diff --git a/.gitmodules b/.gitmodules
index fbe945c5..8dd505f9 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -4,3 +4,6 @@
 [submodule "third_party/doctest"]
 	path = third_party/doctest
 	url = https://github.com/onqtam/doctest
+[submodule "third_party/sparsehash"]
+	path = third_party/sparsehash
+	url = https://github.com/sparsehash/sparsehash
diff --git a/src/query.h b/src/query.h
index 47867489..1c94e54b 100644
--- a/src/query.h
+++ b/src/query.h
@@ -3,6 +3,8 @@
 #include "indexer.h"
 #include "serializer.h"
 
+#include <sparsehash/dense_hash_map>
+
 using Usr = std::string;
 
 struct QueryableFile;
@@ -234,7 +236,12 @@ struct QueryableDatabase {
   std::vector<QueryableVarDef> vars;
 
   // Lookup symbol based on a usr.
-  std::unordered_map<Usr, SymbolIdx> usr_to_symbol;
+  google::dense_hash_map<Usr, SymbolIdx> usr_to_symbol;
+
+  QueryableDatabase() {
+    usr_to_symbol.set_empty_key("");
+  }
+  //std::unordered_map<Usr, SymbolIdx> usr_to_symbol;
 
   // Insert the contents of |update| into |db|.
   void ApplyIndexUpdate(IndexUpdate* update);
diff --git a/third_party/sparsehash b/third_party/sparsehash
new file mode 160000
index 00000000..4cb92402
--- /dev/null
+++ b/third_party/sparsehash
@@ -0,0 +1 @@
+Subproject commit 4cb924025b8c622d1a1e11f4c1e9db15410c75fb
diff --git a/wscript b/wscript
index aceb42e1..63dfb406 100644
--- a/wscript
+++ b/wscript
@@ -88,15 +88,16 @@ def build(bld):
                                excl=['libcxx/*', '*tests/*', 'third_party/*'])
   bld.program(
       source=cc_files,
-      cxxflags=['-std=c++11', '-Wall'],
+      cxxflags=['-O3', '-std=c++11', '-Wall'],
       includes=[
         'third_party/',
         'third_party/doctest/',
         'third_party/rapidjson/include',
+        'third_party/sparsehash/src',
         CLANG_INCLUDE_DIR],
       lib=[
         'clang',
-        #'rt',
+        'rt',
         'pthread'],
       libpath=[CLANG_LIB_DIR],
       rpath=[CLANG_LIB_DIR],