diff --git a/milli/src/update/index_documents/mod.rs b/milli/src/update/index_documents/mod.rs
index 9b1c73b36..93b86617c 100644
--- a/milli/src/update/index_documents/mod.rs
+++ b/milli/src/update/index_documents/mod.rs
@@ -279,9 +279,9 @@ where
         let index_documents_ids = self.index.documents_ids(self.wtxn)?;
         let index_is_empty = index_documents_ids.len() == 0;
         let mut final_documents_ids = RoaringBitmap::new();
-        let mut word_pair_proximity_docids = Vec::new();
-        let mut word_position_docids = Vec::new();
-        let mut word_docids = Vec::new();
+        let mut word_pair_proximity_docids = None;
+        let mut word_position_docids = None;
+        let mut word_docids = None;
 
         let mut databases_seen = 0;
         (self.progress)(UpdateIndexingStep::MergeDataIntoFinalDatabase {
@@ -293,17 +293,17 @@ where
             let typed_chunk = match result? {
                 TypedChunk::WordDocids(chunk) => {
                     let cloneable_chunk = unsafe { as_cloneable_grenad(&chunk)? };
-                    word_docids.push(cloneable_chunk);
+                    word_docids = Some(cloneable_chunk);
                     TypedChunk::WordDocids(chunk)
                 }
                 TypedChunk::WordPairProximityDocids(chunk) => {
                     let cloneable_chunk = unsafe { as_cloneable_grenad(&chunk)? };
-                    word_pair_proximity_docids.push(cloneable_chunk);
+                    word_pair_proximity_docids = Some(cloneable_chunk);
                     TypedChunk::WordPairProximityDocids(chunk)
                 }
                 TypedChunk::WordPositionDocids(chunk) => {
                     let cloneable_chunk = unsafe { as_cloneable_grenad(&chunk)? };
-                    word_position_docids.push(cloneable_chunk);
+                    word_position_docids = Some(cloneable_chunk);
                     TypedChunk::WordPositionDocids(chunk)
                 }
                 otherwise => otherwise,
@@ -345,9 +345,9 @@ where
         self.index.put_documents_ids(self.wtxn, &all_documents_ids)?;
 
         self.execute_prefix_databases(
-            word_docids,
-            word_pair_proximity_docids,
-            word_position_docids,
+            word_docids.unwrap(),
+            word_pair_proximity_docids.unwrap(),
+            word_position_docids.unwrap(),
         )?;
 
         Ok(all_documents_ids.len())
@@ -356,9 +356,9 @@ where
     #[logging_timer::time("IndexDocuments::{}")]
     pub fn execute_prefix_databases(
         self,
-        word_docids: Vec<grenad::Reader<CursorClonableMmap>>,
-        word_pair_proximity_docids: Vec<grenad::Reader<CursorClonableMmap>>,
-        word_position_docids: Vec<grenad::Reader<CursorClonableMmap>>,
+        word_docids: grenad::Reader<CursorClonableMmap>,
+        word_pair_proximity_docids: grenad::Reader<CursorClonableMmap>,
+        word_position_docids: grenad::Reader<CursorClonableMmap>,
     ) -> Result<()>
     where
         F: Fn(UpdateIndexingStep) + Sync,
diff --git a/milli/src/update/word_prefix_docids.rs b/milli/src/update/word_prefix_docids.rs
index 0bb5edb9a..2baaf2f19 100644
--- a/milli/src/update/word_prefix_docids.rs
+++ b/milli/src/update/word_prefix_docids.rs
@@ -1,6 +1,6 @@
 use std::collections::{HashMap, HashSet};
 
-use grenad::{CompressionType, MergerBuilder};
+use grenad::CompressionType;
 use heed::types::ByteSlice;
 
 use crate::update::index_documents::{
@@ -35,7 +35,7 @@ impl<'t, 'u, 'i> WordPrefixDocids<'t, 'u, 'i> {
     #[logging_timer::time("WordPrefixDocids::{}")]
     pub fn execute(
         self,
-        new_word_docids: Vec<grenad::Reader<CursorClonableMmap>>,
+        new_word_docids: grenad::Reader<CursorClonableMmap>,
         new_prefix_fst_words: &[String],
         common_prefix_fst_words: &[&[String]],
         del_prefix_fst_words: &HashSet<Vec<u8>>,
@@ -50,15 +50,10 @@ impl<'t, 'u, 'i> WordPrefixDocids<'t, 'u, 'i> {
             self.max_memory,
         );
 
-        let mut word_docids_merger = MergerBuilder::new(merge_roaring_bitmaps);
-        for reader in new_word_docids {
-            word_docids_merger.push(reader.into_cursor()?);
-        }
-        let mut word_docids_iter = word_docids_merger.build().into_stream_merger_iter()?;
-
+        let mut new_word_docids_iter = new_word_docids.into_cursor()?;
         let mut current_prefixes: Option<&&[String]> = None;
         let mut prefixes_cache = HashMap::new();
-        while let Some((word, data)) = 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 => {
diff --git a/milli/src/update/word_prefix_pair_proximity_docids.rs b/milli/src/update/word_prefix_pair_proximity_docids.rs
index b498d5850..692dd1568 100644
--- a/milli/src/update/word_prefix_pair_proximity_docids.rs
+++ b/milli/src/update/word_prefix_pair_proximity_docids.rs
@@ -1,6 +1,6 @@
 use std::collections::{HashMap, HashSet};
 
