Introduce MTBL parallel merging before LMDB writing

Cargo.lock

@@ -1,5 +1,11 @@
 # This file is automatically @generated by Cargo.
 # It is not intended for manual editing.
+[[package]]
+name = "adler32"
+version = "1.0.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5d2e7343e7fc9de883d1b0341e0b13970f764c14101234857d2ddafa1cb1cac2"
+
 [[package]]
 name = "anyhow"
 version = "1.0.31"
@@ -66,6 +72,9 @@ name = "cc"
 version = "1.0.54"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7bbb73db36c1246e9034e307d0fba23f9a2e251faa47ade70c1bd252220c8311"
+dependencies = [
+ "jobserver",
+]
 
 [[package]]
 name = "cfg-if"
@@ -84,6 +93,21 @@ dependencies = [
  "unicode-width",
 ]
 
+[[package]]
+name = "crc32c"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "77ba37ef26c12988c1cee882d522d65e1d5d2ad8c3864665b88ee92767ed84c5"
+
+[[package]]
+name = "crc32fast"
+version = "1.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ba125de2af0df55319f41944744ad91c71113bf74a4646efff39afe1f6842db1"
+dependencies = [
+ "cfg-if",
+]
+
 [[package]]
 name = "crossbeam-deque"
 version = "0.7.3"
@@ -165,6 +189,18 @@ version = "1.5.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "bb1f6b1ce1c140482ea30ddd3335fc0024ac7ee112895426e0a629a6c20adfe3"
 
+[[package]]
+name = "flate2"
+version = "1.0.14"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2cfff41391129e0a856d6d822600b8d71179d46879e310417eb9c762eb178b42"
+dependencies = [
+ "cfg-if",
+ "crc32fast",
+ "libc",
+ "miniz_oxide",
+]
+
 [[package]]
 name = "fs_extra"
 version = "1.1.0"
@@ -186,6 +222,23 @@ dependencies = [
  "byteorder 1.3.4",
 ]
 
+[[package]]
+name = "getrandom"
+version = "0.1.14"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7abc8dd8451921606d809ba32e95b6111925cd2906060d2dcc29c070220503eb"
+dependencies = [
+ "cfg-if",
+ "libc",
+ "wasi",
+]
+
+[[package]]
+name = "glob"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9b919933a397b79c37e33b77bb2aa3dc8eb6e165ad809e58ff75bc7db2e34574"
+
 [[package]]
 name = "heck"
 version = "0.3.1"
@@ -251,6 +304,15 @@ dependencies = [
  "unicode-normalization",
 ]
 
+[[package]]
+name = "itertools"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "284f18f85651fe11e8a991b2adb42cb078325c996ed026d994719efcfca1d54b"
+dependencies = [
+ "either",
+]
+
 [[package]]
 name = "itoa"
 version = "0.4.5"
@@ -278,6 +340,15 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "jobserver"
+version = "0.1.21"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5c71313ebb9439f74b00d9d2dcec36440beaf57a6aa0623068441dd7cd81a7f2"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "lazy_static"
 version = "1.4.0"
@@ -325,11 +396,15 @@ dependencies = [
  "fxhash",
  "heed",
  "jemallocator",
+ "memmap",
+ "oxidized-mtbl",
  "rayon",
  "roaring",
  "slice-group-by",
  "smallstr",
+ "smallvec",
  "structopt",
+ "tempfile",
 ]
 
 [[package]]
@@ -338,6 +413,16 @@ version = "2.3.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3728d817d99e5ac407411fa471ff9800a778d88a24685968b36824eaf4bee400"
 
+[[package]]
+name = "memmap"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6585fd95e7bb50d6cc31e20d4cf9afb4e2ba16c5846fc76793f11218da9c475b"
+dependencies = [
+ "libc",
+ "winapi",
+]
+
 [[package]]
 name = "memoffset"
 version = "0.5.4"
@@ -347,6 +432,15 @@ dependencies = [
  "autocfg",
 ]
 
+[[package]]
+name = "miniz_oxide"
+version = "0.3.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "aa679ff6578b1cddee93d7e82e263b94a575e0bfced07284eb0c037c1d2416a5"
+dependencies = [
+ "adler32",
+]
+
 [[package]]
 name = "num_cpus"
 version = "1.13.0"
@@ -363,6 +457,18 @@ version = "1.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0b631f7e854af39a1739f401cf34a8a013dfe09eac4fa4dba91e9768bd28168d"
 
