From 072382fa61ff75ad7c08e08de3711cb8b63cc924 Mon Sep 17 00:00:00 2001
From: Kerollmops <clement@meilisearch.com>
Date: Mon, 7 Sep 2020 22:38:49 +0200
Subject: [PATCH] Sort the word docids to make intersections much faster

---
 src/search.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/search.rs b/src/search.rs
index a545ab77d..ffbb9328d 100644
--- a/src/search.rs
+++ b/src/search.rs
@@ -122,6 +122,10 @@ impl<'a> Search<'a> {
         derived_words: &[(HashMap<String, (u8, RoaringBitmap)>, RoaringBitmap)],
     ) -> RoaringBitmap
     {
+        // We sort the derived words by inverse popularity, this way intersections are faster.
+        let mut derived_words: Vec<_> = derived_words.iter().collect();
+        derived_words.sort_unstable_by_key(|(_, docids)| docids.len());
+
         // we do a union between all the docids of each of the derived words,
         // we got N unions (the number of original query words), we then intersect them.
         let mut candidates = RoaringBitmap::new();