[HUDI-223] Adding a way to infer target schema from the dataset after the transformation (#854)

- Adding a way to decouple target and source schema providers
- Adding flattening transformer

hudi-utilities/src/test/java/org/apache/hudi/utilities/TestFlatteningTransformer.java

@@ -0,0 +1,56 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.hudi.utilities;
+
+import static org.junit.Assert.assertEquals;
+
+import org.apache.hudi.utilities.transform.FlatteningTransformer;
+import org.apache.spark.sql.types.DataTypes;
+import org.apache.spark.sql.types.Metadata;
+import org.apache.spark.sql.types.StructField;
+import org.apache.spark.sql.types.StructType;
+import org.junit.Test;
+
+public class TestFlatteningTransformer {
+
+  @Test
+  public void testFlatten() {
+    FlatteningTransformer transformer = new FlatteningTransformer();
+
+    // Init
+    StructField[] nestedStructFields = new StructField[]{
+        new StructField("nestedIntColumn", DataTypes.IntegerType, true, Metadata.empty()),
+        new StructField("nestedStringColumn", DataTypes.StringType, true, Metadata.empty()),
+    };
+
+    StructField[] structFields = new StructField[]{
+        new StructField("intColumn", DataTypes.IntegerType, true, Metadata.empty()),
+        new StructField("stringColumn", DataTypes.StringType, true, Metadata.empty()),
+        new StructField("nestedStruct", DataTypes.createStructType(nestedStructFields), true, Metadata.empty())
+    };
+
+    StructType schema = new StructType(structFields);
+    String flattenedSql = transformer.flattenSchema(schema, null);
+
+    assertEquals("intColumn as intColumn,stringColumn as stringColumn,"
+            + "nestedStruct.nestedIntColumn as nestedStruct_nestedIntColumn,"
+            + "nestedStruct.nestedStringColumn as nestedStruct_nestedStringColumn",
+        flattenedSql);
+  }
+}