diff --git a/src/word_db.cc b/src/word_db.cc
index c64ae01a..1e2d8a04 100644
--- a/src/word_db.cc
+++ b/src/word_db.cc
@@ -103,10 +103,11 @@ void WordDB::on_erase(const Buffer& buffer, BufferCoord begin, BufferCoord end)
 std::vector<String> WordDB::find_prefix(const String& prefix) const
 {
     std::vector<String> res;
-    for (auto& word : m_word_to_lines)
+    for (auto it = m_word_to_lines.lower_bound(prefix); it != m_word_to_lines.end(); ++it)
     {
-        if (prefix_match(word.first, prefix))
-            res.push_back(word.first);
+        if (not prefix_match(it->first, prefix))
+            break;
+        res.push_back(it->first);
     }
     return res;
 }
diff --git a/src/word_db.hh b/src/word_db.hh
index c7f8a83d..bbbfa04c 100644
--- a/src/word_db.hh
+++ b/src/word_db.hh
@@ -22,7 +22,7 @@ public:
     std::vector<String> find_prefix(const String& prefix) const;
 
 private:
-    using WordToLines = std::unordered_map<String, std::vector<LineCount>>;
+    using WordToLines = std::map<String, std::vector<LineCount>>;
     using LineToWords = std::map<LineCount, std::vector<String>>;
 
     void add_words(LineCount line, const String& content);