[GitHub] [incubator-pinot] Jackie-Jiang commented on a change in pull request #6845: Add the complex data type transformer for complex type handling

[GitBox <gi...@apache.org>]

Jackie-Jiang commented on a change in pull request #6845:
URL: https://github.com/apache/incubator-pinot/pull/6845#discussion_r627811328



##########
File path: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/recordtransformer/ComplexTypeTransformer.java
##########
@@ -0,0 +1,229 @@
+/**
+ * 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.pinot.segment.local.recordtransformer;
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import javax.annotation.Nullable;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.ingestion.ComplexTypeConfig;
+import org.apache.pinot.spi.data.readers.GenericRow;
+
+
+/**
+ * A transformer to handle the complex types such as Map and Collection, with flattening and unnesting.
+ * <p>
+ * The map flattening rule will recursively flatten all the map types, except for those under the collection that is not marked as to unnest.
+ *
+ * For example:
+ * <pre>
+ * {
+ *    "t1":{
+ *       "array":[
+ *          {
+ *             "t2":{
+ *                "a":"v1"
+ *             }
+ *          }
+ *       ]
+ *    }
+ * }
+ * </pre>
+ *
+ * flattens to
+ * <pre>
+ * {
+ *    "t1.array":[
+ *       {
+ *          "t2.a":"v1"
+ *       }
+ *    ]
+ * }
+ * <pre/>
+ *
+ * <p>
+ *
+ * The unnesting rule will flatten all the collections provided, which are the paths navigating to the collections. For
+ * the same example above. If the the collectionToUnnest is provided as "t1.array", then the rule will unnest the
+ * previous output to:
+ *
+ * <pre>
+ *  [{
+ *     "t1.arrayt2.a": "v1",
+ *  }]
+ *  * <pre/>
+ *
+ *  Note the unnest rule will output a collection of generic rows under the field {@link GenericRow#MULTIPLE_RECORDS_KEY}.
+ *
+ */
+public class ComplexTypeTransformer implements RecordTransformer {
+  private static final CharSequence DELIMITER = ".";
+  private final List<String> _collectionsToUnnest;
+
+  public ComplexTypeTransformer(TableConfig tableConfig) {

Review comment:
       Here we should collect columns from the schema and the transform and filter function input, then we know to which level we want to flatten the object. The unnest fields should be either one of these columns or the prefix of these columns.
   The current PR assume we always want to flatten to the leave node, which might not be enough for certain use cases. We can address it in a separate PR, but please add some TODOs in the javadoc.

##########
File path: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/recordtransformer/ComplexTypeTransformer.java
##########
@@ -0,0 +1,229 @@
+/**
+ * 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.pinot.segment.local.recordtransformer;
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import javax.annotation.Nullable;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.ingestion.ComplexTypeConfig;
+import org.apache.pinot.spi.data.readers.GenericRow;
+
+
+/**
+ * A transformer to handle the complex types such as Map and Collection, with flattening and unnesting.
+ * <p>
+ * The map flattening rule will recursively flatten all the map types, except for those under the collection that is not marked as to unnest.
+ *
+ * For example:
+ * <pre>
+ * {
+ *    "t1":{
+ *       "array":[
+ *          {
+ *             "t2":{
+ *                "a":"v1"
+ *             }
+ *          }
+ *       ]
+ *    }
+ * }
+ * </pre>
+ *
+ * flattens to
+ * <pre>
+ * {
+ *    "t1.array":[
+ *       {
+ *          "t2.a":"v1"
+ *       }
+ *    ]
+ * }
+ * <pre/>
+ *
+ * <p>
+ *
+ * The unnesting rule will flatten all the collections provided, which are the paths navigating to the collections. For
+ * the same example above. If the the collectionToUnnest is provided as "t1.array", then the rule will unnest the
+ * previous output to:
+ *
+ * <pre>
+ *  [{
+ *     "t1.arrayt2.a": "v1",
+ *  }]
+ *  * <pre/>
+ *
+ *  Note the unnest rule will output a collection of generic rows under the field {@link GenericRow#MULTIPLE_RECORDS_KEY}.
+ *
+ */
+public class ComplexTypeTransformer implements RecordTransformer {
+  private static final CharSequence DELIMITER = ".";
+  private final List<String> _collectionsToUnnest;
+
+  public ComplexTypeTransformer(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() != null
+        && tableConfig.getIngestionConfig().getComplexTypeConfig() != null) {
+      _collectionsToUnnest =
+          tableConfig.getIngestionConfig().getComplexTypeConfig().getUnnestConfig() != null ? tableConfig
+              .getIngestionConfig().getComplexTypeConfig().getUnnestConfig() : new ArrayList<>();
+    } else {
+      _collectionsToUnnest = new ArrayList<>();
+    }
+  }
+
+  @VisibleForTesting
+  public ComplexTypeTransformer(List<String> unnestCollections) {
+    _collectionsToUnnest = new ArrayList<>(unnestCollections);
+    Collections.sort(_collectionsToUnnest);
+  }
+
+  public static boolean isComplexTypeHandlingEnabled(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig().getMode() == null) {
+      return false;
+    }
+    return tableConfig.getIngestionConfig().getComplexTypeConfig().getMode()
+        != ComplexTypeConfig.Mode.NONE;
+  }
+
+  @Nullable
+  @Override
+  public GenericRow transform(GenericRow record) {
+    flattenMap(record, new HashSet<>(record.getFieldToValueMap().keySet()));
+    for (String collection : _collectionsToUnnest) {
+      unnestCollection(record, collection);
+    }
+    return record;
+  }
+
+  private GenericRow unnestCollection(GenericRow record, String column) {
+    if (record.getValue(GenericRow.MULTIPLE_RECORDS_KEY) == null) {
+      List<GenericRow> list = new ArrayList<>();
+      unnestCollection(record, column, list);
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    } else {
+      Collection<GenericRow> records = (Collection) record.getValue(GenericRow.MULTIPLE_RECORDS_KEY);
+      List<GenericRow> list = new ArrayList<>();
+      for (GenericRow innerRecord : records) {
+        unnestCollection(innerRecord, column, list);
+      }
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    }
+    return record;
+  }
+
+  private void unnestCollection(GenericRow record, String column, List<GenericRow> list) {
+    Object value = record.removeValue(column);
+    if (value == null) {
+      // use the record itself
+      list.add(record);
+    } else if (value instanceof Collection) {
+      if (((Collection) value).isEmpty()) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Collection) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    } else if (value.getClass().isArray()) {
+      if (((Object[]) value).length == 0) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Object[]) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    }
+  }
+
+  private GenericRow flattenCollectionItem(GenericRow record, Object obj, String column) {
+    GenericRow copy = record.copy();
+    if (obj instanceof Map) {
+      Map<String, Object> map = (Map<String, Object>) obj;
+      for (Map.Entry<String, Object> entry : new HashSet<>(map.entrySet())) {
+        String flattenName = concat(column, entry.getKey());
+        copy.putValue(flattenName, entry.getValue());
+      }
+    } else {
+      copy.putValue(column, obj);
+    }
+    return copy;
+  }
+
+  @VisibleForTesting
+  protected GenericRow flattenMap(GenericRow record, Collection<String> columns) {

Review comment:
       Add some javadoc?

##########
File path: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/recordtransformer/ComplexTypeTransformer.java
##########
@@ -0,0 +1,229 @@
+/**
+ * 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.pinot.segment.local.recordtransformer;
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import javax.annotation.Nullable;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.ingestion.ComplexTypeConfig;
+import org.apache.pinot.spi.data.readers.GenericRow;
+
+
+/**
+ * A transformer to handle the complex types such as Map and Collection, with flattening and unnesting.
+ * <p>
+ * The map flattening rule will recursively flatten all the map types, except for those under the collection that is not marked as to unnest.
+ *
+ * For example:
+ * <pre>
+ * {
+ *    "t1":{
+ *       "array":[
+ *          {
+ *             "t2":{
+ *                "a":"v1"
+ *             }
+ *          }
+ *       ]
+ *    }
+ * }
+ * </pre>
+ *
+ * flattens to
+ * <pre>
+ * {
+ *    "t1.array":[
+ *       {
+ *          "t2.a":"v1"
+ *       }
+ *    ]
+ * }
+ * <pre/>
+ *
+ * <p>
+ *
+ * The unnesting rule will flatten all the collections provided, which are the paths navigating to the collections. For
+ * the same example above. If the the collectionToUnnest is provided as "t1.array", then the rule will unnest the
+ * previous output to:
+ *
+ * <pre>
+ *  [{
+ *     "t1.arrayt2.a": "v1",
+ *  }]
+ *  * <pre/>
+ *
+ *  Note the unnest rule will output a collection of generic rows under the field {@link GenericRow#MULTIPLE_RECORDS_KEY}.
+ *
+ */
+public class ComplexTypeTransformer implements RecordTransformer {
+  private static final CharSequence DELIMITER = ".";
+  private final List<String> _collectionsToUnnest;

Review comment:
       `_unnestFields`?

