diff --git a/.gitmodules b/.gitmodules
index 8dd505f9..f20df4e9 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -7,3 +7,6 @@
 [submodule "third_party/sparsehash"]
 	path = third_party/sparsehash
 	url = https://github.com/sparsehash/sparsehash
+[submodule "third_party/sparsepp"]
+	path = third_party/sparsepp
+	url = https://github.com/greg7mdp/sparsepp
diff --git a/src/query.cc b/src/query.cc
index b1f856da..6f6b7aba 100644
--- a/src/query.cc
+++ b/src/query.cc
@@ -72,8 +72,9 @@ void AddMergeableRange(
   // time at the cost of some additional memory.
 
   // Build lookup table.
-  google::dense_hash_map<TId, size_t, std::hash<TId>> id_to_index;
-  id_to_index.set_empty_key(TId(-1));
+  //google::dense_hash_map<TId, size_t, std::hash<TId>> id_to_index;
+  //id_to_index.set_empty_key(TId(-1));
+  spp::sparse_hash_map<TId, size_t> id_to_index;
   id_to_index.resize(dest->size());
   for (size_t i = 0; i < dest->size(); ++i)
     id_to_index[(*dest)[i].id] = i;
@@ -356,17 +357,17 @@ IdMap::IdMap(QueryDatabase* query_db, const IdCache& local_ids)
   : local_ids(local_ids) {
   primary_file = GetQueryFileIdFromPath(query_db, local_ids.primary_file);
 
-  cached_type_ids_.set_empty_key(IndexTypeId(-1));
+  //cached_type_ids_.set_empty_key(IndexTypeId(-1));
   cached_type_ids_.resize(local_ids.type_id_to_usr.size());
   for (const auto& entry : local_ids.type_id_to_usr)
     cached_type_ids_[entry.first] = GetQueryTypeIdFromUsr(query_db, entry.second);
 
-  cached_func_ids_.set_empty_key(IndexFuncId(-1));
+  //cached_func_ids_.set_empty_key(IndexFuncId(-1));
   cached_func_ids_.resize(local_ids.func_id_to_usr.size());
   for (const auto& entry : local_ids.func_id_to_usr)
     cached_func_ids_[entry.first] = GetQueryFuncIdFromUsr(query_db, entry.second);
 
-  cached_var_ids_.set_empty_key(IndexVarId(-1));
+  //cached_var_ids_.set_empty_key(IndexVarId(-1));
   cached_var_ids_.resize(local_ids.var_id_to_usr.size());
   for (const auto& entry : local_ids.var_id_to_usr)
     cached_var_ids_[entry.first] = GetQueryVarIdFromUsr(query_db, entry.second);
diff --git a/src/query.h b/src/query.h
index 9b8ccaac..23e3e618 100644
--- a/src/query.h
+++ b/src/query.h
@@ -3,7 +3,7 @@
 #include "indexer.h"
 #include "serializer.h"
 
-#include <sparsehash/dense_hash_map>
+#include <sparsepp/spp.h>
 
 #include <functional>
 
@@ -257,10 +257,11 @@ struct QueryDatabase {
   std::vector<QueryVar> vars;
 
   // Lookup symbol based on a usr.
-  google::dense_hash_map<Usr, SymbolIdx> usr_to_symbol;
+  spp::sparse_hash_map<Usr, SymbolIdx> usr_to_symbol;
+  //google::dense_hash_map<Usr, SymbolIdx> usr_to_symbol;
 
   QueryDatabase() {
-    usr_to_symbol.set_empty_key("");
+    //usr_to_symbol.set_empty_key("");
   }
   //std::unordered_map<Usr, SymbolIdx> usr_to_symbol;
 
@@ -310,7 +311,10 @@ struct IdMap {
   SymbolIdx ToSymbol(IndexFuncId id) const;
   SymbolIdx ToSymbol(IndexVarId id) const;
 private:
-  google::dense_hash_map<IndexTypeId, QueryTypeId, std::hash<IndexTypeId>> cached_type_ids_;
-  google::dense_hash_map<IndexFuncId, QueryFuncId, std::hash<IndexFuncId>> cached_func_ids_;
-  google::dense_hash_map<IndexVarId, QueryVarId, std::hash<IndexVarId>> cached_var_ids_;
+  spp::sparse_hash_map<IndexTypeId, QueryTypeId> cached_type_ids_;
+  spp::sparse_hash_map<IndexFuncId, QueryFuncId> cached_func_ids_;
+  spp::sparse_hash_map<IndexVarId, QueryVarId> cached_var_ids_;
+  //google::dense_hash_map<IndexTypeId, QueryTypeId, std::hash<IndexTypeId>> cached_type_ids_;
+  //google::dense_hash_map<IndexFuncId, QueryFuncId, std::hash<IndexFuncId>> cached_func_ids_;
+  //google::dense_hash_map<IndexVarId, QueryVarId, std::hash<IndexVarId>> cached_var_ids_;
 };
diff --git a/third_party/sparsepp b/third_party/sparsepp
new file mode 160000
index 00000000..b1d54fbe
--- /dev/null
+++ b/third_party/sparsepp
@@ -0,0 +1 @@
+Subproject commit b1d54fbe547cab3a13d181f16f1de758d6827f81