-use grenad::{CompressionType, MergerBuilder};
+use grenad::CompressionType;
 use heed::types::ByteSlice;
 use heed::BytesDecode;
 use log::debug;
@@ -64,7 +64,7 @@ impl<'t, 'u, 'i> WordPrefixPairProximityDocids<'t, 'u, 'i> {
     #[logging_timer::time("WordPrefixPairProximityDocids::{}")]
     pub fn execute(
         self,
-        new_word_pair_proximity_docids: Vec<grenad::Reader<CursorClonableMmap>>,
+        new_word_pair_proximity_docids: grenad::Reader<CursorClonableMmap>,
         new_prefix_fst_words: &[String],
         common_prefix_fst_words: &[&[String]],
         del_prefix_fst_words: &HashSet<Vec<u8>>,
@@ -74,14 +74,7 @@ impl<'t, 'u, 'i> WordPrefixPairProximityDocids<'t, 'u, 'i> {
         let new_prefix_fst_words: Vec<_> =
             new_prefix_fst_words.linear_group_by_key(|x| x.chars().nth(0).unwrap()).collect();
 
-        // We retrieve and merge the created word pair proximities docids entries
-        // for the newly added documents.
-        let mut wppd_merger = MergerBuilder::new(merge_cbo_roaring_bitmaps);
-        for reader in new_word_pair_proximity_docids {
-            wppd_merger.push(reader.into_cursor()?);
-        }
-        let mut wppd_iter = wppd_merger.build().into_stream_merger_iter()?;
-
+        let mut new_wppd_iter = new_word_pair_proximity_docids.into_cursor()?;
         let mut word_prefix_pair_proximity_docids_sorter = create_sorter(
             merge_cbo_roaring_bitmaps,
             self.chunk_compression_type,
@@ -95,7 +88,7 @@ impl<'t, 'u, 'i> WordPrefixPairProximityDocids<'t, 'u, 'i> {
         let mut buffer = Vec::new();
         let mut current_prefixes: Option<&&[String]> = None;
         let mut prefixes_cache = HashMap::new();
-        while let Some((key, data)) = wppd_iter.next()? {
+        while let Some((key, data)) = new_wppd_iter.move_on_next()? {
             let (w1, w2, prox) = StrStrU8Codec::bytes_decode(key).ok_or(heed::Error::Decoding)?;
             if prox > self.max_proximity {
                 continue;
diff --git a/milli/src/update/words_prefix_position_docids.rs b/milli/src/update/words_prefix_position_docids.rs
index 9e15f4d6c..324516325 100644
--- a/milli/src/update/words_prefix_position_docids.rs
+++ b/milli/src/update/words_prefix_position_docids.rs
@@ -2,7 +2,7 @@ use std::collections::{HashMap, HashSet};
 use std::num::NonZeroU32;
 use std::{cmp, str};
 
-use grenad::{CompressionType, MergerBuilder};
+use grenad::CompressionType;
 use heed::types::ByteSlice;
 use heed::{BytesDecode, BytesEncode};
 use log::debug;
@@ -57,7 +57,7 @@ impl<'t, 'u, 'i> WordPrefixPositionDocids<'t, 'u, 'i> {
     #[logging_timer::time("WordPrefixPositionDocids::{}")]
     pub fn execute(
         self,
-        new_word_position_docids: Vec<grenad::Reader<CursorClonableMmap>>,
+        new_word_position_docids: grenad::Reader<CursorClonableMmap>,
         new_prefix_fst_words: &[String],
         common_prefix_fst_words: &[&[String]],
         del_prefix_fst_words: &HashSet<Vec<u8>>,
@@ -72,18 +72,13 @@ impl<'t, 'u, 'i> WordPrefixPositionDocids<'t, 'u, 'i> {
             self.max_memory,
         );
 
-        let mut word_position_docids_merger = MergerBuilder::new(merge_cbo_roaring_bitmaps);
-        for reader in new_word_position_docids {
-            word_position_docids_merger.push(reader.into_cursor()?);
-        }
-        let mut word_position_docids_iter =
-            word_position_docids_merger.build().into_stream_merger_iter()?;
+        let mut new_word_position_docids_iter = new_word_position_docids.into_cursor()?;
 
         // We fetch all the new common prefixes between the previous and new prefix fst.
         let mut buffer = Vec::new();
         let mut current_prefixes: Option<&&[String]> = None;
         let mut prefixes_cache = HashMap::new();
-        while let Some((key, data)) = word_position_docids_iter.next()? {
+        while let Some((key, data)) = new_word_position_docids_iter.move_on_next()? {
             let (word, pos) = StrBEU32Codec::bytes_decode(key).ok_or(heed::Error::Decoding)?;
 
             current_prefixes = match current_prefixes.take() {