##########
File path: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/recordtransformer/ComplexTypeTransformer.java
##########
@@ -0,0 +1,229 @@
+/**
+ * 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.pinot.segment.local.recordtransformer;
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import javax.annotation.Nullable;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.ingestion.ComplexTypeConfig;
+import org.apache.pinot.spi.data.readers.GenericRow;
+
+
+/**
+ * A transformer to handle the complex types such as Map and Collection, with flattening and unnesting.
+ * <p>
+ * The map flattening rule will recursively flatten all the map types, except for those under the collection that is not marked as to unnest.
+ *
+ * For example:
+ * <pre>
+ * {
+ *    "t1":{
+ *       "array":[
+ *          {
+ *             "t2":{
+ *                "a":"v1"
+ *             }
+ *          }
+ *       ]
+ *    }
+ * }
+ * </pre>
+ *
+ * flattens to
+ * <pre>
+ * {
+ *    "t1.array":[
+ *       {
+ *          "t2.a":"v1"
+ *       }
+ *    ]
+ * }
+ * <pre/>
+ *
+ * <p>
+ *
+ * The unnesting rule will flatten all the collections provided, which are the paths navigating to the collections. For
+ * the same example above. If the the collectionToUnnest is provided as "t1.array", then the rule will unnest the
+ * previous output to:
+ *
+ * <pre>
+ *  [{
+ *     "t1.arrayt2.a": "v1",
+ *  }]
+ *  * <pre/>
+ *
+ *  Note the unnest rule will output a collection of generic rows under the field {@link GenericRow#MULTIPLE_RECORDS_KEY}.
+ *
+ */
+public class ComplexTypeTransformer implements RecordTransformer {
+  private static final CharSequence DELIMITER = ".";
+  private final List<String> _collectionsToUnnest;
+
+  public ComplexTypeTransformer(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() != null
+        && tableConfig.getIngestionConfig().getComplexTypeConfig() != null) {
+      _collectionsToUnnest =
+          tableConfig.getIngestionConfig().getComplexTypeConfig().getUnnestConfig() != null ? tableConfig
+              .getIngestionConfig().getComplexTypeConfig().getUnnestConfig() : new ArrayList<>();
+    } else {
+      _collectionsToUnnest = new ArrayList<>();
+    }
+  }
+
+  @VisibleForTesting
+  public ComplexTypeTransformer(List<String> unnestCollections) {
+    _collectionsToUnnest = new ArrayList<>(unnestCollections);
+    Collections.sort(_collectionsToUnnest);
+  }
+
+  public static boolean isComplexTypeHandlingEnabled(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig().getMode() == null) {
+      return false;
+    }
+    return tableConfig.getIngestionConfig().getComplexTypeConfig().getMode()
+        != ComplexTypeConfig.Mode.NONE;
+  }
+
+  @Nullable
+  @Override
+  public GenericRow transform(GenericRow record) {
+    flattenMap(record, new HashSet<>(record.getFieldToValueMap().keySet()));
+    for (String collection : _collectionsToUnnest) {
+      unnestCollection(record, collection);
+    }
+    return record;
+  }
+
+  private GenericRow unnestCollection(GenericRow record, String column) {
+    if (record.getValue(GenericRow.MULTIPLE_RECORDS_KEY) == null) {
+      List<GenericRow> list = new ArrayList<>();
+      unnestCollection(record, column, list);
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    } else {
+      Collection<GenericRow> records = (Collection) record.getValue(GenericRow.MULTIPLE_RECORDS_KEY);
+      List<GenericRow> list = new ArrayList<>();
+      for (GenericRow innerRecord : records) {
+        unnestCollection(innerRecord, column, list);
+      }
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    }
+    return record;
+  }
+
+  private void unnestCollection(GenericRow record, String column, List<GenericRow> list) {
+    Object value = record.removeValue(column);
+    if (value == null) {
+      // use the record itself
+      list.add(record);
+    } else if (value instanceof Collection) {
+      if (((Collection) value).isEmpty()) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Collection) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    } else if (value.getClass().isArray()) {
+      if (((Object[]) value).length == 0) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Object[]) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    }
+  }
+
+  private GenericRow flattenCollectionItem(GenericRow record, Object obj, String column) {
+    GenericRow copy = record.copy();
+    if (obj instanceof Map) {
+      Map<String, Object> map = (Map<String, Object>) obj;
+      for (Map.Entry<String, Object> entry : new HashSet<>(map.entrySet())) {
+        String flattenName = concat(column, entry.getKey());
+        copy.putValue(flattenName, entry.getValue());
+      }
+    } else {
+      copy.putValue(column, obj);
+    }
+    return copy;
+  }
+
+  @VisibleForTesting
+  protected GenericRow flattenMap(GenericRow record, Collection<String> columns) {
+    for (String column : columns) {
+      if (record.getValue(column) instanceof Map) {
+        Map<String, Object> map = (Map) record.removeValue(column);
+        List<String> mapColumns = new ArrayList<>();
+        for (Map.Entry<String, Object> entry : new HashSet<>(map.entrySet())) {
+          String flattenName = concat(column, entry.getKey());
+          record.putValue(flattenName, entry.getValue());
+          mapColumns.add(flattenName);
+        }
+        record = flattenMap(record, mapColumns);
+      } else if (record.getValue(column) instanceof Collection && _collectionsToUnnest.contains(column)) {
+        for (Object inner : (Collection) record.getValue(column)) {
+          if (inner instanceof Map) {

Review comment:
       How about nested array? If not going to address it in this PR, add a TODO here

##########
File path: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/recordtransformer/ComplexTypeTransformer.java
##########
@@ -0,0 +1,229 @@
+/**
+ * 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.pinot.segment.local.recordtransformer;
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import javax.annotation.Nullable;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.ingestion.ComplexTypeConfig;
+import org.apache.pinot.spi.data.readers.GenericRow;
+
+
+/**
+ * A transformer to handle the complex types such as Map and Collection, with flattening and unnesting.
+ * <p>
+ * The map flattening rule will recursively flatten all the map types, except for those under the collection that is not marked as to unnest.
+ *
+ * For example:
+ * <pre>
+ * {
+ *    "t1":{
+ *       "array":[
+ *          {
+ *             "t2":{
+ *                "a":"v1"
+ *             }
+ *          }
+ *       ]
+ *    }
+ * }
+ * </pre>
+ *
+ * flattens to
+ * <pre>
+ * {
+ *    "t1.array":[
+ *       {
+ *          "t2.a":"v1"
+ *       }
+ *    ]
+ * }
+ * <pre/>
+ *
+ * <p>
+ *
+ * The unnesting rule will flatten all the collections provided, which are the paths navigating to the collections. For
+ * the same example above. If the the collectionToUnnest is provided as "t1.array", then the rule will unnest the
+ * previous output to:
+ *
+ * <pre>
+ *  [{
+ *     "t1.arrayt2.a": "v1",
+ *  }]
+ *  * <pre/>
+ *
+ *  Note the unnest rule will output a collection of generic rows under the field {@link GenericRow#MULTIPLE_RECORDS_KEY}.
+ *
+ */
+public class ComplexTypeTransformer implements RecordTransformer {
+  private static final CharSequence DELIMITER = ".";
+  private final List<String> _collectionsToUnnest;
+
+  public ComplexTypeTransformer(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() != null
+        && tableConfig.getIngestionConfig().getComplexTypeConfig() != null) {
+      _collectionsToUnnest =
+          tableConfig.getIngestionConfig().getComplexTypeConfig().getUnnestConfig() != null ? tableConfig
+              .getIngestionConfig().getComplexTypeConfig().getUnnestConfig() : new ArrayList<>();
+    } else {
+      _collectionsToUnnest = new ArrayList<>();
+    }
+  }
+
+  @VisibleForTesting
+  public ComplexTypeTransformer(List<String> unnestCollections) {
+    _collectionsToUnnest = new ArrayList<>(unnestCollections);
+    Collections.sort(_collectionsToUnnest);
+  }
+
+  public static boolean isComplexTypeHandlingEnabled(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig().getMode() == null) {
+      return false;
+    }
+    return tableConfig.getIngestionConfig().getComplexTypeConfig().getMode()
+        != ComplexTypeConfig.Mode.NONE;
+  }
+
+  @Nullable
+  @Override
+  public GenericRow transform(GenericRow record) {
+    flattenMap(record, new HashSet<>(record.getFieldToValueMap().keySet()));
+    for (String collection : _collectionsToUnnest) {
+      unnestCollection(record, collection);
+    }
+    return record;
+  }
+
+  private GenericRow unnestCollection(GenericRow record, String column) {
+    if (record.getValue(GenericRow.MULTIPLE_RECORDS_KEY) == null) {
+      List<GenericRow> list = new ArrayList<>();
+      unnestCollection(record, column, list);
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    } else {
+      Collection<GenericRow> records = (Collection) record.getValue(GenericRow.MULTIPLE_RECORDS_KEY);
+      List<GenericRow> list = new ArrayList<>();
+      for (GenericRow innerRecord : records) {
+        unnestCollection(innerRecord, column, list);
+      }
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    }
+    return record;
+  }
+
+  private void unnestCollection(GenericRow record, String column, List<GenericRow> list) {
+    Object value = record.removeValue(column);
+    if (value == null) {
+      // use the record itself
+      list.add(record);
+    } else if (value instanceof Collection) {
+      if (((Collection) value).isEmpty()) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Collection) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    } else if (value.getClass().isArray()) {
+      if (((Object[]) value).length == 0) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Object[]) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    }
+  }
+
+  private GenericRow flattenCollectionItem(GenericRow record, Object obj, String column) {
+    GenericRow copy = record.copy();
+    if (obj instanceof Map) {
+      Map<String, Object> map = (Map<String, Object>) obj;
+      for (Map.Entry<String, Object> entry : new HashSet<>(map.entrySet())) {
+        String flattenName = concat(column, entry.getKey());
+        copy.putValue(flattenName, entry.getValue());
+      }
+    } else {
+      copy.putValue(column, obj);
+    }
+    return copy;
+  }
+
+  @VisibleForTesting
+  protected GenericRow flattenMap(GenericRow record, Collection<String> columns) {
+    for (String column : columns) {
+      if (record.getValue(column) instanceof Map) {
+        Map<String, Object> map = (Map) record.removeValue(column);
+        List<String> mapColumns = new ArrayList<>();
+        for (Map.Entry<String, Object> entry : new HashSet<>(map.entrySet())) {
+          String flattenName = concat(column, entry.getKey());
+          record.putValue(flattenName, entry.getValue());
+          mapColumns.add(flattenName);

