diff --git a/src/bin/indexer.rs b/src/bin/indexer.rs
index 98c279d85..bed058dfc 100644
--- a/src/bin/indexer.rs
+++ b/src/bin/indexer.rs
@@ -1,5 +1,5 @@
 use std::collections::BTreeSet;
-use std::convert::{TryInto, TryFrom};
+use std::convert::TryFrom;
 use std::fs::File;
 use std::path::PathBuf;
 use std::sync::atomic::{AtomicUsize, Ordering};
@@ -15,7 +15,7 @@ use roaring::RoaringBitmap;
 use structopt::StructOpt;
 
 use mega_mini_indexer::alphanumeric_tokens;
-use mega_mini_indexer::{FastMap4, SmallVec32, BEU32, Index, DocumentId};
+use mega_mini_indexer::{FastMap4, SmallVec32, Index, DocumentId};
 
 #[cfg(target_os = "linux")]
 #[global_allocator]
@@ -136,7 +136,9 @@ impl MtblKvStore {
         }
     }
 
-    fn from_many(stores: Vec<MtblKvStore>) -> anyhow::Result<MtblKvStore> {
+    fn from_many<F>(stores: Vec<MtblKvStore>, mut f: F) -> anyhow::Result<()>
+    where F: FnMut(&[u8], &[u8]) -> anyhow::Result<()>
+    {
         eprintln!("{:?}: Merging {} MTBL stores...", rayon::current_thread_index(), stores.len());
 
         let mmaps: Vec<_> = stores.iter().flat_map(|m| {
@@ -147,21 +149,16 @@ impl MtblKvStore {
             Reader::new(&mmap, ReaderOptions::default()).unwrap()
         }).collect();
 
-        let outfile = tempfile::tempfile()?;
-        let mut out = Writer::new(outfile, None)?;
-
         let opt = MergerOptions { merge: MtblKvStore::merge };
         let mut merger = Merger::new(sources, opt);
 
         let mut iter = merger.iter();
         while let Some((k, v)) = iter.next() {
-            out.add(k, v).unwrap();
+            (f)(k, v)?;
         }
 
-        let out = out.into_inner()?;
-
         eprintln!("{:?}: MTBL stores merged!", rayon::current_thread_index());
-        Ok(MtblKvStore(Some(out)))
+        Ok(())
     }
 }
 
@@ -226,46 +223,32 @@ fn index_csv(mut rdr: csv::Reader<File>) -> anyhow::Result<MtblKvStore> {
 }
 
 // TODO merge with the previous values
-fn writer(wtxn: &mut heed::RwTxn, index: Index, mtbl_store: MtblKvStore) -> anyhow::Result<usize> {
-    let mtbl_store = match mtbl_store.0 {
-        Some(store) => unsafe { memmap::Mmap::map(&store)? },
-        None => return Ok(0),
-    };
-    let mtbl_store = Reader::new(&mtbl_store, ReaderOptions::default()).unwrap();
-
-    // Write the words fst
-    let fst = mtbl_store.get(b"\0words-fst").unwrap();
-    let fst = fst::Set::new(fst)?;
-    index.main.put::<_, Str, ByteSlice>(wtxn, "words-fst", &fst.as_fst().as_bytes())?;
-
-    // Write and merge the headers
-    let headers = mtbl_store.get(b"\0headers").unwrap();
-    index.main.put::<_, Str, ByteSlice>(wtxn, "headers", headers.as_ref())?;
-
-    // Write and merge the postings lists
-    let mut iter = mtbl_store.iter_prefix(&[1]).unwrap();
-    while let Some((word, postings)) = iter.next() {
-        let word = std::str::from_utf8(&word[1..]).unwrap();
-        index.postings_ids.put(wtxn, &word, &postings)?;
+fn writer(wtxn: &mut heed::RwTxn, index: &Index, key: &[u8], val: &[u8]) -> anyhow::Result<()> {
+    if key == b"\0words-fst" {
+        // Write the words fst
+        index.main.put::<_, Str, ByteSlice>(wtxn, "words-fst", val)?;
+    }
+    else if key == b"\0headers" {
+        // Write the headers
+        index.main.put::<_, Str, ByteSlice>(wtxn, "headers", val)?;
+    }
+    else if key.starts_with(&[1]) {
+        // Write the postings lists
+        index.postings_ids.as_polymorph()
+            .put::<_, ByteSlice, ByteSlice>(wtxn, &key[1..], val)?;
+    }
+    else if key.starts_with(&[2]) {
+        // Write the prefix postings lists
+        index.prefix_postings_ids.as_polymorph()
+            .put::<_, ByteSlice, ByteSlice>(wtxn, &key[1..], val)?;
+    }
+    else if key.starts_with(&[3]) {
+        // Write the documents
+        index.documents.as_polymorph()
+            .put::<_, ByteSlice, ByteSlice>(wtxn, &key[1..], val)?;
     }
 
-    // Write and merge the prefix postings lists
-    let mut iter = mtbl_store.iter_prefix(&[2]).unwrap();
-    while let Some((word, postings)) = iter.next() {
-        let word = std::str::from_utf8(&word[1..]).unwrap();
-        index.prefix_postings_ids.put(wtxn, &word, &postings)?;
-    }
-
-    // Write the documents
-    let mut count = 0;
-    let mut iter = mtbl_store.iter_prefix(&[3]).unwrap();
-    while let Some((id_bytes, content)) = iter.next() {
-        let id = id_bytes[1..].try_into().map(u32::from_be_bytes).unwrap();
-        index.documents.put(wtxn, &BEU32::new(id), &content)?;
-        count += 1;
-    }
-
-    Ok(count)
+    Ok(())
 }
 
 fn main() -> anyhow::Result<()> {
@@ -291,18 +274,24 @@ fn main() -> anyhow::Result<()> {
         })
         .collect::<Result<_, _>>()?;
 
-    while stores.len() > 1 {
+    while stores.len() > 3 {
+        let chunk_size = (stores.len() / rayon::current_num_threads()).max(2);
         let s = std::mem::take(&mut stores);
-        stores = s.into_par_iter().chunks(3)
-            .map(MtblKvStore::from_many)
+        stores = s.into_par_iter().chunks(chunk_size)
+            .map(|v| {
+                let outfile = tempfile::tempfile()?;
+                let mut out = Writer::new(outfile, None)?;
+                MtblKvStore::from_many(v, |k, v| Ok(out.add(k, v).unwrap()))?;
+                let out = out.into_inner()?;
+                Ok(MtblKvStore(Some(out))) as anyhow::Result<_>
+            })
             .collect::<Result<_, _>>()?;
     }
 
-    let mtbl_store = stores.pop().unwrap_or_default();
-
     eprintln!("We are writing into LMDB...");
     let mut wtxn = env.write_txn()?;
-    let count = writer(&mut wtxn, index, mtbl_store)?;
+    MtblKvStore::from_many(stores, |k, v| writer(&mut wtxn, &index, k, v))?;
+    let count = index.documents.len(&wtxn)?;
     wtxn.commit()?;
     eprintln!("Wrote {} documents into LMDB", count);