Insertion by one sometimes gives drammatically better results than

components/omnibox/browser/url_index_private_data.cc

Index: components/omnibox/browser/url_index_private_data.cc
diff --git a/components/omnibox/browser/url_index_private_data.cc b/components/omnibox/browser/url_index_private_data.cc
index b9f2a93ea945bc2e73e33afdb396d0bd0ef8a975..2671573ba1e12cba45be7267fd7f4a6bdfeed7a7 100644
--- a/components/omnibox/browser/url_index_private_data.cc
+++ b/components/omnibox/browser/url_index_private_data.cc
@@ -590,6 +590,15 @@ HistoryIDSet URLIndexPrivateData::HistoryIDsForTerm(
   // the sets from each word.
   // We use |buffer| because it's more efficient to collect everything and then
   // construct a flat_set than to insert elements one by one.
+
+#define BUFFER 1
+#define BY_ONE 2
+#define MERGE 3
+
+#define METHOD MERGE
+
+#if METHOD == BUFFER
+
   HistoryIDVector buffer;
   for (WordID word_id : word_id_set) {
     WordIDHistoryMap::iterator word_iter = word_id_history_map_.find(word_id);
@@ -602,6 +611,34 @@ HistoryIDSet URLIndexPrivateData::HistoryIDsForTerm(
   HistoryIDSet history_id_set(buffer.begin(), buffer.end(),
                               base::KEEP_FIRST_OF_DUPES);
 
+#elif METHOD == BY_ONE
+
+  HistoryIDSet history_id_set;
+
+  for (WordID word_id : word_id_set) {
+    WordIDHistoryMap::iterator word_iter = word_id_history_map_.find(word_id);
+    if (word_iter != word_id_history_map_.end()) {
+      HistoryIDSet& word_history_id_set(word_iter->second);
+      history_id_set.insert_by_one(word_history_id_set.begin(),
+                                   word_history_id_set.end());
+    }
+  }
+
+#else  // MERGE
+
+  HistoryIDSet history_id_set;
+
+  for (WordID word_id : word_id_set) {
+    WordIDHistoryMap::iterator word_iter = word_id_history_map_.find(word_id);
+    if (word_iter != word_id_history_map_.end()) {
+      HistoryIDSet& word_history_id_set(word_iter->second);
+      history_id_set.insert_merge(word_history_id_set.begin(),
+                                  word_history_id_set.end());
+    }
+  }
+
+#endif
+
   // Record a new cache entry for this word if the term is longer than
   // a single character.
   if (term_length > 1)