+[[package]]
+name = "oxidized-mtbl"
+version = "0.1.0"
+source = "git+https://github.com/Kerollmops/oxidized-mtbl.git?rev=8918476#8918476f61f4430890d067db7b4a6cfb2d549c43"
+dependencies = [
+ "byteorder 1.3.4",
+ "crc32c",
+ "flate2",
+ "snap",
+ "zstd",
+]
+
 [[package]]
 name = "page_size"
 version = "0.4.2"
@@ -385,6 +491,12 @@ version = "0.3.17"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "05da548ad6865900e60eaba7f589cc0783590a92e940c26953ff81ddbab2d677"
 
+[[package]]
+name = "ppv-lite86"
+version = "0.2.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "237a5ed80e274dbc66f86bd59c1e25edc039660be53194b5fe0a482e0f2612ea"
+
 [[package]]
 name = "proc-macro-error"
 version = "1.0.2"
@@ -429,6 +541,47 @@ dependencies = [
  "proc-macro2",
 ]
 
+[[package]]
+name = "rand"
+version = "0.7.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6a6b1679d49b24bbfe0c803429aa1874472f50d9b363131f0e89fc356b544d03"
+dependencies = [
+ "getrandom",
+ "libc",
+ "rand_chacha",
+ "rand_core",
+ "rand_hc",
+]
+
+[[package]]
+name = "rand_chacha"
+version = "0.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f4c8ed856279c9737206bf725bf36935d8666ead7aa69b52be55af369d193402"
+dependencies = [
+ "ppv-lite86",
+ "rand_core",
+]
+
+[[package]]
+name = "rand_core"
+version = "0.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "90bde5296fc891b0cef12a6d03ddccc162ce7b2aff54160af9338f8d40df6d19"
+dependencies = [
+ "getrandom",
+]
+
+[[package]]
+name = "rand_hc"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ca3129af7b92a17112d59ad498c6f81eaf463253766b90396d39ea7a39d6613c"
+dependencies = [
+ "rand_core",
+]
+
 [[package]]
 name = "rayon"
 version = "1.3.0"
@@ -453,6 +606,12 @@ dependencies = [
  "num_cpus",
 ]
 
+[[package]]
+name = "redox_syscall"
+version = "0.1.56"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2439c63f3f6139d1b57529d16bc3b8bb855230c8efcc5d3a896c8bea7c3b1e84"
+
 [[package]]
 name = "regex-automata"
 version = "0.1.9"
@@ -462,6 +621,15 @@ dependencies = [
  "byteorder 1.3.4",
 ]
 
+[[package]]
+name = "remove_dir_all"
+version = "0.5.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4a83fa3702a688b9359eccba92d153ac33fd2e8462f9e0e3fdf155239ea7792e"
+dependencies = [
+ "winapi",
+]
+
 [[package]]
 name = "roaring"
 version = "0.5.2"
@@ -521,6 +689,12 @@ version = "1.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c7cb5678e1615754284ec264d9bb5b4c27d2018577fd90ac0ceb578591ed5ee4"
 
+[[package]]
+name = "snap"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f7fb9b0bb877b35a1cc1474a3b43d9c226a2625311760cdda2cbccbc0c7a8376"
+
 [[package]]
 name = "structopt"
 version = "0.3.14"
@@ -579,6 +753,20 @@ dependencies = [
  "unicode-xid",
 ]
 
+[[package]]
+name = "tempfile"
+version = "3.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7a6e24d9338a0a5be79593e2fa15a648add6138caa803e2d5bc782c371732ca9"
+dependencies = [
+ "cfg-if",
+ "libc",
+ "rand",
+ "redox_syscall",
+ "remove_dir_all",
+ "winapi",
+]
+
 [[package]]
 name = "textwrap"
 version = "0.11.0"
@@ -641,6 +829,12 @@ version = "0.9.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b5a972e5669d67ba988ce3dc826706fb0a8b01471c088cb0b6110b805cc36aed"
 
