Use smartstring to store the external id in our hashmap

We need to store all the external id (primary key) in a hashmap
associated to their internal id during.
The smartstring remove heap allocation / memory usage and should
improve the cache locality.

milli/Cargo.toml

@@ -32,6 +32,7 @@ serde_json = { version = "1.0.79", features = ["preserve_order"] }
 slice-group-by = "0.3.0"
 smallstr =  { version = "0.3.0", features = ["serde"] }
 smallvec = "1.8.0"
+smartstring = "1.0.1"
 tempfile = "3.3.0"
 time = { version = "0.3.7", features = ["serde-well-known", "formatting", "parsing", "macros"] }
 uuid = { version = "0.8.2", features = ["v4"] }

milli/src/update/index_documents/mod.rs

@@ -1109,8 +1109,11 @@ mod tests {
 
         let mut big_object = HashMap::new();
         big_object.insert(S("id"), "wow");
-        let content: String =
-            (0..=u16::MAX).into_iter().map(|p| p.to_string()).reduce(|a, b| a + " " + &b).unwrap();
+        let content: String = (0..=u16::MAX)
+            .into_iter()
+            .map(|p| p.to_string())
+            .reduce(|a, b| a + " " + b.as_ref())
+            .unwrap();
         big_object.insert("content".to_string(), &content);
 
         let mut cursor = Cursor::new(Vec::new());

milli/src/update/index_documents/transform.rs

@@ -11,6 +11,7 @@ use itertools::Itertools;
 use obkv::{KvReader, KvWriter};
 use roaring::RoaringBitmap;
 use serde_json::{Map, Value};
+use smartstring::SmartString;
 
 use super::helpers::{create_sorter, create_writer, keep_latest_obkv, merge_obkvs, MergeFn};
 use super::{IndexDocumentsMethod, IndexerConfig};
@@ -55,7 +56,8 @@ pub struct Transform<'a, 'i> {
     flattened_sorter: grenad::Sorter<MergeFn>,
     replaced_documents_ids: RoaringBitmap,
     new_documents_ids: RoaringBitmap,
-    new_external_documents_ids_builder: FxHashMap<Vec<u8>, u64>,
+    // To increase the cache locality and the heap usage we use smartstring.
+    new_external_documents_ids_builder: FxHashMap<SmartString<smartstring::Compact>, u64>,
     documents_count: usize,
 }
 
@@ -254,10 +256,7 @@ impl<'a, 'i> Transform<'a, 'i> {
                     None => {
                         // if the document has already been inserted in this
                         // batch we need to get its docid
-                        match self
-                            .new_external_documents_ids_builder
-                            .entry(external_id.as_bytes().to_vec())
-                        {
+                        match self.new_external_documents_ids_builder.entry(external_id.into()) {
                             Entry::Occupied(entry) => (*entry.get() as u32, false),
                             // if the document has never been encountered we give it a new docid
                             // and push this new docid to the external documents ids builder