diff --git a/src/blob/mod.rs b/src/blob/mod.rs
index 76f28fef2..05f9367c4 100644
--- a/src/blob/mod.rs
+++ b/src/blob/mod.rs
@@ -3,7 +3,7 @@ pub mod positive;
 pub mod negative;
 
 pub use self::positive::{PositiveBlob, PositiveBlobBuilder};
-pub use self::negative::{NegativeBlob, NegativeBlobBuilder};
+pub use self::negative::NegativeBlob;
 pub use self::ops::OpBuilder;
 
 use std::fmt;
diff --git a/src/blob/negative/blob.rs b/src/blob/negative/blob.rs
index 53e50b021..425be4cf6 100644
--- a/src/blob/negative/blob.rs
+++ b/src/blob/negative/blob.rs
@@ -1,11 +1,10 @@
-use std::io::Write;
 use std::path::Path;
 use std::error::Error;
 
-use crate::DocumentId;
-use crate::data::{DocIds, DocIdsBuilder};
-use serde::ser::{Serialize, Serializer};
 use serde::de::{self, Deserialize, Deserializer};
+use serde::ser::{Serialize, Serializer};
+use crate::data::DocIds;
+use crate::DocumentId;
 
 pub struct NegativeBlob {
     doc_ids: DocIds,
@@ -55,33 +54,3 @@ impl<'de> Deserialize<'de> for NegativeBlob {
         NegativeBlob::from_bytes(bytes).map_err(de::Error::custom)
     }
 }
-
-pub struct NegativeBlobBuilder<W> {
-    doc_ids: DocIdsBuilder<W>,
-}
-
-impl<W: Write> NegativeBlobBuilder<W> {
-    pub fn new(wrt: W) -> Self {
-        Self { doc_ids: DocIdsBuilder::new(wrt) }
-    }
-
-    pub fn insert(&mut self, doc: DocumentId) -> bool {
-        self.doc_ids.insert(doc)
-    }
-
-    pub fn finish(self) -> Result<(), Box<Error>> {
-        self.into_inner().map(|_| ())
-    }
-
-    pub fn into_inner(self) -> Result<W, Box<Error>> {
-        // FIXME insert a magic number that indicates if the endianess
-        //       of the input is the same as the machine that is reading it.
-        Ok(self.doc_ids.into_inner()?)
-    }
-}
-
-impl NegativeBlobBuilder<Vec<u8>> {
-    pub fn build(self) -> Result<NegativeBlob, Box<Error>> {
-        self.into_inner().and_then(|ids| NegativeBlob::from_bytes(ids))
-    }
-}
diff --git a/src/blob/negative/mod.rs b/src/blob/negative/mod.rs
index 56c9f4ef4..ce0000da0 100644
--- a/src/blob/negative/mod.rs
+++ b/src/blob/negative/mod.rs
@@ -1,5 +1,5 @@
 mod blob;
 mod ops;
 
-pub use self::blob::{NegativeBlob, NegativeBlobBuilder};
+pub use self::blob::NegativeBlob;
 pub use self::ops::OpBuilder;
diff --git a/src/data/doc_ids.rs b/src/data/doc_ids.rs
index f93fe495e..d5650cce6 100644
--- a/src/data/doc_ids.rs
+++ b/src/data/doc_ids.rs
@@ -1,11 +1,9 @@
-use std::collections::BTreeSet;
 use std::slice::from_raw_parts;
 use std::error::Error;
 use std::path::Path;
 use std::sync::Arc;
 use std::{io, mem};
 
-use byteorder::{NativeEndian, WriteBytesExt};
 use fst::raw::MmapReadOnly;
 use serde::ser::{Serialize, Serializer};
 