+[[package]]
+name = "wasi"
+version = "0.9.0+wasi-snapshot-preview1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cccddf32554fecc6acb585f82a32a72e28b48f8c4c1883ddfeeeaa96f7d8e519"
+
 [[package]]
 name = "winapi"
 version = "0.3.8"
@@ -683,3 +877,34 @@ dependencies = [
  "syn",
  "synstructure",
 ]
+
+[[package]]
+name = "zstd"
+version = "0.5.2+zstd.1.4.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "644352b10ce7f333d6e0af85bd4f5322dc449416dc1211c6308e95bca8923db4"
+dependencies = [
+ "zstd-safe",
+]
+
+[[package]]
+name = "zstd-safe"
+version = "2.0.4+zstd.1.4.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7113c0c9aed2c55181f2d9f5b0a36e7d2c0183b11c058ab40b35987479efe4d7"
+dependencies = [
+ "libc",
+ "zstd-sys",
+]
+
+[[package]]
+name = "zstd-sys"
+version = "1.4.16+zstd.1.4.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c442965efc45353be5a9b9969c9b0872fff6828c7e06d118dda2cb2d0bb11d5a"
+dependencies = [
+ "cc",
+ "glob",
+ "itertools",
+ "libc",
+]

Cargo.toml

@@ -8,16 +8,20 @@ edition = "2018"
 anyhow = "1.0.28"
 bitpacking = "0.8.2"
 byteorder = "1.3.4"
-roaring = "0.5.2"
 csv = "1.1.3"
 fst = "0.4.3"
 fxhash = "0.2.1"
 heed = { version = "0.8.0", default-features = false, features = ["lmdb"] }
 jemallocator = "0.3.2"
+memmap = "0.7.0"
+oxidized-mtbl = { git = "https://github.com/Kerollmops/oxidized-mtbl.git", rev = "8918476" }
 rayon = "1.3.0"
+roaring = "0.5.2"
 slice-group-by = "0.2.6"
 smallstr = "0.2.0"
+smallvec = "1.4.0"
 structopt = { version = "0.3.14", default-features = false }
+tempfile = "3.1.0"
 
 [profile.release]
 debug = true

src/main.rs

@@ -1,23 +1,25 @@
-use std::collections::hash_map::Entry;
 use std::collections::{HashMap, BTreeSet};
 use std::convert::TryFrom;
+use std::convert::TryInto;
 use std::fs::File;
 use std::hash::BuildHasherDefault;
 use std::path::PathBuf;
 use std::sync::atomic::{AtomicUsize, Ordering};
 
-use anyhow::{ensure, Context};
+use anyhow::Context;
-use roaring::RoaringBitmap;
+use fst::{Streamer, IntoStreamer};
-use fst::IntoStreamer;
 use fxhash::FxHasher32;
-use heed::{EnvOpenOptions, PolyDatabase, Database};
 use heed::types::*;
+use heed::{EnvOpenOptions, PolyDatabase, Database};
+use oxidized_mtbl::{Reader, ReaderOptions, Writer, Merger, MergerOptions};
 use rayon::prelude::*;
+use roaring::RoaringBitmap;
 use slice_group_by::StrGroupBy;
 use structopt::StructOpt;
 
 pub type FastMap4<K, V> = HashMap<K, V, BuildHasherDefault<FxHasher32>>;
 pub type SmallString32 = smallstr::SmallString<[u8; 32]>;
+pub type SmallVec32 = smallvec::SmallVec<[u8; 32]>;
 pub type BEU32 = heed::zerocopy::U32<heed::byteorder::BE>;
 pub type DocumentId = u32;
 