Review comment:
       Add to the next iteration only when the value is `Collection` or `Map` to avoid redundant map lookups

##########
File path: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/recordtransformer/ComplexTypeTransformer.java
##########
@@ -0,0 +1,229 @@
+/**
+ * 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.pinot.segment.local.recordtransformer;
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import javax.annotation.Nullable;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.ingestion.ComplexTypeConfig;
+import org.apache.pinot.spi.data.readers.GenericRow;
+
+
+/**
+ * A transformer to handle the complex types such as Map and Collection, with flattening and unnesting.
+ * <p>
+ * The map flattening rule will recursively flatten all the map types, except for those under the collection that is not marked as to unnest.
+ *
+ * For example:
+ * <pre>
+ * {
+ *    "t1":{
+ *       "array":[
+ *          {
+ *             "t2":{
+ *                "a":"v1"
+ *             }
+ *          }
+ *       ]
+ *    }
+ * }
+ * </pre>
+ *
+ * flattens to
+ * <pre>
+ * {
+ *    "t1.array":[
+ *       {
+ *          "t2.a":"v1"
+ *       }
+ *    ]
+ * }
+ * <pre/>
+ *
+ * <p>
+ *
+ * The unnesting rule will flatten all the collections provided, which are the paths navigating to the collections. For
+ * the same example above. If the the collectionToUnnest is provided as "t1.array", then the rule will unnest the
+ * previous output to:
+ *
+ * <pre>
+ *  [{
+ *     "t1.arrayt2.a": "v1",
+ *  }]
+ *  * <pre/>
+ *
+ *  Note the unnest rule will output a collection of generic rows under the field {@link GenericRow#MULTIPLE_RECORDS_KEY}.
+ *
+ */
+public class ComplexTypeTransformer implements RecordTransformer {
+  private static final CharSequence DELIMITER = ".";
+  private final List<String> _collectionsToUnnest;
+
+  public ComplexTypeTransformer(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() != null
+        && tableConfig.getIngestionConfig().getComplexTypeConfig() != null) {
+      _collectionsToUnnest =
+          tableConfig.getIngestionConfig().getComplexTypeConfig().getUnnestConfig() != null ? tableConfig
+              .getIngestionConfig().getComplexTypeConfig().getUnnestConfig() : new ArrayList<>();
+    } else {
+      _collectionsToUnnest = new ArrayList<>();
+    }
+  }
+
+  @VisibleForTesting
+  public ComplexTypeTransformer(List<String> unnestCollections) {
+    _collectionsToUnnest = new ArrayList<>(unnestCollections);
+    Collections.sort(_collectionsToUnnest);
+  }
+
+  public static boolean isComplexTypeHandlingEnabled(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig().getMode() == null) {
+      return false;
+    }
+    return tableConfig.getIngestionConfig().getComplexTypeConfig().getMode()
+        != ComplexTypeConfig.Mode.NONE;
+  }
+
+  @Nullable
+  @Override
+  public GenericRow transform(GenericRow record) {
+    flattenMap(record, new HashSet<>(record.getFieldToValueMap().keySet()));
+    for (String collection : _collectionsToUnnest) {
+      unnestCollection(record, collection);
+    }
+    return record;
+  }
+
+  private GenericRow unnestCollection(GenericRow record, String column) {
+    if (record.getValue(GenericRow.MULTIPLE_RECORDS_KEY) == null) {
+      List<GenericRow> list = new ArrayList<>();
+      unnestCollection(record, column, list);
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    } else {
+      Collection<GenericRow> records = (Collection) record.getValue(GenericRow.MULTIPLE_RECORDS_KEY);
+      List<GenericRow> list = new ArrayList<>();
+      for (GenericRow innerRecord : records) {
+        unnestCollection(innerRecord, column, list);
+      }
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    }
+    return record;
+  }
+
+  private void unnestCollection(GenericRow record, String column, List<GenericRow> list) {
+    Object value = record.removeValue(column);
+    if (value == null) {
+      // use the record itself
+      list.add(record);
+    } else if (value instanceof Collection) {
+      if (((Collection) value).isEmpty()) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Collection) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    } else if (value.getClass().isArray()) {
+      if (((Object[]) value).length == 0) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Object[]) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    }
+  }
+
+  private GenericRow flattenCollectionItem(GenericRow record, Object obj, String column) {
+    GenericRow copy = record.copy();
+    if (obj instanceof Map) {
+      Map<String, Object> map = (Map<String, Object>) obj;
+      for (Map.Entry<String, Object> entry : new HashSet<>(map.entrySet())) {

Review comment:
       No need to make a new `HashSet`

##########
File path: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/recordtransformer/ComplexTypeTransformer.java
##########
@@ -0,0 +1,229 @@
+/**
+ * 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.pinot.segment.local.recordtransformer;
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import javax.annotation.Nullable;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.ingestion.ComplexTypeConfig;
+import org.apache.pinot.spi.data.readers.GenericRow;
+
+
+/**
+ * A transformer to handle the complex types such as Map and Collection, with flattening and unnesting.
+ * <p>
+ * The map flattening rule will recursively flatten all the map types, except for those under the collection that is not marked as to unnest.
+ *
+ * For example:
+ * <pre>
+ * {
+ *    "t1":{
+ *       "array":[
+ *          {
+ *             "t2":{
+ *                "a":"v1"
+ *             }
+ *          }
+ *       ]
+ *    }
+ * }
+ * </pre>
+ *
+ * flattens to
+ * <pre>
+ * {
+ *    "t1.array":[
+ *       {
+ *          "t2.a":"v1"
+ *       }
+ *    ]
+ * }
+ * <pre/>
+ *
+ * <p>
+ *
+ * The unnesting rule will flatten all the collections provided, which are the paths navigating to the collections. For
+ * the same example above. If the the collectionToUnnest is provided as "t1.array", then the rule will unnest the
+ * previous output to:
+ *
+ * <pre>
+ *  [{
+ *     "t1.arrayt2.a": "v1",
+ *  }]
+ *  * <pre/>
+ *
+ *  Note the unnest rule will output a collection of generic rows under the field {@link GenericRow#MULTIPLE_RECORDS_KEY}.
+ *
+ */
+public class ComplexTypeTransformer implements RecordTransformer {
+  private static final CharSequence DELIMITER = ".";
+  private final List<String> _collectionsToUnnest;
+
+  public ComplexTypeTransformer(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() != null
+        && tableConfig.getIngestionConfig().getComplexTypeConfig() != null) {
+      _collectionsToUnnest =
+          tableConfig.getIngestionConfig().getComplexTypeConfig().getUnnestConfig() != null ? tableConfig
+              .getIngestionConfig().getComplexTypeConfig().getUnnestConfig() : new ArrayList<>();
+    } else {
+      _collectionsToUnnest = new ArrayList<>();
+    }
+  }
+
+  @VisibleForTesting
+  public ComplexTypeTransformer(List<String> unnestCollections) {
+    _collectionsToUnnest = new ArrayList<>(unnestCollections);
+    Collections.sort(_collectionsToUnnest);
+  }
+
+  public static boolean isComplexTypeHandlingEnabled(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig().getMode() == null) {
+      return false;
+    }
+    return tableConfig.getIngestionConfig().getComplexTypeConfig().getMode()
+        != ComplexTypeConfig.Mode.NONE;
+  }
+
+  @Nullable
+  @Override
+  public GenericRow transform(GenericRow record) {
+    flattenMap(record, new HashSet<>(record.getFieldToValueMap().keySet()));
+    for (String collection : _collectionsToUnnest) {
+      unnestCollection(record, collection);
+    }
+    return record;
+  }
+
+  private GenericRow unnestCollection(GenericRow record, String column) {
+    if (record.getValue(GenericRow.MULTIPLE_RECORDS_KEY) == null) {
+      List<GenericRow> list = new ArrayList<>();
+      unnestCollection(record, column, list);
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    } else {
+      Collection<GenericRow> records = (Collection) record.getValue(GenericRow.MULTIPLE_RECORDS_KEY);
+      List<GenericRow> list = new ArrayList<>();
+      for (GenericRow innerRecord : records) {
+        unnestCollection(innerRecord, column, list);
+      }
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    }
+    return record;
+  }
+
+  private void unnestCollection(GenericRow record, String column, List<GenericRow> list) {
+    Object value = record.removeValue(column);
+    if (value == null) {
+      // use the record itself
+      list.add(record);
+    } else if (value instanceof Collection) {
+      if (((Collection) value).isEmpty()) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Collection) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    } else if (value.getClass().isArray()) {
+      if (((Object[]) value).length == 0) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Object[]) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    }
+  }
+
+  private GenericRow flattenCollectionItem(GenericRow record, Object obj, String column) {
+    GenericRow copy = record.copy();
+    if (obj instanceof Map) {
+      Map<String, Object> map = (Map<String, Object>) obj;
+      for (Map.Entry<String, Object> entry : new HashSet<>(map.entrySet())) {
+        String flattenName = concat(column, entry.getKey());
+        copy.putValue(flattenName, entry.getValue());
+      }
+    } else {
+      copy.putValue(column, obj);
+    }
+    return copy;
+  }
+
+  @VisibleForTesting
+  protected GenericRow flattenMap(GenericRow record, Collection<String> columns) {
+    for (String column : columns) {
+      if (record.getValue(column) instanceof Map) {
+        Map<String, Object> map = (Map) record.removeValue(column);
+        List<String> mapColumns = new ArrayList<>();
+        for (Map.Entry<String, Object> entry : new HashSet<>(map.entrySet())) {
+          String flattenName = concat(column, entry.getKey());
+          record.putValue(flattenName, entry.getValue());
+          mapColumns.add(flattenName);
+        }
+        record = flattenMap(record, mapColumns);
+      } else if (record.getValue(column) instanceof Collection && _collectionsToUnnest.contains(column)) {
+        for (Object inner : (Collection) record.getValue(column)) {
+          if (inner instanceof Map) {
+            Map<String, Object> innerMap = (Map<String, Object>) inner;
+            flattenMap(column, innerMap, new HashSet<>(innerMap.keySet()));
+          }
+        }
+      }
+    }
+    return record;
+  }
+
+  private void flattenMap(String path, Map<String, Object> map, Collection<String> fields) {
+    for (String field : fields) {
+      if (map.get(field) instanceof Map) {