@@ -57,28 +55,3 @@ impl Serialize for DocIds {
         self.data.as_ref().serialize(serializer)
     }
 }
-
-pub struct DocIdsBuilder<W> {
-    doc_ids: BTreeSet<DocumentId>, // TODO: prefer a linked-list
-    wrt: W,
-}
-
-impl<W: io::Write> DocIdsBuilder<W> {
-    pub fn new(wrt: W) -> Self {
-        Self {
-            doc_ids: BTreeSet::new(),
-            wrt: wrt,
-        }
-    }
-
-    pub fn insert(&mut self, doc: DocumentId) -> bool {
-        self.doc_ids.insert(doc)
-    }
-
-    pub fn into_inner(mut self) -> io::Result<W> {
-        for id in self.doc_ids {
-            self.wrt.write_u64::<NativeEndian>(id)?;
-        }
-        Ok(self.wrt)
-    }
-}
diff --git a/src/data/mod.rs b/src/data/mod.rs
index 4cd21b65e..bf810de93 100644
--- a/src/data/mod.rs
+++ b/src/data/mod.rs
@@ -6,7 +6,7 @@ use std::sync::Arc;
 
 use fst::raw::MmapReadOnly;
 
-pub use self::doc_ids::{DocIds, DocIdsBuilder};
+pub use self::doc_ids::DocIds;
 pub use self::doc_indexes::{DocIndexes, DocIndexesBuilder, RawDocIndexesBuilder};
 
 #[derive(Clone)]
diff --git a/src/index/update/.DS_Store b/src/index/update/.DS_Store
new file mode 100644
index 000000000..445e2e61a
Binary files /dev/null and b/src/index/update/.DS_Store differ
diff --git a/src/index/update/mod.rs b/src/index/update/mod.rs
index 3c3601f26..104ec469f 100644
--- a/src/index/update/mod.rs
+++ b/src/index/update/mod.rs
@@ -1,11 +1,20 @@
+use std::io::{Cursor, Write};
 use std::path::PathBuf;
 use std::error::Error;
 
-mod negative_update;
-mod positive_update;
+use byteorder::{NetworkEndian, WriteBytesExt};
 
-pub use self::positive_update::{PositiveUpdateBuilder, NewState};
-pub use self::negative_update::NegativeUpdateBuilder;
+use crate::index::schema::SchemaAttr;
+use crate::DocumentId;
+
+mod negative;
+mod positive;
+
+pub use self::positive::{PositiveUpdateBuilder, NewState};
+pub use self::negative::NegativeUpdateBuilder;
+
+const DOC_KEY_LEN:      usize = 4 + std::mem::size_of::<u64>();
+const DOC_KEY_ATTR_LEN: usize = DOC_KEY_LEN + 1 + std::mem::size_of::<u32>();
 
