diff --git a/src/engine/enginequery.h b/src/engine/enginequery.h
--- a/src/engine/enginequery.h
+++ b/src/engine/enginequery.h
@@ -43,7 +43,7 @@
     EngineQuery();
     EngineQuery(const QByteArray& term, int pos = 0);
     EngineQuery(const QByteArray& term, Operation op, int pos = 0);
-    EngineQuery(const QVector<EngineQuery> subQueries, Operation op);
+    EngineQuery(const QVector<EngineQuery> &subQueries, Operation op);
 
     QByteArray term() const {
         return m_term;
diff --git a/src/engine/enginequery.cpp b/src/engine/enginequery.cpp
--- a/src/engine/enginequery.cpp
+++ b/src/engine/enginequery.cpp
@@ -42,7 +42,7 @@
 {
 }
 
-EngineQuery::EngineQuery(const QVector<EngineQuery> subQueries, Operation op)
+EngineQuery::EngineQuery(const QVector<EngineQuery> &subQueries, Operation op)
     : m_pos(0)
     , m_op(op)
     , m_subQueries(subQueries)
diff --git a/src/engine/idtreedb.h b/src/engine/idtreedb.h
--- a/src/engine/idtreedb.h
+++ b/src/engine/idtreedb.h
@@ -38,7 +38,7 @@
     static MDB_dbi create(MDB_txn* txn);
     static MDB_dbi open(MDB_txn* txn);
 
-    void put(quint64 docId, const QVector<quint64> subDocIds);
+    void put(quint64 docId, const QVector<quint64> &subDocIds);
     QVector<quint64> get(quint64 docId);
     void del(quint64 docId);
 
diff --git a/src/engine/idtreedb.cpp b/src/engine/idtreedb.cpp
--- a/src/engine/idtreedb.cpp
+++ b/src/engine/idtreedb.cpp
@@ -58,7 +58,7 @@
     return dbi;
 }
 
-void IdTreeDB::put(quint64 docId, const QVector<quint64> subDocIds)
+void IdTreeDB::put(quint64 docId, const QVector<quint64> &subDocIds)
 {
     Q_ASSERT(!subDocIds.isEmpty());
     Q_ASSERT(!subDocIds.contains(0));