Update join relationship of orders table to selectin

closes #6791 by slightly improving performance in this area.

freqtrade/persistence/trade_model.py

@@ -1144,7 +1144,8 @@ class Trade(_DECL_BASE, LocalTrade):
 
     id = Column(Integer, primary_key=True)
 
-    orders = relationship("Order", order_by="Order.id", cascade="all, delete-orphan", lazy="joined")
+    orders = relationship("Order", order_by="Order.id", cascade="all, delete-orphan",
+                          lazy="selectin", innerjoin=True)
 
     exchange = Column(String(25), nullable=False)
     pair = Column(String(25), nullable=False, index=True)

tests/persistence/test_persistence.py

@@ -2197,15 +2197,16 @@ def test_get_trades__query(fee, is_short):
     # without orders there should be no join issued.
     query1 = Trade.get_trades([], include_orders=False)
 
-    assert "JOIN orders" in str(query)
-    assert "JOIN orders" not in str(query1)
+    # Empty "with-options -> default - selectin"
+    assert query._with_options == ()
+    assert query1._with_options != ()
 
     create_mock_trades(fee, is_short)
     query = Trade.get_trades([])
     query1 = Trade.get_trades([], include_orders=False)
 
-    assert "JOIN orders" in str(query)
-    assert "JOIN orders" not in str(query1)
+    assert query._with_options == ()
+    assert query1._with_options != ()
 
 
 def test_get_trades_backtest():