diff --git a/src/indexer.hh b/src/indexer.hh
index fb762ac1..62401ab3 100644
--- a/src/indexer.hh
+++ b/src/indexer.hh
@@ -143,6 +143,9 @@ void Reflect(BinaryWriter &visitor, SymbolRef &value);
 void Reflect(BinaryWriter &visitor, Use &value);
 void Reflect(BinaryWriter &visitor, DeclRef &value);
 
+template <typename T>
+using VectorAdapter = std::vector<T, std::allocator<T>>;
+
 template <typename D> struct NameMixin {
   std::string_view Name(bool qualified) const {
     auto self = static_cast<const D *>(this);
@@ -156,7 +159,8 @@ template <typename D> struct NameMixin {
   }
 };
 
-struct FuncDef : NameMixin<FuncDef> {
+template <template <typename T> class V>
+struct FuncDef : NameMixin<FuncDef<V>> {
   // General metadata.
   const char *detailed_name = "";
   const char *hover = "";
@@ -164,11 +168,11 @@ struct FuncDef : NameMixin<FuncDef> {
   Maybe<DeclRef> spell;
 
   // Method this method overrides.
-  std::vector<Usr> bases;
+  V<Usr> bases;
   // Local variables or parameters.
-  std::vector<Usr> vars;
+  V<Usr> vars;
   // Functions that this function calls.
-  std::vector<SymbolRef> callees;
+  V<SymbolRef> callees;
 
   int file_id = -1; // not serialized
   int16_t qual_name_offset = 0;
@@ -178,14 +182,15 @@ struct FuncDef : NameMixin<FuncDef> {
   SymbolKind parent_kind = SymbolKind::Unknown;
   uint8_t storage = clang::SC_None;
 
-  std::vector<Usr> GetBases() const { return bases; }
+  const Usr *bases_begin() const { return bases.begin(); }
+  const Usr *bases_end() const { return bases.end(); }
 };
-REFLECT_STRUCT(FuncDef, detailed_name, hover, comments, spell, bases, vars,
-                    callees, qual_name_offset, short_name_offset,
-                    short_name_size, kind, parent_kind, storage);
+REFLECT_STRUCT(FuncDef<VectorAdapter>, detailed_name, hover, comments, spell,
+               bases, vars, callees, qual_name_offset, short_name_offset,
+               short_name_size, kind, parent_kind, storage);
 
 struct IndexFunc : NameMixin<IndexFunc> {
-  using Def = FuncDef;
+  using Def = FuncDef<VectorAdapter>;
   Usr usr;
   Def def;
   std::vector<DeclRef> declarations;
@@ -193,17 +198,18 @@ struct IndexFunc : NameMixin<IndexFunc> {
   std::vector<Use> uses;
 };
 
-struct TypeDef : NameMixin<TypeDef> {
+template <template <typename T> class V>
+struct TypeDef : NameMixin<TypeDef<V>> {
   const char *detailed_name = "";
   const char *hover = "";
   const char *comments = "";
   Maybe<DeclRef> spell;
 
-  std::vector<Usr> bases;
+  V<Usr> bases;
   // Types, functions, and variables defined in this type.
-  std::vector<Usr> funcs;
-  std::vector<Usr> types;
-  std::vector<std::pair<Usr, int64_t>> vars;
+  V<Usr> funcs;
+  V<Usr> types;
+  V<std::pair<Usr, int64_t>> vars;
 
   // If set, then this is the same underlying type as the given value (ie, this
   // type comes from a using or typedef statement).
@@ -215,14 +221,15 @@ struct TypeDef : NameMixin<TypeDef> {
   SymbolKind kind = SymbolKind::Unknown;
   SymbolKind parent_kind = SymbolKind::Unknown;
 
-  std::vector<Usr> GetBases() const { return bases; }
+  const Usr *bases_begin() const { return bases.begin(); }
+  const Usr *bases_end() const { return bases.end(); }
 };
-REFLECT_STRUCT(TypeDef, detailed_name, hover, comments, spell, bases,
-                    funcs, types, vars, alias_of, qual_name_offset,
-                    short_name_offset, short_name_size, kind, parent_kind);
+REFLECT_STRUCT(TypeDef<VectorAdapter>, detailed_name, hover, comments, spell,
+               bases, funcs, types, vars, alias_of, qual_name_offset,
+               short_name_offset, short_name_size, kind, parent_kind);
 
 struct IndexType {
-  using Def = TypeDef;
+  using Def = TypeDef<VectorAdapter>;
   Usr usr;
   Def def;
   std::vector<DeclRef> declarations;
@@ -260,7 +267,8 @@ struct VarDef : NameMixin<VarDef> {
             storage == clang::SC_Register);
   }
 