 pub struct Update {
     path: PathBuf,
@@ -20,3 +29,27 @@ impl Update {
         self.path
     }
 }
+
+// "doc-{ID_8_BYTES}"
+fn raw_document_key(id: DocumentId) -> [u8; DOC_KEY_LEN] {
+    let mut key = [0; DOC_KEY_LEN];
+
+    let mut wtr = Cursor::new(&mut key[..]);
+    wtr.write_all(b"doc-").unwrap();
+    wtr.write_u64::<NetworkEndian>(id).unwrap();
+
+    key
+}
+
+// "doc-{ID_8_BYTES}-{ATTR_4_BYTES}"
+fn raw_document_key_attr(id: DocumentId, attr: SchemaAttr) -> [u8; DOC_KEY_ATTR_LEN] {
+    let mut key = [0; DOC_KEY_ATTR_LEN];
+    let raw_key = raw_document_key(id);
+
+    let mut wtr = Cursor::new(&mut key[..]);
+    wtr.write_all(&raw_key).unwrap();
+    wtr.write_all(b"-").unwrap();
+    wtr.write_u32::<NetworkEndian>(attr.as_u32()).unwrap();
+
+    key
+}
diff --git a/src/index/update/negative/mod.rs b/src/index/update/negative/mod.rs
new file mode 100644
index 000000000..bad19c918
--- /dev/null
+++ b/src/index/update/negative/mod.rs
@@ -0,0 +1,4 @@
+mod update;
+mod unordered_builder;
+
+pub use self::update::NegativeUpdateBuilder;
diff --git a/src/index/update/negative/unordered_builder.rs b/src/index/update/negative/unordered_builder.rs
new file mode 100644
index 000000000..b73ecd2e3
--- /dev/null
+++ b/src/index/update/negative/unordered_builder.rs
@@ -0,0 +1,37 @@
+use std::collections::BTreeSet;
+use std::io;
+
+use byteorder::{NativeEndian, WriteBytesExt};
+
+use crate::DocumentId;
+
+pub struct UnorderedNegativeBlobBuilder<W> {
+    doc_ids: BTreeSet<DocumentId>, // TODO: prefer a linked-list
+    wrt: W,
+}
+
+impl UnorderedNegativeBlobBuilder<Vec<u8>> {
+    pub fn memory() -> Self {
+        UnorderedNegativeBlobBuilder::new(Vec::new())
+    }
+}
+
+impl<W: io::Write> UnorderedNegativeBlobBuilder<W> {
+    pub fn new(wrt: W) -> Self {
+        Self {
+            doc_ids: BTreeSet::new(),
+            wrt: wrt,
+        }
+    }
+
+    pub fn insert(&mut self, doc: DocumentId) -> bool {
+        self.doc_ids.insert(doc)
+    }
+
+    pub fn into_inner(mut self) -> io::Result<W> {
+        for id in self.doc_ids {
+            self.wrt.write_u64::<NativeEndian>(id)?;
+        }
+        Ok(self.wrt)
+    }
+}
diff --git a/src/index/update/negative_update.rs b/src/index/update/negative/update.rs
similarity index 70%
rename from src/index/update/negative_update.rs
rename to src/index/update/negative/update.rs
index 342fde98c..54355c6e0 100644
--- a/src/index/update/negative_update.rs
+++ b/src/index/update/negative/update.rs
@@ -1,37 +1,24 @@
 use std::path::PathBuf;
 use std::error::Error;
-use std::io::{Cursor, Write};
 
-use byteorder::{NetworkEndian, WriteBytesExt};
 use ::rocksdb::rocksdb_options;
 
-use crate::data::{DocIds, DocIdsBuilder};
+use crate::index::update::negative::unordered_builder::UnorderedNegativeBlobBuilder;
+use crate::index::update::{Update, raw_document_key};
 use crate::blob::{Blob, NegativeBlob};
-use crate::index::update::Update;
 use crate::index::DATA_INDEX;
 use crate::DocumentId;
 
-const DOC_KEY_LEN: usize = 4 + std::mem::size_of::<u64>();
-
-// "doc-ID_8_BYTES"
-fn raw_document_key(id: DocumentId) -> [u8; DOC_KEY_LEN] {
-    let mut key = [0; DOC_KEY_LEN];
-    let mut rdr = Cursor::new(&mut key[..]);
-    rdr.write_all(b"doc-").unwrap();
-    rdr.write_u64::<NetworkEndian>(id).unwrap();
-    key
-}
-
 pub struct NegativeUpdateBuilder {
     path: PathBuf,
-    doc_ids: DocIdsBuilder<Vec<u8>>,
+    doc_ids: UnorderedNegativeBlobBuilder<Vec<u8>>,
 }
 
 impl NegativeUpdateBuilder {
     pub fn new<P: Into<PathBuf>>(path: P) -> NegativeUpdateBuilder {
         NegativeUpdateBuilder {
             path: path.into(),
-            doc_ids: DocIdsBuilder::new(Vec::new()),
+            doc_ids: UnorderedNegativeBlobBuilder::memory(),
         }
     }
 
@@ -45,10 +32,11 @@ impl NegativeUpdateBuilder {
         let mut file_writer = rocksdb::SstFileWriter::new(env_options, column_family_options);
         file_writer.open(&self.path.to_string_lossy())?;
 
