diff --git a/processing/src/main/java/io/druid/query/UnionDataSource.java b/processing/src/main/java/io/druid/query/UnionDataSource.java
index 83b9f0acbe7..003ce86be47 100644
--- a/processing/src/main/java/io/druid/query/UnionDataSource.java
+++ b/processing/src/main/java/io/druid/query/UnionDataSource.java
@@ -101,4 +101,12 @@ public class UnionDataSource implements DataSource
   {
     return dataSources.hashCode();
   }
+
+  @Override
+  public String toString()
+  {
+    return "UnionDataSource{" +
+           "dataSources=" + dataSources +
+           '}';
+  }
 }