-  std::vector<Usr> GetBases() const { return {}; }
+  const Usr *bases_begin() const { return nullptr; }
+  const Usr *bases_end() const { return nullptr; }
 };
 REFLECT_STRUCT(VarDef, detailed_name, hover, comments, spell, type,
                     qual_name_offset, short_name_offset, short_name_size, kind,
diff --git a/src/messages/textDocument_references.cc b/src/messages/textDocument_references.cc
index e9858dfc..9289b15a 100644
--- a/src/messages/textDocument_references.cc
+++ b/src/messages/textDocument_references.cc
@@ -16,8 +16,12 @@ limitations under the License.
 #include "message_handler.hh"
 #include "query.hh"
 
+#include <llvm/ADT/iterator_range.h>
+
 #include <unordered_set>
 
+using namespace llvm;
+
 namespace ccls {
 namespace {
 struct ReferenceParam : public TextDocumentPositionParam {
@@ -80,7 +84,7 @@ void MessageHandler::textDocument_references(JsonReader &reader,
           if (def.spell) {
             parent_kind = GetSymbolKind(db, sym);
             if (param.base)
-              for (Usr usr : def.GetBases())
+              for (Usr usr : make_range(def.bases_begin(), def.bases_end()))
                 if (!seen.count(usr)) {
                   seen.insert(usr);
                   stack.push_back(usr);
diff --git a/src/query.cc b/src/query.cc
index 6a2d1b5b..6adfb424 100644
--- a/src/query.cc
+++ b/src/query.cc
@@ -81,6 +81,51 @@ bool TryReplaceDef(llvm::SmallVectorImpl<Q> &def_list, Q &&def) {
 