Review comment:
       Cache `map.get(field)`

##########
File path: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/recordtransformer/ComplexTypeTransformer.java
##########
@@ -0,0 +1,184 @@
+/**
+ * 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.pinot.segment.local.recordtransformer;
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import javax.annotation.Nullable;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.ingestion.ComplexTypeHandlingConfig;
+import org.apache.pinot.spi.data.readers.GenericRow;
+
+
+public class ComplexTypeTransformer implements RecordTransformer {
+  private static final CharSequence DELIMITER = ".";
+  private final List<String> _collectionsToUnnest;
+
+  public ComplexTypeTransformer(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() != null
+        && tableConfig.getIngestionConfig().getComplexTypeHandlingConfig() != null) {
+      _collectionsToUnnest =
+          tableConfig.getIngestionConfig().getComplexTypeHandlingConfig().getUnnestConfig() != null ? tableConfig
+              .getIngestionConfig().getComplexTypeHandlingConfig().getUnnestConfig() : new ArrayList<>();
+    } else {
+      _collectionsToUnnest = new ArrayList<>();
+    }
+  }
+
+  @VisibleForTesting
+  public ComplexTypeTransformer(List<String> unnestCollections) {
+    _collectionsToUnnest = new ArrayList<>(unnestCollections);
+    Collections.sort(_collectionsToUnnest);
+  }
+
+  public static boolean isComplexTypeHandlingEnabled(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeHandlingConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeHandlingConfig().getMode() == null) {
+      return false;
+    }
+    return tableConfig.getIngestionConfig().getComplexTypeHandlingConfig().getMode()
+        != ComplexTypeHandlingConfig.Mode.NONE;
+  }
+
+  @Nullable
+  @Override
+  public GenericRow transform(GenericRow record) {
+    flattenMap(record, new HashSet<>(record.getFieldToValueMap().keySet()));
+    for (String collection : _collectionsToUnnest) {
+      unnestCollection(record, collection);
+    }
+    return record;
+  }
+
+  private GenericRow unnestCollection(GenericRow record, String column) {
+    if (record.getValue(GenericRow.MULTIPLE_RECORDS_KEY) == null) {
+      List<GenericRow> list = new ArrayList<>();
+      unnestCollection(record, column, list);
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    } else {
+      Collection<GenericRow> records = (Collection) record.getValue(GenericRow.MULTIPLE_RECORDS_KEY);
+      List<GenericRow> list = new ArrayList<>();
+      for (GenericRow innerRecord : records) {
+        unnestCollection(innerRecord, column, list);
+      }
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    }
+    return record;
+  }
+
+  private void unnestCollection(GenericRow record, String column, List<GenericRow> list) {
+    Object value = record.removeValue(column);
+    if (value == null) {
+      // use the record itself
+      list.add(record);
+      return;
+    } else if (value instanceof Collection) {
+      if (((Collection) value).isEmpty()) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Collection) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    } else if (value.getClass().isArray()) {
+      if (((Object[]) value).length == 0) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Object[]) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    }
+  }
+
+  private GenericRow flattenCollectionItem(GenericRow record, Object obj, String column) {
+    GenericRow copy = record.copy();
+    if (obj instanceof Map) {
+      Map<String, Object> map = (Map<String, Object>) obj;
+      for (Map.Entry<String, Object> entry : new HashSet<>(map.entrySet())) {
+        String flattenName = concat(column, entry.getKey());
+        copy.putValue(flattenName, entry.getValue());
+      }
+    } else {
+      copy.putValue(column, obj);
+    }
+    return copy;
+  }
+
+  @VisibleForTesting
+  protected GenericRow flattenMap(GenericRow record, Collection<String> columns) {

Review comment:
       Do we need to return a `GenericRow` here? It is always in-place modification

##########
File path: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/recordtransformer/ComplexTypeTransformer.java
##########
@@ -0,0 +1,184 @@
+/**
+ * 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.pinot.segment.local.recordtransformer;
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import javax.annotation.Nullable;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.ingestion.ComplexTypeHandlingConfig;
+import org.apache.pinot.spi.data.readers.GenericRow;
+
+
+public class ComplexTypeTransformer implements RecordTransformer {
+  private static final CharSequence DELIMITER = ".";
+  private final List<String> _collectionsToUnnest;
+
+  public ComplexTypeTransformer(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() != null
+        && tableConfig.getIngestionConfig().getComplexTypeHandlingConfig() != null) {
+      _collectionsToUnnest =
+          tableConfig.getIngestionConfig().getComplexTypeHandlingConfig().getUnnestConfig() != null ? tableConfig
+              .getIngestionConfig().getComplexTypeHandlingConfig().getUnnestConfig() : new ArrayList<>();
+    } else {
+      _collectionsToUnnest = new ArrayList<>();
+    }
+  }
+
+  @VisibleForTesting
+  public ComplexTypeTransformer(List<String> unnestCollections) {
+    _collectionsToUnnest = new ArrayList<>(unnestCollections);
+    Collections.sort(_collectionsToUnnest);
+  }
+
+  public static boolean isComplexTypeHandlingEnabled(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeHandlingConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeHandlingConfig().getMode() == null) {
+      return false;
+    }
+    return tableConfig.getIngestionConfig().getComplexTypeHandlingConfig().getMode()
+        != ComplexTypeHandlingConfig.Mode.NONE;
+  }
+
+  @Nullable
+  @Override
+  public GenericRow transform(GenericRow record) {
+    flattenMap(record, new HashSet<>(record.getFieldToValueMap().keySet()));

Review comment:
       Since we are going to remove entry and add new entries at the same time, there are 2 ways: 1. use iterator and only remove entries, keep the new entries in a separate map and merge 2 maps in the end; 2. put the keys in a separate list (faster than set for iteration).
   Since both approaches require extra data structure, I feel it is okay to just cache the keys (suggest changing to list though).

##########
File path: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/recordtransformer/ComplexTypeTransformer.java
##########
@@ -0,0 +1,230 @@
+/**
+ * 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.pinot.segment.local.recordtransformer;
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import javax.annotation.Nullable;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.ingestion.ComplexTypeHandlingConfig;
+import org.apache.pinot.spi.data.readers.GenericRow;
+
+
+/**
+ * A transformer to handle the complex types such as Map and Collection, with flattening and unnesting.
+ * <p>
+ * The map flattening rule will recursively flatten all the map types, except for those under the collection that is not marked as to unnest.
+ *
+ * For example:
+ * <pre>
+ * {
+ *    "t1":{
+ *       "array":[
+ *          {
+ *             "t2":{
+ *                "a":"v1"
+ *             }
+ *          }
+ *       ]
+ *    }
+ * }
+ * </pre>
+ *
+ * flattens to
+ * <pre>
+ * {
+ *    "t1.array":[
+ *       {
+ *          "t2.a":"v1"
+ *       }
+ *    ]
+ * }
+ * <pre/>
+ *
+ * <p>
+ *
+ * The unnesting rule will flatten all the collections provided, which are the paths navigating to the collections. For
+ * the same example above. If the the collectionToUnnest is provided as "t1.array", then the rule will unnest the
+ * previous output to:
+ *
+ * <pre>
+ *  [{
+ *     "t1.arrayt2.a": "v1",
+ *  }]
+ *  * <pre/>
+ *
+ *  Note the unnest rule will output a collection of generic rows under the field {@link GenericRow#MULTIPLE_RECORDS_KEY}.
+ *
+ */
+public class ComplexTypeTransformer implements RecordTransformer {
+  private static final CharSequence DELIMITER = ".";
+  private final List<String> _collectionsToUnnest;
+
+  public ComplexTypeTransformer(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() != null
+        && tableConfig.getIngestionConfig().getComplexTypeHandlingConfig() != null) {
+      _collectionsToUnnest =
+          tableConfig.getIngestionConfig().getComplexTypeHandlingConfig().getUnnestConfig() != null ? tableConfig
+              .getIngestionConfig().getComplexTypeHandlingConfig().getUnnestConfig() : new ArrayList<>();
+    } else {
+      _collectionsToUnnest = new ArrayList<>();
+    }
+  }
+
+  @VisibleForTesting
+  public ComplexTypeTransformer(List<String> unnestCollections) {
+    _collectionsToUnnest = new ArrayList<>(unnestCollections);
+    Collections.sort(_collectionsToUnnest);