-        // write the data-index aka negative blob
         let bytes = self.doc_ids.into_inner()?;
-        let doc_ids = DocIds::from_bytes(bytes)?;
-        let blob = Blob::Negative(NegativeBlob::from_raw(doc_ids));
+        let negative_blob = NegativeBlob::from_bytes(bytes)?;
+        let blob = Blob::Negative(negative_blob);
+
+        // write the data-index aka negative blob
         let bytes = bincode::serialize(&blob)?;
         file_writer.merge(DATA_INDEX, &bytes)?;
 
diff --git a/src/index/update/positive/mod.rs b/src/index/update/positive/mod.rs
new file mode 100644
index 000000000..e05bd9dff
--- /dev/null
+++ b/src/index/update/positive/mod.rs
@@ -0,0 +1,4 @@
+mod update;
+mod unordered_builder;
+
+pub use self::update::{PositiveUpdateBuilder, NewState};
diff --git a/src/index/update/positive/unordered_builder.rs b/src/index/update/positive/unordered_builder.rs
new file mode 100644
index 000000000..5b75fe28d
--- /dev/null
+++ b/src/index/update/positive/unordered_builder.rs
@@ -0,0 +1,45 @@
+use std::collections::BTreeMap;
+use std::error::Error;
+use std::io::Write;
+
+use crate::blob::positive::PositiveBlobBuilder;
+use crate::DocIndex;
+
+pub struct UnorderedPositiveBlobBuilder<W, X> {
+    builder: PositiveBlobBuilder<W, X>,
+    map: BTreeMap<Vec<u8>, Vec<DocIndex>>,
+}
+
+impl UnorderedPositiveBlobBuilder<Vec<u8>, Vec<u8>> {
+    pub fn memory() -> Self {
+        Self {
+            builder: PositiveBlobBuilder::memory(),
+            map: BTreeMap::new(),
+        }
+    }
+}
+
+impl<W: Write, X: Write> UnorderedPositiveBlobBuilder<W, X> {
+    pub fn new(map_wtr: W, doc_wtr: X) -> Result<Self, Box<Error>> {
+        Ok(UnorderedPositiveBlobBuilder {
+            builder: PositiveBlobBuilder::new(map_wtr, doc_wtr)?,
+            map: BTreeMap::new(),
+        })
+    }
+
+    pub fn insert<K: Into<Vec<u8>>>(&mut self, input: K, doc_index: DocIndex) {
+        self.map.entry(input.into()).or_insert_with(Vec::new).push(doc_index);
+    }
+
+    pub fn finish(self) -> Result<(), Box<Error>> {
+        self.into_inner().map(drop)
+    }
+
+    pub fn into_inner(mut self) -> Result<(W, X), Box<Error>> {
+        for (key, mut doc_indexes) in self.map {
+            doc_indexes.sort_unstable();
+            self.builder.insert(&key, &doc_indexes)?;
+        }
+        self.builder.into_inner()
+    }
+}
diff --git a/src/index/update/positive/update.rs b/src/index/update/positive/update.rs
new file mode 100644
index 000000000..8551867b1
--- /dev/null
+++ b/src/index/update/positive/update.rs
@@ -0,0 +1,110 @@
+use std::collections::BTreeMap;
+use std::path::PathBuf;
+use std::error::Error;
+
+use ::rocksdb::rocksdb_options;
+
+use crate::index::update::positive::unordered_builder::UnorderedPositiveBlobBuilder;
+use crate::index::schema::{SchemaProps, Schema, SchemaAttr};
+use crate::index::update::{Update, raw_document_key_attr};
+use crate::blob::positive::PositiveBlob;
+use crate::tokenizer::TokenizerBuilder;
+use crate::{DocumentId, DocIndex};
+use crate::index::DATA_INDEX;
+use crate::blob::Blob;
+
+pub enum NewState {
+    Updated {
+        value: String,
+        props: SchemaProps,
+    },
+    Removed,
+}
+
+pub struct PositiveUpdateBuilder<B> {
+    path: PathBuf,
+    schema: Schema,
+    tokenizer_builder: B,
+    new_states: BTreeMap<(DocumentId, SchemaAttr), NewState>,
+}
+
+impl<B> PositiveUpdateBuilder<B> {
+    pub fn new<P: Into<PathBuf>>(path: P, schema: Schema, tokenizer_builder: B) -> PositiveUpdateBuilder<B> {
+        PositiveUpdateBuilder {
+            path: path.into(),
+            schema: schema,
+            tokenizer_builder: tokenizer_builder,