 } // namespace
 
+template <typename T> Vec<T> Convert(const std::vector<T> &o) {
+  Vec<T> r{std::make_unique<T[]>(o.size()), (int)o.size()};
+  std::copy(o.begin(), o.end(), r.begin());
+  return r;
+}
+
+QueryFunc::Def Convert(const IndexFunc::Def &o) {
+  QueryFunc::Def r;
+  r.detailed_name = o.detailed_name;
+  r.hover = o.hover;
+  r.comments = o.comments;
+  r.spell = o.spell;
+  r.bases = Convert(o.bases);
+  r.vars = Convert(o.vars);
+  r.callees = Convert(o.callees);
+  // no file_id
+  r.qual_name_offset = o.qual_name_offset;
+  r.short_name_offset = o.short_name_offset;
+  r.short_name_size = o.short_name_size;
+  r.kind = o.kind;
+  r.parent_kind = o.parent_kind;
+  r.storage = o.storage;
+  return r;
+}
+
+QueryType::Def Convert(const IndexType::Def &o) {
+  QueryType::Def r;
+  r.detailed_name = o.detailed_name;
+  r.hover = o.hover;
+  r.comments = o.comments;
+  r.spell = o.spell;
+  r.bases = Convert(o.bases);
+  r.funcs = Convert(o.funcs);
+  r.types = Convert(o.types);
+  r.vars = Convert(o.vars);
+  r.alias_of = o.alias_of;
+  // no file_id
+  r.qual_name_offset = o.qual_name_offset;
+  r.short_name_offset = o.short_name_offset;
+  r.short_name_size = o.short_name_size;
+  r.kind = o.kind;
+  r.parent_kind = o.parent_kind;
+  return r;
+}
+
 IndexUpdate IndexUpdate::CreateDelta(IndexFile *previous, IndexFile *current) {
   IndexUpdate r;
   static IndexFile empty(current->path, "<empty>");
@@ -94,7 +139,7 @@ IndexUpdate IndexUpdate::CreateDelta(IndexFile *previous, IndexFile *current) {
   for (auto &it : previous->usr2func) {
     auto &func = it.second;
     if (func.def.detailed_name[0])
-      r.funcs_removed.emplace_back(func.usr, func.def);
+      r.funcs_removed.emplace_back(func.usr, Convert(func.def));
     r.funcs_declarations[func.usr].first = std::move(func.declarations);
     r.funcs_uses[func.usr].first = std::move(func.uses);
     r.funcs_derived[func.usr].first = std::move(func.derived);
@@ -102,7 +147,7 @@ IndexUpdate IndexUpdate::CreateDelta(IndexFile *previous, IndexFile *current) {
   for (auto &it : current->usr2func) {
     auto &func = it.second;
     if (func.def.detailed_name[0])
-      r.funcs_def_update.emplace_back(it.first, func.def);
+      r.funcs_def_update.emplace_back(it.first, Convert(func.def));
     r.funcs_declarations[func.usr].second = std::move(func.declarations);
     r.funcs_uses[func.usr].second = std::move(func.uses);
     r.funcs_derived[func.usr].second = std::move(func.derived);
@@ -112,7 +157,7 @@ IndexUpdate IndexUpdate::CreateDelta(IndexFile *previous, IndexFile *current) {
   for (auto &it : previous->usr2type) {
     auto &type = it.second;
     if (type.def.detailed_name[0])
-      r.types_removed.emplace_back(type.usr, type.def);
+      r.types_removed.emplace_back(type.usr, Convert(type.def));
     r.types_declarations[type.usr].first = std::move(type.declarations);
     r.types_uses[type.usr].first = std::move(type.uses);
     r.types_derived[type.usr].first = std::move(type.derived);
@@ -121,7 +166,7 @@ IndexUpdate IndexUpdate::CreateDelta(IndexFile *previous, IndexFile *current) {
   for (auto &it : current->usr2type) {
     auto &type = it.second;
     if (type.def.detailed_name[0])
-      r.types_def_update.emplace_back(it.first, type.def);
+      r.types_def_update.emplace_back(it.first, Convert(type.def));
     r.types_declarations[type.usr].second = std::move(type.declarations);
     r.types_uses[type.usr].second = std::move(type.uses);
     r.types_derived[type.usr].second = std::move(type.derived);
@@ -214,8 +259,10 @@ void DB::ApplyIndexUpdate(IndexUpdate *u) {
 #define REMOVE_ADD(C, F)                                                       \
   for (auto &it : u->C##s_##F) {                                               \
     auto R = C##_usr.try_emplace({it.first}, C##_usr.size());                  \
-    if (R.second)                                                              \
-      C##s.emplace_back().usr = it.first;                                      \
+    if (R.second) {                                                            \
+      C##s.emplace_back();                                                     \
+      C##s.back().usr = it.first;                                              \
+    }                                                                          \
     auto &entity = C##s[R.first->second];                                      \
     RemoveRange(entity.F, it.second.first);                                    \
     AddRange(entity.F, it.second.second);                                      \
@@ -262,8 +309,10 @@ void DB::ApplyIndexUpdate(IndexUpdate *u) {
           llvm::DenseMap<Usr, int, DenseMapInfoForUsr> &entity_usr,
           auto &entities, auto &p, bool hint_implicit) {
         auto R = entity_usr.try_emplace(usr, entity_usr.size());
-        if (R.second)
-          entities.emplace_back().usr = usr;
+        if (R.second) {
+          entities.emplace_back();
+          entities.back().usr = usr;
+        }
         auto &entity = entities[R.first->second];
         for (Use &use : p.first) {
           if (hint_implicit && use.role & Role::Implicit) {
@@ -497,10 +546,10 @@ int ComputeRangeSize(const Range &range) {
   return range.end.column - range.start.column;
 }
 
-template <typename Q>
+template <typename Q, typename C>
 std::vector<Use>
 GetDeclarations(llvm::DenseMap<Usr, int, DenseMapInfoForUsr> &entity_usr,
-                std::vector<Q> &entities, const std::vector<Usr> &usrs) {
+                llvm::SmallVectorImpl<Q> &entities, const C &usrs) {
   std::vector<Use> ret;
   ret.reserve(usrs.size());
   for (Usr usr : usrs) {
@@ -528,6 +577,9 @@ Maybe<DeclRef> GetDefinitionSpell(DB *db, SymbolIdx sym) {
 std::vector<Use> GetFuncDeclarations(DB *db, const std::vector<Usr> &usrs) {
   return GetDeclarations(db->func_usr, db->funcs, usrs);
 }
+std::vector<Use> GetFuncDeclarations(DB *db, const Vec<Usr> &usrs) {
+  return GetDeclarations(db->func_usr, db->funcs, usrs);
+}
 std::vector<Use> GetTypeDeclarations(DB *db, const std::vector<Usr> &usrs) {
   return GetDeclarations(db->type_usr, db->types, usrs);
 }
diff --git a/src/query.hh b/src/query.hh
index 92befe88..25667521 100644
--- a/src/query.hh
+++ b/src/query.hh
@@ -77,7 +77,7 @@ template <typename T>
 using Update =
     std::unordered_map<Usr, std::pair<std::vector<T>, std::vector<T>>>;
 