@@ -39,100 +41,126 @@ struct Opt {
     files_to_index: Vec<PathBuf>,
 }
 
-fn union_postings_ids(_key: &[u8], old_value: Option<&[u8]>, new_value: RoaringBitmap) -> Option<Vec<u8>> {
-    let result = match old_value {
-        Some(bytes) => {
-            let mut old_value = RoaringBitmap::deserialize_from(bytes).unwrap();
-            old_value.union_with(&new_value);
-            old_value
-        },
-        None => new_value,
-    };
-
-    let mut vec = Vec::new();
-    result.serialize_into(&mut vec).unwrap();
-    Some(vec)
-}
-
-fn union_words_fst(key: &[u8], old_value: Option<&[u8]>, new_value: &fst::Set<Vec<u8>>) -> Option<Vec<u8>> {
-    if key != b"words-fst" { unimplemented!() }
-
-    // Do an union of the old and the new set of words.
-    let mut builder = fst::set::OpBuilder::new();
-
-    let old_words = old_value.map(|v| fst::Set::new(v).unwrap());
-    let old_words = old_words.as_ref().map(|v| v.into_stream());
-    if let Some(old_words) = old_words {
-        builder.push(old_words);
-    }
-
-    builder.push(new_value);
-
-    let op = builder.r#union();
-    let mut build = fst::SetBuilder::memory();
-    build.extend_stream(op.into_stream()).unwrap();
-
-    Some(build.into_inner().unwrap())
-}
-
 fn alphanumeric_tokens(string: &str) -> impl Iterator<Item = &str> {
     let is_alphanumeric = |s: &&str| s.chars().next().map_or(false, char::is_alphanumeric);
     string.linear_group_by_key(|c| c.is_alphanumeric()).filter(is_alphanumeric)
 }
 
-#[derive(Default)]
 struct Indexed {
     fst: fst::Set<Vec<u8>>,
-    postings_ids: FastMap4<SmallString32, RoaringBitmap>,
+    postings_ids: FastMap4<SmallVec32, RoaringBitmap>,
     headers: Vec<u8>,
     documents: Vec<(DocumentId, Vec<u8>)>,
 }
 
-impl Indexed {
+#[derive(Default)]
-    fn merge_with(mut self, mut other: Indexed) -> Indexed {
+struct MtblKvStore(Option<File>);
 
-        // Union of the two FSTs
+impl MtblKvStore {
-        let op = fst::set::OpBuilder::new()
+    fn from_indexed(mut indexed: Indexed) -> anyhow::Result<MtblKvStore> {
-            .add(self.fst.into_stream())
+        let outfile = tempfile::tempfile()?;
-            .add(other.fst.into_stream())
+        let mut out = Writer::new(outfile, None)?;
-            .r#union();
 
-        let mut build = fst::SetBuilder::memory();
+        out.add(b"\0headers", indexed.headers)?;
-        build.extend_stream(op.into_stream()).unwrap();
+        out.add(b"\0words-fst", indexed.fst.as_fst().as_bytes())?;
-        let fst = build.into_set();
 
-        // Merge the postings by unions
+        // postings ids keys are all prefixed by a '1'
-        for (word, mut postings) in other.postings_ids {
+        let mut key = vec![1];
-            match self.postings_ids.entry(word) {
+        let mut buffer = Vec::new();
-                Entry::Occupied(mut entry) => {
+        // We must write the postings ids in order for mtbl therefore
-                    let old = entry.get();
+        // we iterate over the fst to read the words in order
-                    postings.union_with(&old);
+        let mut stream = indexed.fst.stream();
-                    entry.insert(postings);
+        while let Some(word) = stream.next() {
-                },
+            key.truncate(1);
-                Entry::Vacant(entry) => {
+            key.extend_from_slice(word);
-                    entry.insert(postings);
+            if let Some(ids) = indexed.postings_ids.remove(word) {
-                },
+                buffer.clear();
+                ids.serialize_into(&mut buffer)?;
+                out.add(&key, &buffer).unwrap();
             }
         }
 
-        // assert headers are valid
+        // postings ids keys are all prefixed by a '2'
-        if !self.headers.is_empty() {
+        key[0] = 2;
-            assert_eq!(self.headers, other.headers);
+        indexed.documents.sort_unstable();
+        for (id, content) in indexed.documents {
+            key.truncate(1);
+            key.extend_from_slice(&id.to_be_bytes());
+            out.add(&key, content).unwrap();
         }
 
-        // extend the documents
+        let out = out.into_inner()?;
-        self.documents.append(&mut other.documents);
+        Ok(MtblKvStore(Some(out)))
+    }
 