Review comment:
       +1. We should probably use a set because we need to do per-record lookups

##########
File path: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/recordtransformer/ComplexTypeTransformer.java
##########
@@ -0,0 +1,229 @@
+/**
+ * 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.pinot.segment.local.recordtransformer;
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import javax.annotation.Nullable;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.ingestion.ComplexTypeConfig;
+import org.apache.pinot.spi.data.readers.GenericRow;
+
+
+/**
+ * A transformer to handle the complex types such as Map and Collection, with flattening and unnesting.
+ * <p>
+ * The map flattening rule will recursively flatten all the map types, except for those under the collection that is not marked as to unnest.
+ *
+ * For example:
+ * <pre>
+ * {
+ *    "t1":{
+ *       "array":[
+ *          {
+ *             "t2":{
+ *                "a":"v1"
+ *             }
+ *          }
+ *       ]
+ *    }
+ * }
+ * </pre>
+ *
+ * flattens to
+ * <pre>
+ * {
+ *    "t1.array":[
+ *       {
+ *          "t2.a":"v1"
+ *       }
+ *    ]
+ * }
+ * <pre/>
+ *
+ * <p>
+ *
+ * The unnesting rule will flatten all the collections provided, which are the paths navigating to the collections. For
+ * the same example above. If the the collectionToUnnest is provided as "t1.array", then the rule will unnest the
+ * previous output to:
+ *
+ * <pre>
+ *  [{
+ *     "t1.arrayt2.a": "v1",
+ *  }]
+ *  * <pre/>
+ *
+ *  Note the unnest rule will output a collection of generic rows under the field {@link GenericRow#MULTIPLE_RECORDS_KEY}.
+ *
+ */
+public class ComplexTypeTransformer implements RecordTransformer {
+  private static final CharSequence DELIMITER = ".";
+  private final List<String> _collectionsToUnnest;
+
+  public ComplexTypeTransformer(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() != null
+        && tableConfig.getIngestionConfig().getComplexTypeConfig() != null) {
+      _collectionsToUnnest =
+          tableConfig.getIngestionConfig().getComplexTypeConfig().getUnnestConfig() != null ? tableConfig
+              .getIngestionConfig().getComplexTypeConfig().getUnnestConfig() : new ArrayList<>();
+    } else {
+      _collectionsToUnnest = new ArrayList<>();
+    }
+  }
+
+  @VisibleForTesting
+  public ComplexTypeTransformer(List<String> unnestCollections) {
+    _collectionsToUnnest = new ArrayList<>(unnestCollections);
+    Collections.sort(_collectionsToUnnest);
+  }
+
+  public static boolean isComplexTypeHandlingEnabled(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig().getMode() == null) {
+      return false;
+    }
+    return tableConfig.getIngestionConfig().getComplexTypeConfig().getMode()
+        != ComplexTypeConfig.Mode.NONE;
+  }
+
+  @Nullable
+  @Override
+  public GenericRow transform(GenericRow record) {
+    flattenMap(record, new HashSet<>(record.getFieldToValueMap().keySet()));
+    for (String collection : _collectionsToUnnest) {
+      unnestCollection(record, collection);
+    }
+    return record;
+  }
+
+  private GenericRow unnestCollection(GenericRow record, String column) {
+    if (record.getValue(GenericRow.MULTIPLE_RECORDS_KEY) == null) {

Review comment:
       Cache `record.getValue(GenericRow.MULTIPLE_RECORDS_KEY)`

##########
File path: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/recordtransformer/ComplexTypeTransformer.java
##########
@@ -0,0 +1,229 @@
+/**
+ * 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.pinot.segment.local.recordtransformer;
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import javax.annotation.Nullable;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.ingestion.ComplexTypeConfig;
+import org.apache.pinot.spi.data.readers.GenericRow;
+
+
+/**
+ * A transformer to handle the complex types such as Map and Collection, with flattening and unnesting.
+ * <p>
+ * The map flattening rule will recursively flatten all the map types, except for those under the collection that is not marked as to unnest.
+ *
+ * For example:
+ * <pre>
+ * {
+ *    "t1":{
+ *       "array":[
+ *          {
+ *             "t2":{
+ *                "a":"v1"
+ *             }
+ *          }
+ *       ]
+ *    }
+ * }
+ * </pre>
+ *
+ * flattens to
+ * <pre>
+ * {
+ *    "t1.array":[
+ *       {
+ *          "t2.a":"v1"
+ *       }
+ *    ]
+ * }
+ * <pre/>
+ *
+ * <p>
+ *
+ * The unnesting rule will flatten all the collections provided, which are the paths navigating to the collections. For
+ * the same example above. If the the collectionToUnnest is provided as "t1.array", then the rule will unnest the
+ * previous output to:
+ *
+ * <pre>
+ *  [{
+ *     "t1.arrayt2.a": "v1",
+ *  }]
+ *  * <pre/>
+ *
+ *  Note the unnest rule will output a collection of generic rows under the field {@link GenericRow#MULTIPLE_RECORDS_KEY}.
+ *
+ */
+public class ComplexTypeTransformer implements RecordTransformer {
+  private static final CharSequence DELIMITER = ".";
+  private final List<String> _collectionsToUnnest;
+
+  public ComplexTypeTransformer(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() != null
+        && tableConfig.getIngestionConfig().getComplexTypeConfig() != null) {
+      _collectionsToUnnest =
+          tableConfig.getIngestionConfig().getComplexTypeConfig().getUnnestConfig() != null ? tableConfig
+              .getIngestionConfig().getComplexTypeConfig().getUnnestConfig() : new ArrayList<>();
+    } else {
+      _collectionsToUnnest = new ArrayList<>();
+    }
+  }
+
+  @VisibleForTesting
+  public ComplexTypeTransformer(List<String> unnestCollections) {
+    _collectionsToUnnest = new ArrayList<>(unnestCollections);
+    Collections.sort(_collectionsToUnnest);
+  }
+
+  public static boolean isComplexTypeHandlingEnabled(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig().getMode() == null) {
+      return false;
+    }
+    return tableConfig.getIngestionConfig().getComplexTypeConfig().getMode()
+        != ComplexTypeConfig.Mode.NONE;
+  }
+
+  @Nullable
+  @Override
+  public GenericRow transform(GenericRow record) {
+    flattenMap(record, new HashSet<>(record.getFieldToValueMap().keySet()));
+    for (String collection : _collectionsToUnnest) {
+      unnestCollection(record, collection);
+    }
+    return record;
+  }
+
+  private GenericRow unnestCollection(GenericRow record, String column) {
+    if (record.getValue(GenericRow.MULTIPLE_RECORDS_KEY) == null) {
+      List<GenericRow> list = new ArrayList<>();
+      unnestCollection(record, column, list);
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    } else {
+      Collection<GenericRow> records = (Collection) record.getValue(GenericRow.MULTIPLE_RECORDS_KEY);
+      List<GenericRow> list = new ArrayList<>();
+      for (GenericRow innerRecord : records) {
+        unnestCollection(innerRecord, column, list);
+      }
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    }
+    return record;
+  }
+
+  private void unnestCollection(GenericRow record, String column, List<GenericRow> list) {
+    Object value = record.removeValue(column);
+    if (value == null) {
+      // use the record itself
+      list.add(record);
+    } else if (value instanceof Collection) {
+      if (((Collection) value).isEmpty()) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Collection) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    } else if (value.getClass().isArray()) {
+      if (((Object[]) value).length == 0) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Object[]) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    }
+  }
+
+  private GenericRow flattenCollectionItem(GenericRow record, Object obj, String column) {
+    GenericRow copy = record.copy();
+    if (obj instanceof Map) {
+      Map<String, Object> map = (Map<String, Object>) obj;
+      for (Map.Entry<String, Object> entry : new HashSet<>(map.entrySet())) {
+        String flattenName = concat(column, entry.getKey());
+        copy.putValue(flattenName, entry.getValue());
+      }
+    } else {
+      copy.putValue(column, obj);
+    }
+    return copy;
+  }
+
+  @VisibleForTesting
+  protected GenericRow flattenMap(GenericRow record, Collection<String> columns) {
+    for (String column : columns) {
+      if (record.getValue(column) instanceof Map) {
+        Map<String, Object> map = (Map) record.removeValue(column);
+        List<String> mapColumns = new ArrayList<>();
+        for (Map.Entry<String, Object> entry : new HashSet<>(map.entrySet())) {
+          String flattenName = concat(column, entry.getKey());
+          record.putValue(flattenName, entry.getValue());
+          mapColumns.add(flattenName);
+        }
+        record = flattenMap(record, mapColumns);
+      } else if (record.getValue(column) instanceof Collection && _collectionsToUnnest.contains(column)) {
+        for (Object inner : (Collection) record.getValue(column)) {
+          if (inner instanceof Map) {
+            Map<String, Object> innerMap = (Map<String, Object>) inner;
+            flattenMap(column, innerMap, new HashSet<>(innerMap.keySet()));