-struct QueryFunc : QueryEntity<QueryFunc, FuncDef> {
+struct QueryFunc : QueryEntity<QueryFunc, FuncDef<Vec>> {
   Usr usr;
   llvm::SmallVector<Def, 1> def;
   std::vector<DeclRef> declarations;
@@ -85,7 +85,7 @@ struct QueryFunc : QueryEntity<QueryFunc, FuncDef> {
   std::vector<Use> uses;
 };
 
-struct QueryType : QueryEntity<QueryType, TypeDef> {
+struct QueryType : QueryEntity<QueryType, TypeDef<Vec>> {
   Usr usr;
   llvm::SmallVector<Def, 1> def;
   std::vector<DeclRef> declarations;
@@ -158,9 +158,9 @@ struct DB {
   std::vector<QueryFile> files;
   llvm::StringMap<int> name2file_id;
   llvm::DenseMap<Usr, int, DenseMapInfoForUsr> func_usr, type_usr, var_usr;
-  std::vector<QueryFunc> funcs;
-  std::vector<QueryType> types;
-  std::vector<QueryVar> vars;
+  llvm::SmallVector<QueryFunc, 0> funcs;
+  llvm::SmallVector<QueryType, 0> types;
+  llvm::SmallVector<QueryVar, 0> vars;
 
   void clear();
 
@@ -199,6 +199,7 @@ Maybe<DeclRef> GetDefinitionSpell(DB *db, SymbolIdx sym);
 // Get defining declaration (if exists) or an arbitrary declaration (otherwise)
 // for each id.
 std::vector<Use> GetFuncDeclarations(DB *, const std::vector<Usr> &);
+std::vector<Use> GetFuncDeclarations(DB *, const Vec<Usr> &);
 std::vector<Use> GetTypeDeclarations(DB *, const std::vector<Usr> &);
 std::vector<DeclRef> GetVarDeclarations(DB *, const std::vector<Usr> &, unsigned);
 
@@ -268,8 +269,8 @@ void EachOccurrence(DB *db, SymbolIdx sym, bool include_decl, Fn &&fn) {
 
 SymbolKind GetSymbolKind(DB *db, SymbolIdx sym);
 
-template <typename Fn>
-void EachDefinedFunc(DB *db, const std::vector<Usr> &usrs, Fn &&fn) {
+template <typename C, typename Fn>
+void EachDefinedFunc(DB *db, const C &usrs, Fn &&fn) {
   for (Usr usr : usrs) {
     auto &obj = db->Func(usr);
     if (!obj.def.empty())
diff --git a/src/utils.hh b/src/utils.hh
index 6485d44a..7834837e 100644
--- a/src/utils.hh
+++ b/src/utils.hh
@@ -140,4 +140,27 @@ public:
   bool operator==(const Maybe &o) const { return storage == o.storage; }
   bool operator!=(const Maybe &o) const { return !(*this == o); }
 };
+
+template <typename T> struct Vec {
+  std::unique_ptr<T[]> a;
+  int s = 0;
+#if !(__clang__ || __GNUC__ > 7 || __GNUC__ == 7 && __GNUC_MINOR__ >= 4)
+  // Work around a bug in GCC<7.4 that optional<IndexUpdate> would not be
+  // construtible.
+  Vec() = default;
+  Vec(const Vec &o) : a(std::make_unique<T[]>(o.s)), s(o.s) {
+    std::copy(o.a.get(), o.a.get() + o.s, a.get());
+  }
+  Vec(Vec &&) = default;
+  Vec &operator=(Vec &&) = default;
+  Vec(std::unique_ptr<T[]> a, int s) : a(std::move(a)), s(s) {}
+#endif
+  const T *begin() const { return a.get(); }
+  T *begin() { return a.get(); }
+  const T *end() const { return a.get() + s; }
+  T *end() { return a.get() + s; }
+  int size() const { return s; }
+  const T &operator[](size_t i) const { return a.get()[i]; }
+  T &operator[](size_t i) { return a.get()[i]; }
+};
 } // namespace ccls