-        Indexed {
+    fn merge_with(self, other: MtblKvStore) -> anyhow::Result<MtblKvStore> {
-            fst,
+        let (left, right) = match (self.0, other.0) {
-            postings_ids: self.postings_ids,
+            (Some(left), Some(right)) => (left, right),
-            headers: self.headers,
+            (Some(left), None) => return Ok(MtblKvStore(Some(left))),
-            documents: self.documents,
+            (None, Some(right)) => return Ok(MtblKvStore(Some(right))),
+            (None, None) => return Ok(MtblKvStore(None)),
+        };
+
+        let left = unsafe { memmap::Mmap::map(&left)? };
+        let right = unsafe { memmap::Mmap::map(&right)? };
+
+        let left = Reader::new(&left, ReaderOptions::default()).unwrap();
+        let right = Reader::new(&right, ReaderOptions::default()).unwrap();
+
+        fn merge(key: &[u8], left: &[u8], right: &[u8]) -> Option<Vec<u8>> {
+            if key == b"\0words-fst" {
+                let left_fst = fst::Set::new(left).unwrap();
+                let right_fst = fst::Set::new(right).unwrap();
+
+                // Union of the two FSTs
+                let op = fst::set::OpBuilder::new()
+                    .add(left_fst.into_stream())
+                    .add(right_fst.into_stream())
+                    .r#union();
+
+                let mut build = fst::SetBuilder::memory();
+                build.extend_stream(op.into_stream()).unwrap();
+                Some(build.into_inner().unwrap())
+            }
+            else if key == b"\0headers" {
+                assert_eq!(left, right);
+                Some(left.to_vec())
+            }
+            else if key.starts_with(&[1]) {
+                let mut left = RoaringBitmap::deserialize_from(left).unwrap();
+                let right = RoaringBitmap::deserialize_from(right).unwrap();
+                left.union_with(&right);
+                let mut vec = Vec::new();
+                left.serialize_into(&mut vec).unwrap();
+                Some(vec)
+            }
+            else if key.starts_with(&[2]) {
+                assert_eq!(left, right);
+                Some(left.to_vec())
+            }
+            else {
+                panic!("wut? {:?}", key)
+            }
         }
+
+        let outfile = tempfile::tempfile()?;
+        let mut out = Writer::new(outfile, None)?;
+
+        let sources = vec![left, right];
+        let opt = MergerOptions { 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();
+        }
+
+        let out = out.into_inner()?;
+        Ok(MtblKvStore(Some(out)))
     }
 }
 
-fn index_csv(mut rdr: csv::Reader<File>) -> anyhow::Result<Indexed> {
+fn index_csv(mut rdr: csv::Reader<File>) -> anyhow::Result<MtblKvStore> {
     const MAX_POSITION: usize = 1000;
     const MAX_ATTRIBUTES: usize = u32::max_value() as usize / MAX_POSITION;
 
@@ -153,7 +181,7 @@ fn index_csv(mut rdr: csv::Reader<File>) -> anyhow::Result<Indexed> {
         for (_attr, content) in document.iter().enumerate().take(MAX_ATTRIBUTES) {
             for (_pos, word) in alphanumeric_tokens(&content).enumerate().take(MAX_POSITION) {
                 if !word.is_empty() && word.len() < 500 { // LMDB limits
-                    postings_ids.entry(SmallString32::from(word))
+                    postings_ids.entry(SmallVec32::from(word.as_bytes()))
                         .or_insert_with(RoaringBitmap::new)
                         .insert(document_id);
                 }
@@ -173,44 +201,51 @@ fn index_csv(mut rdr: csv::Reader<File>) -> anyhow::Result<Indexed> {
         new_words.insert(word.clone());
     }
 
-    let new_words_fst = fst::Set::from_iter(new_words.iter().map(SmallString32::as_str))?;
+    let new_words_fst = fst::Set::from_iter(new_words.iter().map(SmallVec32::as_ref))?;
 
-    Ok(Indexed { fst: new_words_fst, headers, postings_ids, documents })
+    let indexed = Indexed { fst: new_words_fst, headers, postings_ids, documents };
+
+    MtblKvStore::from_indexed(indexed)
 }
 