Review comment:
       Use `List` instead of `Set`

##########
File path: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/recordtransformer/ComplexTypeTransformer.java
##########
@@ -0,0 +1,229 @@
+/**
+ * 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.pinot.segment.local.recordtransformer;
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import javax.annotation.Nullable;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.ingestion.ComplexTypeConfig;
+import org.apache.pinot.spi.data.readers.GenericRow;
+
+
+/**
+ * A transformer to handle the complex types such as Map and Collection, with flattening and unnesting.
+ * <p>
+ * The map flattening rule will recursively flatten all the map types, except for those under the collection that is not marked as to unnest.
+ *
+ * For example:
+ * <pre>
+ * {
+ *    "t1":{
+ *       "array":[
+ *          {
+ *             "t2":{
+ *                "a":"v1"
+ *             }
+ *          }
+ *       ]
+ *    }
+ * }
+ * </pre>
+ *
+ * flattens to
+ * <pre>
+ * {
+ *    "t1.array":[
+ *       {
+ *          "t2.a":"v1"
+ *       }
+ *    ]
+ * }
+ * <pre/>
+ *
+ * <p>
+ *
+ * The unnesting rule will flatten all the collections provided, which are the paths navigating to the collections. For
+ * the same example above. If the the collectionToUnnest is provided as "t1.array", then the rule will unnest the
+ * previous output to:
+ *
+ * <pre>
+ *  [{
+ *     "t1.arrayt2.a": "v1",
+ *  }]
+ *  * <pre/>
+ *
+ *  Note the unnest rule will output a collection of generic rows under the field {@link GenericRow#MULTIPLE_RECORDS_KEY}.
+ *
+ */
+public class ComplexTypeTransformer implements RecordTransformer {
+  private static final CharSequence DELIMITER = ".";
+  private final List<String> _collectionsToUnnest;
+
+  public ComplexTypeTransformer(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() != null
+        && tableConfig.getIngestionConfig().getComplexTypeConfig() != null) {
+      _collectionsToUnnest =
+          tableConfig.getIngestionConfig().getComplexTypeConfig().getUnnestConfig() != null ? tableConfig
+              .getIngestionConfig().getComplexTypeConfig().getUnnestConfig() : new ArrayList<>();
+    } else {
+      _collectionsToUnnest = new ArrayList<>();
+    }
+  }
+
+  @VisibleForTesting
+  public ComplexTypeTransformer(List<String> unnestCollections) {
+    _collectionsToUnnest = new ArrayList<>(unnestCollections);
+    Collections.sort(_collectionsToUnnest);
+  }
+
+  public static boolean isComplexTypeHandlingEnabled(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig().getMode() == null) {
+      return false;
+    }
+    return tableConfig.getIngestionConfig().getComplexTypeConfig().getMode()
+        != ComplexTypeConfig.Mode.NONE;
+  }
+
+  @Nullable
+  @Override
+  public GenericRow transform(GenericRow record) {
+    flattenMap(record, new HashSet<>(record.getFieldToValueMap().keySet()));
+    for (String collection : _collectionsToUnnest) {
+      unnestCollection(record, collection);
+    }
+    return record;
+  }
+
+  private GenericRow unnestCollection(GenericRow record, String column) {
+    if (record.getValue(GenericRow.MULTIPLE_RECORDS_KEY) == null) {
+      List<GenericRow> list = new ArrayList<>();
+      unnestCollection(record, column, list);
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    } else {
+      Collection<GenericRow> records = (Collection) record.getValue(GenericRow.MULTIPLE_RECORDS_KEY);
+      List<GenericRow> list = new ArrayList<>();
+      for (GenericRow innerRecord : records) {
+        unnestCollection(innerRecord, column, list);
+      }
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    }
+    return record;
+  }
+
+  private void unnestCollection(GenericRow record, String column, List<GenericRow> list) {
+    Object value = record.removeValue(column);
+    if (value == null) {
+      // use the record itself
+      list.add(record);
+    } else if (value instanceof Collection) {
+      if (((Collection) value).isEmpty()) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Collection) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    } else if (value.getClass().isArray()) {
+      if (((Object[]) value).length == 0) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Object[]) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }

Review comment:
       What if the value is not a collection or an array?

##########
File path: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/recordtransformer/ComplexTypeTransformer.java
##########
@@ -0,0 +1,229 @@
+/**
+ * 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.pinot.segment.local.recordtransformer;
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import javax.annotation.Nullable;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.ingestion.ComplexTypeConfig;
+import org.apache.pinot.spi.data.readers.GenericRow;
+
+
+/**
+ * A transformer to handle the complex types such as Map and Collection, with flattening and unnesting.
+ * <p>
+ * The map flattening rule will recursively flatten all the map types, except for those under the collection that is not marked as to unnest.
+ *
+ * For example:
+ * <pre>
+ * {
+ *    "t1":{
+ *       "array":[
+ *          {
+ *             "t2":{
+ *                "a":"v1"
+ *             }
+ *          }
+ *       ]
+ *    }
+ * }
+ * </pre>
+ *
+ * flattens to
+ * <pre>
+ * {
+ *    "t1.array":[
+ *       {
+ *          "t2.a":"v1"
+ *       }
+ *    ]
+ * }
+ * <pre/>
+ *
+ * <p>
+ *
+ * The unnesting rule will flatten all the collections provided, which are the paths navigating to the collections. For
+ * the same example above. If the the collectionToUnnest is provided as "t1.array", then the rule will unnest the
+ * previous output to:
+ *
+ * <pre>
+ *  [{
+ *     "t1.arrayt2.a": "v1",
+ *  }]
+ *  * <pre/>
+ *
+ *  Note the unnest rule will output a collection of generic rows under the field {@link GenericRow#MULTIPLE_RECORDS_KEY}.
+ *
+ */
+public class ComplexTypeTransformer implements RecordTransformer {
+  private static final CharSequence DELIMITER = ".";
+  private final List<String> _collectionsToUnnest;
+
+  public ComplexTypeTransformer(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() != null
+        && tableConfig.getIngestionConfig().getComplexTypeConfig() != null) {
+      _collectionsToUnnest =
+          tableConfig.getIngestionConfig().getComplexTypeConfig().getUnnestConfig() != null ? tableConfig
+              .getIngestionConfig().getComplexTypeConfig().getUnnestConfig() : new ArrayList<>();
+    } else {
+      _collectionsToUnnest = new ArrayList<>();
+    }
+  }
+
+  @VisibleForTesting
+  public ComplexTypeTransformer(List<String> unnestCollections) {
+    _collectionsToUnnest = new ArrayList<>(unnestCollections);
+    Collections.sort(_collectionsToUnnest);
+  }
+
+  public static boolean isComplexTypeHandlingEnabled(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig().getMode() == null) {
+      return false;
+    }
+    return tableConfig.getIngestionConfig().getComplexTypeConfig().getMode()
+        != ComplexTypeConfig.Mode.NONE;
+  }
+
+  @Nullable
+  @Override
+  public GenericRow transform(GenericRow record) {
+    flattenMap(record, new HashSet<>(record.getFieldToValueMap().keySet()));
+    for (String collection : _collectionsToUnnest) {
+      unnestCollection(record, collection);
+    }
+    return record;
+  }
+
+  private GenericRow unnestCollection(GenericRow record, String column) {
+    if (record.getValue(GenericRow.MULTIPLE_RECORDS_KEY) == null) {
+      List<GenericRow> list = new ArrayList<>();
+      unnestCollection(record, column, list);
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    } else {
+      Collection<GenericRow> records = (Collection) record.getValue(GenericRow.MULTIPLE_RECORDS_KEY);
+      List<GenericRow> list = new ArrayList<>();
+      for (GenericRow innerRecord : records) {
+        unnestCollection(innerRecord, column, list);
+      }
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    }
+    return record;
+  }
+
+  private void unnestCollection(GenericRow record, String column, List<GenericRow> list) {
+    Object value = record.removeValue(column);
+    if (value == null) {
+      // use the record itself
+      list.add(record);
+    } else if (value instanceof Collection) {
+      if (((Collection) value).isEmpty()) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Collection) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    } else if (value.getClass().isArray()) {
+      if (((Object[]) value).length == 0) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Object[]) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    }
+  }
+
+  private GenericRow flattenCollectionItem(GenericRow record, Object obj, String column) {
+    GenericRow copy = record.copy();
+    if (obj instanceof Map) {
+      Map<String, Object> map = (Map<String, Object>) obj;
+      for (Map.Entry<String, Object> entry : new HashSet<>(map.entrySet())) {
+        String flattenName = concat(column, entry.getKey());
+        copy.putValue(flattenName, entry.getValue());
+      }
+    } else {
+      copy.putValue(column, obj);
+    }
+    return copy;
+  }
+
+  @VisibleForTesting
+  protected GenericRow flattenMap(GenericRow record, Collection<String> columns) {
+    for (String column : columns) {
+      if (record.getValue(column) instanceof Map) {
+        Map<String, Object> map = (Map) record.removeValue(column);
+        List<String> mapColumns = new ArrayList<>();
+        for (Map.Entry<String, Object> entry : new HashSet<>(map.entrySet())) {
+          String flattenName = concat(column, entry.getKey());
+          record.putValue(flattenName, entry.getValue());
+          mapColumns.add(flattenName);
+        }
+        record = flattenMap(record, mapColumns);
+      } else if (record.getValue(column) instanceof Collection && _collectionsToUnnest.contains(column)) {
+        for (Object inner : (Collection) record.getValue(column)) {
+          if (inner instanceof Map) {
+            Map<String, Object> innerMap = (Map<String, Object>) inner;
+            flattenMap(column, innerMap, new HashSet<>(innerMap.keySet()));
+          }
+        }
+      }
+    }
+    return record;
+  }
+
+  private void flattenMap(String path, Map<String, Object> map, Collection<String> fields) {
+    for (String field : fields) {
+      if (map.get(field) instanceof Map) {
+        Map<String, Object> innerMap = (Map<String, Object>) map.remove(field);
+        List<String> innerMapFields = new ArrayList<>();
+        for (Map.Entry<String, Object> innerEntry : new HashSet<>(innerMap.entrySet())) {
+          map.put(concat(field, innerEntry.getKey()), innerEntry.getValue());
+          innerMapFields.add(concat(field, innerEntry.getKey()));
+        }
+        if (!innerMapFields.isEmpty()) {
+          flattenMap(concat(path, field), map, innerMapFields);
+        }
+      } else if (map.get(field) instanceof Collection && _collectionsToUnnest.contains(concat(path, field))) {
+        for (Object inner : (Collection) map.get(field)) {
+          if (inner instanceof Map) {

