Avoid of a closure allocation in case node mapping already contains a clone

Author: AlexUstinov

Orm/Xtensive.Orm/Sql/Dml/SqlTableRef.cs

@@ -35,16 +35,19 @@ internal override object Clone(SqlNodeCloneContext context)
         return context.NodeMapping[this];
       }
 
-      {
-        var clone = new SqlTableRef {Name = Name, DataTable = DataTable};
-        context.NodeMapping[this] = clone;
-        var columnClones = new List<SqlTableColumn>(columns.Count);
-        columnClones.AddRange(columns.Select(column => (SqlTableColumn) column.Clone(context)));
+      return CreateClone(context);
+    }
+
+    private SqlTableRef CreateClone(SqlNodeCloneContext context)
+    {
+      var clone = new SqlTableRef {Name = Name, DataTable = DataTable};
+      context.NodeMapping[this] = clone;
+      var columnClones = new List<SqlTableColumn>(columns.Count);
+      columnClones.AddRange(columns.Select(column => (SqlTableColumn) column.Clone(context)));
 
-        clone.columns = new SqlTableColumnCollection(columnClones);
+      clone.columns = new SqlTableColumnCollection(columnClones);
 
-        return clone;
-      }
+      return clone;
     }
 
     public override void AcceptVisitor(ISqlVisitor visitor) => visitor.Visit(this);