+            new_states: BTreeMap::new(),
+        }
+    }
+
+    // TODO value must be a field that can be indexed
+    pub fn update_field(&mut self, id: DocumentId, field: SchemaAttr, value: String) {
+        let state = NewState::Updated { value, props: self.schema.props(field) };
+        self.new_states.insert((id, field), state);
+    }
+
+    pub fn remove_field(&mut self, id: DocumentId, field: SchemaAttr) {
+        self.new_states.insert((id, field), NewState::Removed);
+    }
+}
+
+impl<B> PositiveUpdateBuilder<B>
+where B: TokenizerBuilder
+{
+    pub fn build(self) -> Result<Update, Box<Error>> {
+        let env_options = rocksdb_options::EnvOptions::new();
+        let column_family_options = rocksdb_options::ColumnFamilyOptions::new();
+        let mut file_writer = rocksdb::SstFileWriter::new(env_options, column_family_options);
+        file_writer.open(&self.path.to_string_lossy())?;
+
+        let mut builder = UnorderedPositiveBlobBuilder::memory();
+        for ((document_id, attr), state) in &self.new_states {
+            let value = match state {
+                NewState::Updated { value, props } if props.is_indexed() => value,
+                _ => continue,
+            };
+
+            for (index, word) in self.tokenizer_builder.build(value) {
+                let doc_index = DocIndex {
+                    document_id: *document_id,
+                    attribute: attr.as_u32() as u8,
+                    attribute_index: index as u32,
+                };
+
+                // insert the exact representation
+                let word_lower = word.to_lowercase();
+
+                // and the unidecoded lowercased version
+                let word_unidecoded = unidecode::unidecode(word).to_lowercase();
+                if word_lower != word_unidecoded {
+                    builder.insert(word_unidecoded, doc_index);
+                }
+
+                builder.insert(word_lower, doc_index);
+            }
+        }
+
+        let (blob_fst_map, blob_doc_idx) = builder.into_inner()?;
+        let positive_blob = PositiveBlob::from_bytes(blob_fst_map, blob_doc_idx)?;
+        let blob = Blob::Positive(positive_blob);
+
+        // write the data-index aka positive blob
+        let bytes = bincode::serialize(&blob)?;
+        file_writer.merge(DATA_INDEX, &bytes)?;
+
+        // write all the documents fields updates
+        for ((id, attr), state) in self.new_states {
+            let key = raw_document_key_attr(id, attr);
+            match state {
+                NewState::Updated { value, props } => if props.is_stored() {
+                    file_writer.put(&key, value.as_bytes())?
+                },
+                NewState::Removed => file_writer.delete(&key)?,
+            }
+        }
+
+        file_writer.finish()?;
+        Update::open(self.path)
+    }
+}
diff --git a/src/index/update/positive_update.rs b/src/index/update/positive_update.rs
deleted file mode 100644
index 1e6d38316..000000000
--- a/src/index/update/positive_update.rs
+++ /dev/null
@@ -1,116 +0,0 @@
-use std::collections::BTreeMap;
-use std::path::PathBuf;
-use std::error::Error;
-
-use ::rocksdb::rocksdb_options;
-
-use crate::index::update::Update;
-use crate::index::schema::{SchemaProps, Schema, SchemaAttr};
-use crate::tokenizer::TokenizerBuilder;
-use crate::DocumentId;
-
-pub enum NewState {
-    Updated {
-        value: String,
-        props: SchemaProps,
-    },
-    Removed,
-}
-
-pub struct PositiveUpdateBuilder<B> {
-    path: PathBuf,
-    schema: Schema,
-    tokenizer_builder: B,
-    new_states: BTreeMap<(DocumentId, SchemaAttr), NewState>,
-}
-