Review comment:
       Nested array handling

##########
File path: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/recordtransformer/ComplexTypeTransformer.java
##########
@@ -0,0 +1,229 @@
+/**
+ * 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.pinot.segment.local.recordtransformer;
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import javax.annotation.Nullable;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.ingestion.ComplexTypeConfig;
+import org.apache.pinot.spi.data.readers.GenericRow;
+
+
+/**
+ * A transformer to handle the complex types such as Map and Collection, with flattening and unnesting.
+ * <p>
+ * The map flattening rule will recursively flatten all the map types, except for those under the collection that is not marked as to unnest.
+ *
+ * For example:
+ * <pre>
+ * {
+ *    "t1":{
+ *       "array":[
+ *          {
+ *             "t2":{
+ *                "a":"v1"
+ *             }
+ *          }
+ *       ]
+ *    }
+ * }
+ * </pre>
+ *
+ * flattens to
+ * <pre>
+ * {
+ *    "t1.array":[
+ *       {
+ *          "t2.a":"v1"
+ *       }
+ *    ]
+ * }
+ * <pre/>
+ *
+ * <p>
+ *
+ * The unnesting rule will flatten all the collections provided, which are the paths navigating to the collections. For
+ * the same example above. If the the collectionToUnnest is provided as "t1.array", then the rule will unnest the
+ * previous output to:
+ *
+ * <pre>
+ *  [{
+ *     "t1.arrayt2.a": "v1",
+ *  }]
+ *  * <pre/>
+ *
+ *  Note the unnest rule will output a collection of generic rows under the field {@link GenericRow#MULTIPLE_RECORDS_KEY}.
+ *
+ */
+public class ComplexTypeTransformer implements RecordTransformer {
+  private static final CharSequence DELIMITER = ".";
+  private final List<String> _collectionsToUnnest;
+
+  public ComplexTypeTransformer(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() != null
+        && tableConfig.getIngestionConfig().getComplexTypeConfig() != null) {
+      _collectionsToUnnest =
+          tableConfig.getIngestionConfig().getComplexTypeConfig().getUnnestConfig() != null ? tableConfig
+              .getIngestionConfig().getComplexTypeConfig().getUnnestConfig() : new ArrayList<>();
+    } else {
+      _collectionsToUnnest = new ArrayList<>();
+    }
+  }
+
+  @VisibleForTesting
+  public ComplexTypeTransformer(List<String> unnestCollections) {
+    _collectionsToUnnest = new ArrayList<>(unnestCollections);
+    Collections.sort(_collectionsToUnnest);
+  }
+
+  public static boolean isComplexTypeHandlingEnabled(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig().getMode() == null) {
+      return false;
+    }
+    return tableConfig.getIngestionConfig().getComplexTypeConfig().getMode()
+        != ComplexTypeConfig.Mode.NONE;
+  }
+
+  @Nullable
+  @Override
+  public GenericRow transform(GenericRow record) {
+    flattenMap(record, new HashSet<>(record.getFieldToValueMap().keySet()));
+    for (String collection : _collectionsToUnnest) {
+      unnestCollection(record, collection);
+    }
+    return record;
+  }
+
+  private GenericRow unnestCollection(GenericRow record, String column) {
+    if (record.getValue(GenericRow.MULTIPLE_RECORDS_KEY) == null) {
+      List<GenericRow> list = new ArrayList<>();
+      unnestCollection(record, column, list);
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    } else {
+      Collection<GenericRow> records = (Collection) record.getValue(GenericRow.MULTIPLE_RECORDS_KEY);
+      List<GenericRow> list = new ArrayList<>();
+      for (GenericRow innerRecord : records) {
+        unnestCollection(innerRecord, column, list);
+      }
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    }
+    return record;
+  }
+
+  private void unnestCollection(GenericRow record, String column, List<GenericRow> list) {
+    Object value = record.removeValue(column);
+    if (value == null) {
+      // use the record itself
+      list.add(record);
+    } else if (value instanceof Collection) {
+      if (((Collection) value).isEmpty()) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Collection) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    } else if (value.getClass().isArray()) {
+      if (((Object[]) value).length == 0) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Object[]) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    }
+  }
+
+  private GenericRow flattenCollectionItem(GenericRow record, Object obj, String column) {
+    GenericRow copy = record.copy();
+    if (obj instanceof Map) {
+      Map<String, Object> map = (Map<String, Object>) obj;
+      for (Map.Entry<String, Object> entry : new HashSet<>(map.entrySet())) {
+        String flattenName = concat(column, entry.getKey());
+        copy.putValue(flattenName, entry.getValue());
+      }
+    } else {
+      copy.putValue(column, obj);
+    }
+    return copy;
+  }
+
+  @VisibleForTesting
+  protected GenericRow flattenMap(GenericRow record, Collection<String> columns) {
+    for (String column : columns) {
+      if (record.getValue(column) instanceof Map) {

Review comment:
       Cache `record.getValue(column)` to avoid extra map lookups

##########
File path: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/recordtransformer/ComplexTypeTransformer.java
##########
@@ -0,0 +1,229 @@
+/**
+ * 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.pinot.segment.local.recordtransformer;
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import javax.annotation.Nullable;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.ingestion.ComplexTypeConfig;
+import org.apache.pinot.spi.data.readers.GenericRow;
+
+
+/**
+ * A transformer to handle the complex types such as Map and Collection, with flattening and unnesting.
+ * <p>
+ * The map flattening rule will recursively flatten all the map types, except for those under the collection that is not marked as to unnest.
+ *
+ * For example:
+ * <pre>
+ * {
+ *    "t1":{
+ *       "array":[
+ *          {
+ *             "t2":{
+ *                "a":"v1"
+ *             }
+ *          }
+ *       ]
+ *    }
+ * }
+ * </pre>
+ *
+ * flattens to
+ * <pre>
+ * {
+ *    "t1.array":[
+ *       {
+ *          "t2.a":"v1"
+ *       }
+ *    ]
+ * }
+ * <pre/>
+ *
+ * <p>
+ *
+ * The unnesting rule will flatten all the collections provided, which are the paths navigating to the collections. For
+ * the same example above. If the the collectionToUnnest is provided as "t1.array", then the rule will unnest the
+ * previous output to:
+ *
+ * <pre>
+ *  [{
+ *     "t1.arrayt2.a": "v1",
+ *  }]
+ *  * <pre/>
+ *
+ *  Note the unnest rule will output a collection of generic rows under the field {@link GenericRow#MULTIPLE_RECORDS_KEY}.
+ *
+ */
+public class ComplexTypeTransformer implements RecordTransformer {
+  private static final CharSequence DELIMITER = ".";
+  private final List<String> _collectionsToUnnest;
+
+  public ComplexTypeTransformer(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() != null
+        && tableConfig.getIngestionConfig().getComplexTypeConfig() != null) {
+      _collectionsToUnnest =
+          tableConfig.getIngestionConfig().getComplexTypeConfig().getUnnestConfig() != null ? tableConfig
+              .getIngestionConfig().getComplexTypeConfig().getUnnestConfig() : new ArrayList<>();
+    } else {
+      _collectionsToUnnest = new ArrayList<>();
+    }
+  }
+
+  @VisibleForTesting
+  public ComplexTypeTransformer(List<String> unnestCollections) {
+    _collectionsToUnnest = new ArrayList<>(unnestCollections);
+    Collections.sort(_collectionsToUnnest);
+  }
+
+  public static boolean isComplexTypeHandlingEnabled(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig().getMode() == null) {
+      return false;
+    }
+    return tableConfig.getIngestionConfig().getComplexTypeConfig().getMode()
+        != ComplexTypeConfig.Mode.NONE;
+  }
+
+  @Nullable
+  @Override
+  public GenericRow transform(GenericRow record) {
+    flattenMap(record, new HashSet<>(record.getFieldToValueMap().keySet()));
+    for (String collection : _collectionsToUnnest) {
+      unnestCollection(record, collection);
+    }
+    return record;
+  }
+
+  private GenericRow unnestCollection(GenericRow record, String column) {
+    if (record.getValue(GenericRow.MULTIPLE_RECORDS_KEY) == null) {
+      List<GenericRow> list = new ArrayList<>();
+      unnestCollection(record, column, list);
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    } else {
+      Collection<GenericRow> records = (Collection) record.getValue(GenericRow.MULTIPLE_RECORDS_KEY);
+      List<GenericRow> list = new ArrayList<>();
+      for (GenericRow innerRecord : records) {
+        unnestCollection(innerRecord, column, list);
+      }
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    }
+    return record;
+  }
+
+  private void unnestCollection(GenericRow record, String column, List<GenericRow> list) {
+    Object value = record.removeValue(column);
+    if (value == null) {
+      // use the record itself
+      list.add(record);
+    } else if (value instanceof Collection) {
+      if (((Collection) value).isEmpty()) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Collection) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    } else if (value.getClass().isArray()) {
+      if (((Object[]) value).length == 0) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Object[]) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    }
+  }
+
+  private GenericRow flattenCollectionItem(GenericRow record, Object obj, String column) {
+    GenericRow copy = record.copy();
+    if (obj instanceof Map) {
+      Map<String, Object> map = (Map<String, Object>) obj;
+      for (Map.Entry<String, Object> entry : new HashSet<>(map.entrySet())) {
+        String flattenName = concat(column, entry.getKey());
+        copy.putValue(flattenName, entry.getValue());
+      }
+    } else {
+      copy.putValue(column, obj);
+    }
+    return copy;
+  }
+
+  @VisibleForTesting
+  protected GenericRow flattenMap(GenericRow record, Collection<String> columns) {
+    for (String column : columns) {
+      if (record.getValue(column) instanceof Map) {
+        Map<String, Object> map = (Map) record.removeValue(column);
+        List<String> mapColumns = new ArrayList<>();
+        for (Map.Entry<String, Object> entry : new HashSet<>(map.entrySet())) {
+          String flattenName = concat(column, entry.getKey());
+          record.putValue(flattenName, entry.getValue());
+          mapColumns.add(flattenName);
+        }
+        record = flattenMap(record, mapColumns);
+      } else if (record.getValue(column) instanceof Collection && _collectionsToUnnest.contains(column)) {
+        for (Object inner : (Collection) record.getValue(column)) {
+          if (inner instanceof Map) {
+            Map<String, Object> innerMap = (Map<String, Object>) inner;
+            flattenMap(column, innerMap, new HashSet<>(innerMap.keySet()));
+          }
+        }
+      }
+    }
+    return record;
+  }
+
+  private void flattenMap(String path, Map<String, Object> map, Collection<String> fields) {
+    for (String field : fields) {
+      if (map.get(field) instanceof Map) {
+        Map<String, Object> innerMap = (Map<String, Object>) map.remove(field);
+        List<String> innerMapFields = new ArrayList<>();
+        for (Map.Entry<String, Object> innerEntry : new HashSet<>(innerMap.entrySet())) {
+          map.put(concat(field, innerEntry.getKey()), innerEntry.getValue());
+          innerMapFields.add(concat(field, innerEntry.getKey()));
+        }
+        if (!innerMapFields.isEmpty()) {
+          flattenMap(concat(path, field), map, innerMapFields);
+        }
+      } else if (map.get(field) instanceof Collection && _collectionsToUnnest.contains(concat(path, field))) {
+        for (Object inner : (Collection) map.get(field)) {
+          if (inner instanceof Map) {
+            Map<String, Object> innerMap = (Map<String, Object>) inner;
+            flattenMap(concat(path, field), innerMap, new HashSet<>(innerMap.keySet()));

