avoid creating a set if no where values are removed

activerecord/lib/active_record/relation/merger.rb

@@ -131,6 +131,8 @@ def merge_single_values
       end
 
       def filter_binds(lhs_binds, removed_wheres)
+        return lhs_binds if removed_wheres.empty?
+
         set = Set.new removed_wheres.map { |x| x.left.name }
         lhs_binds.dup.delete_if { |col,_| set.include? col.name }
       end

activerecord/test/cases/relations_test.rb

@@ -1556,4 +1556,16 @@ def test_merging_removes_rhs_bind_parameters
     merged = left.merge(right)
     assert_equal [], merged.bind_values
   end
+
+  def test_merging_keeps_lhs_bind_parameters
+    column = Post.columns_hash['id']
+    binds = [[column, 20]]
+
+    right  = Post.where(id: Arel::Nodes::BindParam.new('?'))
+    right.bind_values += binds
+    left   = Post.where(id: 10)
+
+    merged = left.merge(right)
+    assert_equal binds, merged.bind_values
+  end
 end