-impl<B> PositiveUpdateBuilder<B> {
-    pub fn new<P: Into<PathBuf>>(path: P, schema: Schema, tokenizer_builder: B) -> PositiveUpdateBuilder<B> {
-        PositiveUpdateBuilder {
-            path: path.into(),
-            schema: schema,
-            tokenizer_builder: tokenizer_builder,
-            new_states: BTreeMap::new(),
-        }
-    }
-
-    // TODO value must be a field that can be indexed
-    pub fn update_field(&mut self, id: DocumentId, field: SchemaAttr, value: String) {
-        let state = NewState::Updated { value, props: self.schema.props(field) };
-        self.new_states.insert((id, field), state);
-    }
-
-    pub fn remove_field(&mut self, id: DocumentId, field: SchemaAttr) {
-        self.new_states.insert((id, field), NewState::Removed);
-    }
-}
-
-impl<B> PositiveUpdateBuilder<B>
-where B: TokenizerBuilder
-{
-    pub fn build(self) -> Result<Update, Box<Error>> {
-        let env_options = rocksdb_options::EnvOptions::new();
-        let column_family_options = rocksdb_options::ColumnFamilyOptions::new();
-        let mut file_writer = rocksdb::SstFileWriter::new(env_options, column_family_options);
-        file_writer.open(&self.path.to_string_lossy())?;
-
-        // let mut builder = PositiveBlobBuilder::new(Vec::new(), Vec::new());
-        // for ((document_id, field), state) in &self.new_states {
-        //     let value = match state {
-        //         NewState::Updated { value, props } if props.is_indexed() => value,
-        //         _ => continue,
-        //     };
-
-        //     for (index, word) in self.tokenizer_builder.build(value) {
-        //         let doc_index = DocIndex {
-        //             document_id: *document_id,
-        //             attribute: field.as_u32() as u8,
-        //             attribute_index: index as u32,
-        //         };
-        //         // insert the exact representation
-        //         let word_lower = word.to_lowercase();
-
-        //         // and the unidecoded lowercased version
-        //         let word_unidecoded = unidecode::unidecode(word).to_lowercase();
-        //         if word_lower != word_unidecoded {
-        //             builder.insert(word_unidecoded, doc_index);
-        //         }
-
-        //         builder.insert(word_lower, doc_index);
-        //     }
-        // }
-        // let (blob_fst_map, blob_doc_idx) = builder.into_inner()?;
-
-        // // write the doc-idx
-        // let blob_key = Identifier::blob(blob_info.name).document_indexes().build();
-        // file_writer.put(&blob_key, &blob_doc_idx)?;
-
-        // // write the fst
-        // let blob_key = Identifier::blob(blob_info.name).fst_map().build();
-        // file_writer.put(&blob_key, &blob_fst_map)?;
-
-        // {
-        //     // write the blob name to be merged
-        //     let mut buffer = Vec::new();
-        //     blob_info.write_into(&mut buffer);
-        //     let data_key = Identifier::data().blobs_order().build();
-        //     file_writer.merge(&data_key, &buffer)?;
-        // }
-
-        // // write all the documents fields updates
-        // for ((id, attr), state) in self.new_states {
-        //     let key = Identifier::document(id).attribute(attr).build();
-        //     match state {
-        //         NewState::Updated { value, props } => if props.is_stored() {
-        //             file_writer.put(&key, value.as_bytes())?
-        //         },
-        //         NewState::Removed => file_writer.delete(&key)?,
-        //     }
-        // }
-
-        // file_writer.finish()?;
-        // Update::open(self.path)
-
-        unimplemented!()
-    }
-}