Review comment:
       `List`

##########
File path: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/recordtransformer/ComplexTypeTransformer.java
##########
@@ -0,0 +1,229 @@
+/**
+ * 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.pinot.segment.local.recordtransformer;
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import javax.annotation.Nullable;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.ingestion.ComplexTypeConfig;
+import org.apache.pinot.spi.data.readers.GenericRow;
+
+
+/**
+ * A transformer to handle the complex types such as Map and Collection, with flattening and unnesting.
+ * <p>
+ * The map flattening rule will recursively flatten all the map types, except for those under the collection that is not marked as to unnest.
+ *
+ * For example:
+ * <pre>
+ * {
+ *    "t1":{
+ *       "array":[
+ *          {
+ *             "t2":{
+ *                "a":"v1"
+ *             }
+ *          }
+ *       ]
+ *    }
+ * }
+ * </pre>
+ *
+ * flattens to
+ * <pre>
+ * {
+ *    "t1.array":[
+ *       {
+ *          "t2.a":"v1"
+ *       }
+ *    ]
+ * }
+ * <pre/>
+ *
+ * <p>
+ *
+ * The unnesting rule will flatten all the collections provided, which are the paths navigating to the collections. For
+ * the same example above. If the the collectionToUnnest is provided as "t1.array", then the rule will unnest the
+ * previous output to:
+ *
+ * <pre>
+ *  [{
+ *     "t1.arrayt2.a": "v1",
+ *  }]
+ *  * <pre/>
+ *
+ *  Note the unnest rule will output a collection of generic rows under the field {@link GenericRow#MULTIPLE_RECORDS_KEY}.
+ *
+ */
+public class ComplexTypeTransformer implements RecordTransformer {
+  private static final CharSequence DELIMITER = ".";
+  private final List<String> _collectionsToUnnest;
+
+  public ComplexTypeTransformer(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() != null
+        && tableConfig.getIngestionConfig().getComplexTypeConfig() != null) {
+      _collectionsToUnnest =
+          tableConfig.getIngestionConfig().getComplexTypeConfig().getUnnestConfig() != null ? tableConfig
+              .getIngestionConfig().getComplexTypeConfig().getUnnestConfig() : new ArrayList<>();
+    } else {
+      _collectionsToUnnest = new ArrayList<>();
+    }
+  }
+
+  @VisibleForTesting
+  public ComplexTypeTransformer(List<String> unnestCollections) {
+    _collectionsToUnnest = new ArrayList<>(unnestCollections);
+    Collections.sort(_collectionsToUnnest);
+  }
+
+  public static boolean isComplexTypeHandlingEnabled(TableConfig tableConfig) {
+    if (tableConfig.getIngestionConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig() == null
+        || tableConfig.getIngestionConfig().getComplexTypeConfig().getMode() == null) {
+      return false;
+    }
+    return tableConfig.getIngestionConfig().getComplexTypeConfig().getMode()
+        != ComplexTypeConfig.Mode.NONE;
+  }
+
+  @Nullable
+  @Override
+  public GenericRow transform(GenericRow record) {
+    flattenMap(record, new HashSet<>(record.getFieldToValueMap().keySet()));
+    for (String collection : _collectionsToUnnest) {
+      unnestCollection(record, collection);
+    }
+    return record;
+  }
+
+  private GenericRow unnestCollection(GenericRow record, String column) {
+    if (record.getValue(GenericRow.MULTIPLE_RECORDS_KEY) == null) {
+      List<GenericRow> list = new ArrayList<>();
+      unnestCollection(record, column, list);
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    } else {
+      Collection<GenericRow> records = (Collection) record.getValue(GenericRow.MULTIPLE_RECORDS_KEY);
+      List<GenericRow> list = new ArrayList<>();
+      for (GenericRow innerRecord : records) {
+        unnestCollection(innerRecord, column, list);
+      }
+      record.putValue(GenericRow.MULTIPLE_RECORDS_KEY, list);
+    }
+    return record;
+  }
+
+  private void unnestCollection(GenericRow record, String column, List<GenericRow> list) {
+    Object value = record.removeValue(column);
+    if (value == null) {
+      // use the record itself
+      list.add(record);
+    } else if (value instanceof Collection) {
+      if (((Collection) value).isEmpty()) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Collection) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    } else if (value.getClass().isArray()) {
+      if (((Object[]) value).length == 0) {
+        // use the record itself
+        list.add(record);
+      } else {
+        for (Object obj : (Object[]) value) {
+          GenericRow copy = flattenCollectionItem(record, obj, column);
+          list.add(copy);
+        }
+      }
+    }
+  }
+
+  private GenericRow flattenCollectionItem(GenericRow record, Object obj, String column) {
+    GenericRow copy = record.copy();
+    if (obj instanceof Map) {
+      Map<String, Object> map = (Map<String, Object>) obj;
+      for (Map.Entry<String, Object> entry : new HashSet<>(map.entrySet())) {
+        String flattenName = concat(column, entry.getKey());
+        copy.putValue(flattenName, entry.getValue());
+      }
+    } else {
+      copy.putValue(column, obj);
+    }
+    return copy;
+  }
+
+  @VisibleForTesting
+  protected GenericRow flattenMap(GenericRow record, Collection<String> columns) {
+    for (String column : columns) {
+      if (record.getValue(column) instanceof Map) {
+        Map<String, Object> map = (Map) record.removeValue(column);
+        List<String> mapColumns = new ArrayList<>();
+        for (Map.Entry<String, Object> entry : new HashSet<>(map.entrySet())) {
+          String flattenName = concat(column, entry.getKey());
+          record.putValue(flattenName, entry.getValue());
+          mapColumns.add(flattenName);
+        }
+        record = flattenMap(record, mapColumns);
+      } else if (record.getValue(column) instanceof Collection && _collectionsToUnnest.contains(column)) {
+        for (Object inner : (Collection) record.getValue(column)) {
+          if (inner instanceof Map) {
+            Map<String, Object> innerMap = (Map<String, Object>) inner;
+            flattenMap(column, innerMap, new HashSet<>(innerMap.keySet()));
+          }
+        }
+      }
+    }
+    return record;
+  }
+
+  private void flattenMap(String path, Map<String, Object> map, Collection<String> fields) {
+    for (String field : fields) {
+      if (map.get(field) instanceof Map) {
+        Map<String, Object> innerMap = (Map<String, Object>) map.remove(field);
+        List<String> innerMapFields = new ArrayList<>();
+        for (Map.Entry<String, Object> innerEntry : new HashSet<>(innerMap.entrySet())) {
+          map.put(concat(field, innerEntry.getKey()), innerEntry.getValue());

Review comment:
       Cache `concat(field, innerEntry.getKey()`, and check the value before putting it to the list for the next iteration




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