diff --git a/crates/milli/src/update/new/words_prefix_docids.rs b/crates/milli/src/update/new/words_prefix_docids.rs
index ffc0c5048..5454d815e 100644
--- a/crates/milli/src/update/new/words_prefix_docids.rs
+++ b/crates/milli/src/update/new/words_prefix_docids.rs
@@ -4,7 +4,7 @@ use std::io::{BufReader, BufWriter, Read, Seek, Write};
 
 use hashbrown::HashMap;
 use heed::types::Bytes;
-use heed::{BytesDecode, Database, RoTxn, RwTxn};
+use heed::{BytesDecode, Database, Error, RoTxn, RwTxn};
 use rayon::iter::{IntoParallelIterator, ParallelIterator as _};
 use roaring::MultiOps;
 use tempfile::tempfile;
@@ -171,35 +171,54 @@ impl WordPrefixIntegerDocids {
         prefixes: &HashSet<Prefix>,
     ) -> Result<()> {
         // We fetch the docids associated to the newly added word prefix fst only.
-        // We use a HashMap to store the docids associated to each position, may be RAM consuming.
-        let mut integer_docids = HashMap::new();
-        let mut key_buffer = Vec::new();
-        for prefix in prefixes {
-            let prefix = prefix.as_bytes();
-            for result in self.database.prefix_iter(wtxn, prefix)? {
-                let (key, data) = result?;
-                let (_word, pos) =
-                    StrBEU16Codec::bytes_decode(key).map_err(heed::Error::Decoding)?;
+        // And collect the CboRoaringBitmaps pointers in an HashMap.
+        let frozen = FrozenPrefixIntegerBitmaps::from_prefixes(self.database, wtxn, prefixes)?;
 
-                match integer_docids.get_mut(&pos) {
-                    Some(docids) => {
-                        *docids |= &data;
-                    }
-                    None => {
-                        integer_docids.insert(pos, data);
-                    }
-                }
+        // We access this HashMap in parallel to compute the *union* of all
+        // of them and *serialize* them into files. There is one file by CPU.
+        let local_entries = ThreadLocal::with_capacity(rayon::current_num_threads());
+        prefixes.into_par_iter().map(AsRef::as_ref).try_for_each(|prefix| {
+            let refcell = local_entries.get_or_try(|| {
+                tempfile().map(BufWriter::new).map(|f| RefCell::new((Vec::new(), f, Vec::new())))
+            })?;
+
+            let mut refmut = refcell.borrow_mut_or_yield();
+            let (ref mut index, ref mut file, ref mut buffer) = *refmut;
+
+            for (&pos, bitmaps_bytes) in frozen.bitmaps(prefix).unwrap() {
+                let output = bitmaps_bytes
+                    .iter()
+                    .map(|bytes| CboRoaringBitmapCodec::deserialize_from(bytes))
+                    .union()?;
+
+                buffer.clear();
+                CboRoaringBitmapCodec::serialize_into(&output, buffer);
+                index.push(PrefixIntegerEntry { prefix, pos, serialized_length: buffer.len() });
+                file.write_all(buffer)?;
             }
 
-            for (pos, docids) in integer_docids.iter_mut() {
-                if !docids.is_empty() {
-                    key_buffer.clear();
-                    key_buffer.extend_from_slice(prefix);
-                    key_buffer.push(0);
-                    key_buffer.extend_from_slice(&pos.to_be_bytes());
-                    self.prefix_database.put(wtxn, &key_buffer, docids)?;
-                }
-                docids.clear();
+            Result::Ok(())
+        })?;
+
+        drop(frozen);
+
+        // We iterate over all the collected and serialized bitmaps through
+        // the files and entries to eventually put them in the final database.
+        let mut key_buffer = Vec::new();
+        for refcell in local_entries {
+            let (index, file, mut buffer) = refcell.into_inner();
+            let mut file = file.into_inner().map_err(|e| e.into_error())?;
+            file.rewind()?;
+            let mut file = BufReader::new(file);
+            for PrefixIntegerEntry { prefix, pos, serialized_length } in index {
+                buffer.resize(serialized_length, 0);
+                file.read_exact(&mut buffer)?;
+
+                key_buffer.clear();
+                key_buffer.extend_from_slice(prefix.as_bytes());
+                key_buffer.push(0);
+                key_buffer.extend_from_slice(&pos.to_be_bytes());
+                self.prefix_database.remap_data_type::<Bytes>().put(wtxn, &key_buffer, &buffer)?;
             }
         }
 
@@ -207,6 +226,48 @@ impl WordPrefixIntegerDocids {
     }
 }
 
+/// Represents a prefix and the lenght the bitmap takes on disk.
+struct PrefixIntegerEntry<'a> {
+    prefix: &'a str,
+    pos: u16,
+    serialized_length: usize,
+}
+
+/// TODO doc
+struct FrozenPrefixIntegerBitmaps<'a, 'rtxn> {
+    prefixes_bitmaps: HashMap<&'a str, HashMap<u16, Vec<&'rtxn [u8]>>>,
+}
+
+impl<'a, 'rtxn> FrozenPrefixIntegerBitmaps<'a, 'rtxn> {
+    #[tracing::instrument(level = "trace", skip_all, target = "indexing::prefix")]
+    pub fn from_prefixes(
+        database: Database<Bytes, CboRoaringBitmapCodec>,
+        rtxn: &'rtxn RoTxn,
+        prefixes: &'a HashSet<Prefix>,
+    ) -> heed::Result<Self> {
+        let database = database.remap_data_type::<Bytes>();
+
+        let mut prefixes_bitmaps = HashMap::new();
+        for prefix in prefixes {
+            let mut positions = HashMap::new();
+            for result in database.prefix_iter(rtxn, prefix.as_bytes())? {
+                let (key, bytes) = result?;
+                let (_word, pos) = StrBEU16Codec::bytes_decode(key).map_err(Error::Decoding)?;
+                positions.entry(pos).or_insert_with(Vec::new).push(bytes);
+            }
+            assert!(prefixes_bitmaps.insert(prefix.as_str(), positions).is_none());
+        }
+
+        Ok(Self { prefixes_bitmaps })
+    }
+
+    pub fn bitmaps(&self, key: &'a str) -> Option<&HashMap<u16, Vec<&'rtxn [u8]>>> {
+        self.prefixes_bitmaps.get(&key)
+    }
+}
+
+unsafe impl<'a, 'rtxn> Sync for FrozenPrefixIntegerBitmaps<'a, 'rtxn> {}
+
 #[tracing::instrument(level = "trace", skip_all, target = "indexing::prefix")]
 fn delete_prefixes(
     wtxn: &mut RwTxn,