+// TODO merge with the previous values
 fn writer(
     wtxn: &mut heed::RwTxn,
     main: PolyDatabase,
     postings_ids: Database<Str, ByteSlice>,
     documents: Database<OwnedType<BEU32>, ByteSlice>,
-    indexed: Indexed,
+    mtbl_store: MtblKvStore,
 ) -> anyhow::Result<usize>
 {
-    // Write and merge the words fst
+    let mtbl_store = match mtbl_store.0 {
-    let old_value = main.get::<_, Str, ByteSlice>(wtxn, "words-fst")?;
+        Some(store) => unsafe { memmap::Mmap::map(&store)? },
-    let new_value = union_words_fst(b"words-fst", old_value, &indexed.fst)
+        None => return Ok(0),
-        .context("error while do a words-fst union")?;
+    };
-    main.put::<_, Str, ByteSlice>(wtxn, "words-fst", &new_value)?;
+    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)?;
+    main.put::<_, Str, ByteSlice>(wtxn, "words-fst", &fst.as_fst().as_bytes())?;
 
     // Write and merge the headers
-    if let Some(old_headers) = main.get::<_, Str, ByteSlice>(wtxn, "headers")? {
+    let headers = mtbl_store.get(b"\0headers").unwrap();
-        ensure!(old_headers == &*indexed.headers, "headers differs from the previous ones");
+    main.put::<_, Str, ByteSlice>(wtxn, "headers", headers.as_ref())?;
-    }
-    main.put::<_, Str, ByteSlice>(wtxn, "headers", &indexed.headers)?;
 
     // Write and merge the postings lists
-    for (word, postings) in indexed.postings_ids {
+    let mut iter = mtbl_store.iter_prefix(&[1]).unwrap();
-        let old_value = postings_ids.get(wtxn, word.as_str())?;
+    while let Some((word, postings)) = iter.next() {
-        let new_value = union_postings_ids(word.as_bytes(), old_value, postings)
+        let word = std::str::from_utf8(&word[1..]).unwrap();
-            .context("error while do a words-fst union")?;
+        postings_ids.put(wtxn, &word, &postings)?;
-        postings_ids.put(wtxn, &word, &new_value)?;
     }
 
-    let count = indexed.documents.len();
-
     // Write the documents
-    for (id, content) in indexed.documents {
+    let mut count = 0;
+    let mut iter = mtbl_store.iter_prefix(&[2]).unwrap();
+    while let Some((id_bytes, content)) = iter.next() {
+        let id = id_bytes[1..].try_into().map(u32::from_be_bytes).unwrap();
         documents.put(wtxn, &BEU32::new(id), &content)?;
+        count += 1;
     }
 
     Ok(count)
@@ -232,29 +267,23 @@ fn main() -> anyhow::Result<()> {
 
     let res = opt.files_to_index
         .into_par_iter()
-        .try_fold(|| Indexed::default(), |acc, path| {
+        .try_fold(MtblKvStore::default, |acc, path| {
             let rdr = csv::Reader::from_path(path)?;
-            let indexed = index_csv(rdr)?;
+            let mtbl_store = index_csv(rdr)?;
-            Ok(acc.merge_with(indexed)) as anyhow::Result<Indexed>
+            acc.merge_with(mtbl_store)
-        })
-        .map(|indexed| match indexed {
-            Ok(indexed) => {
-                let tid = rayon::current_thread_index();
-                eprintln!("{:?}: A new step to write into LMDB", tid);
-                let mut wtxn = env.write_txn()?;
-                let count = writer(&mut wtxn, main, postings_ids, documents, indexed)?;
-                wtxn.commit()?;
-                eprintln!("{:?}: Wrote {} documents into LMDB", tid, count);
-                Ok(count)
-            },
-            Err(e) => Err(e),
         })
         .inspect(|_| {
             eprintln!("Total number of documents seen so far is {}", ID_GENERATOR.load(Ordering::Relaxed))
         })
-        .try_reduce(|| 0, |a, b| Ok(a + b));
+        .try_reduce(MtblKvStore::default, MtblKvStore::merge_with);
 
-    println!("indexed {:?} documents", res);
+    let mtbl_store = res?;
+
+    eprintln!("We are writing into LMDB...");
+    let mut wtxn = env.write_txn()?;
+    let count = writer(&mut wtxn, main, postings_ids, documents, mtbl_store)?;
+    wtxn.commit()?;
+    eprintln!("Wrote {} documents into LMDB", count);
 
     Ok(())
 }