                &common_prefix_fst_words,
+                &del_prefix_fst_words,
+            )?;
+        }
+
+        if let Some(exact_word_docids) = exact_word_docids {
+            execute_word_prefix_docids(
+                self.wtxn,
+                exact_word_docids,
+                self.index.exact_word_docids.clone(),
+                self.index.exact_word_prefix_docids.clone(),
+                &self.indexer_config,
                 &new_prefix_fst_words,
                 &common_prefix_fst_words,
                 &del_prefix_fst_words,
@@ -516,6 +518,32 @@ where
     }
 }
 
+/// Run the word prefix docids update operation.
+fn execute_word_prefix_docids(
+    txn: &mut heed::RwTxn,
+    reader: grenad::Reader<Cursor<ClonableMmap>>,
+    word_docids_db: Database<Str, RoaringBitmapCodec>,
+    word_prefix_docids_db: Database<Str, RoaringBitmapCodec>,
+    indexer_config: &IndexerConfig,
+    new_prefix_fst_words: &[String],
+    common_prefix_fst_words: &[&[String]],
+    del_prefix_fst_words: &HashSet<Vec<u8>>,
+) -> Result<()> {
+    let cursor = reader.into_cursor()?;
+    let mut builder = WordPrefixDocids::new(txn, word_docids_db, word_prefix_docids_db);
+    builder.chunk_compression_type = indexer_config.chunk_compression_type;
+    builder.chunk_compression_level = indexer_config.chunk_compression_level;
+    builder.max_nb_chunks = indexer_config.max_nb_chunks;
+    builder.max_memory = indexer_config.max_memory;
+    builder.execute(
+        cursor,
+        &new_prefix_fst_words,
+        &common_prefix_fst_words,
+        &del_prefix_fst_words,
+    )?;
+    Ok(())
+}
+
 #[cfg(test)]
 mod tests {
     use std::io::Cursor;
diff --git a/milli/src/update/word_prefix_docids.rs b/milli/src/update/word_prefix_docids.rs
index b166812a5..2887b5583 100644
--- a/milli/src/update/word_prefix_docids.rs
+++ b/milli/src/update/word_prefix_docids.rs
@@ -23,12 +23,12 @@ impl<'t, 'u, 'i> WordPrefixDocids<'t, 'u, 'i> {
     pub fn new(
         wtxn: &'t mut heed::RwTxn<'i, 'u>,
         word_docids: Database<Str, RoaringBitmapCodec>,
-        word_prefixes_docids: Database<Str, RoaringBitmapCodec>,
+        word_prefix_docids: Database<Str, RoaringBitmapCodec>,
     ) -> WordPrefixDocids<'t, 'u, 'i> {
         WordPrefixDocids {
             wtxn,
             word_docids,
-            word_prefix_docids: word_prefixes_docids,
+            word_prefix_docids,
             chunk_compression_type: CompressionType::None,
             chunk_compression_level: None,
             max_nb_chunks: None,
@@ -39,7 +39,7 @@ impl<'t, 'u, 'i> WordPrefixDocids<'t, 'u, 'i> {
     #[logging_timer::time("WordPrefixDocids::{}")]
     pub fn execute(
         self,
-        mut new_word_docids_iter: grenad::MergerIter<CursorClonableMmap, MergeFn>,
+        mut new_word_docids_iter: grenad::ReaderCursor<CursorClonableMmap>,
         new_prefix_fst_words: &[String],
         common_prefix_fst_words: &[&[String]],
         del_prefix_fst_words: &HashSet<Vec<u8>>,
@@ -57,7 +57,7 @@ impl<'t, 'u, 'i> WordPrefixDocids<'t, 'u, 'i> {
         if !common_prefix_fst_words.is_empty() {
             let mut current_prefixes: Option<&&[String]> = None;
             let mut prefixes_cache = HashMap::new();
-            while let Some((word, data)) = new_word_docids_iter.next()? {
+            while let Some((word, data)) = new_word_docids_iter.move_on_next()? {
                 current_prefixes = match current_prefixes.take() {
                     Some(prefixes) if word.starts_with(&prefixes[0].as_bytes()) => Some(prefixes),
                     _otherwise => {