raghavyadav01 commented on code in PR #18643:
URL: https://github.com/apache/pinot/pull/18643#discussion_r3574843837


##########
pinot-segment-local/src/main/java/org/apache/pinot/segment/local/segment/creator/impl/openstruct/OpenStructColumnSplitter.java:
##########
@@ -0,0 +1,538 @@
+/**
+ * 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.segment.creator.impl.openstruct;
+
+import com.google.common.base.Utf8;
+import java.io.File;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.LinkedHashMap;
+import java.util.LinkedHashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import javax.annotation.Nullable;
+import org.apache.commons.configuration2.PropertiesConfiguration;
+import org.apache.pinot.common.utils.DataSchema.ColumnDataType;
+import org.apache.pinot.segment.local.segment.creator.impl.BaseSegmentCreator;
+import 
org.apache.pinot.segment.local.segment.creator.impl.SegmentDictionaryCreator;
+import 
org.apache.pinot.segment.local.segment.creator.impl.fwd.SingleValueVarByteRawIndexCreator;
+import 
org.apache.pinot.segment.local.segment.creator.impl.nullvalue.NullValueVectorCreator;
+import 
org.apache.pinot.segment.local.segment.creator.impl.stats.AbstractColumnStatisticsCollector;
+import 
org.apache.pinot.segment.local.segment.creator.impl.stats.StatsCollectorUtil;
+import 
org.apache.pinot.segment.local.segment.index.dictionary.DictionaryIndexType;
+import 
org.apache.pinot.segment.local.segment.index.openstruct.OpenStructSupportedIndexes;
+import org.apache.pinot.segment.spi.V1Constants;
+import org.apache.pinot.segment.spi.compression.ChunkCompressionType;
+import org.apache.pinot.segment.spi.creator.IndexCreationContext;
+import org.apache.pinot.segment.spi.index.DictionaryIndexConfig;
+import org.apache.pinot.segment.spi.index.FieldIndexConfigs;
+import org.apache.pinot.segment.spi.index.FieldIndexConfigsUtil;
+import org.apache.pinot.segment.spi.index.ForwardIndexConfig;
+import org.apache.pinot.segment.spi.index.IndexCreator;
+import org.apache.pinot.segment.spi.index.IndexService;
+import org.apache.pinot.segment.spi.index.IndexType;
+import org.apache.pinot.segment.spi.index.StandardIndexes;
+import 
org.apache.pinot.segment.spi.index.creator.ColumnarOpenStructIndexCreator;
+import org.apache.pinot.spi.config.table.FieldConfig;
+import org.apache.pinot.spi.config.table.IndexConfig;
+import org.apache.pinot.spi.config.table.IndexingConfig;
+import org.apache.pinot.spi.config.table.OpenStructIndexConfig;
+import org.apache.pinot.spi.data.ComplexFieldSpec;
+import org.apache.pinot.spi.data.DimensionFieldSpec;
+import org.apache.pinot.spi.data.FieldSpec;
+import org.apache.pinot.spi.data.FieldSpec.DataType;
+import org.apache.pinot.spi.data.OpenStructNaming;
+import org.apache.pinot.spi.data.OpenStructTypeInference;
+import org.apache.pinot.spi.utils.JsonUtils;
+import org.apache.pinot.spi.utils.PinotDataType;
+import org.roaringbitmap.RoaringBitmap;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+
+/// Splits an OPEN_STRUCT column into per-key materialized columns using 
standard Pinot index
+/// creators. Dense keys become independent virtual columns; remaining keys go 
into a single
+/// synthetic JSON column for sparse storage.
+///
+/// Lifecycle: instantiated by `BaseSegmentCreator` for OPEN_STRUCT columns. 
Receives
+/// per-doc `Map<String, Object>` values via [#add(Map, int)], accumulates in 
memory,
+/// then on [#seal()] writes per-key column files using standard creators.
+public class OpenStructColumnSplitter implements 
ColumnarOpenStructIndexCreator {
+
+  private static final Logger LOGGER = 
LoggerFactory.getLogger(OpenStructColumnSplitter.class);
+
+  private final File _indexDir;
+  private final String _columnName;
+  private final Map<String, FieldSpec> _childFieldSpecs;
+  private final OpenStructIndexConfig _config;
+  private final int _maxDenseKeys;
+
+  // Per-key accumulation
+  private final Map<String, RoaringBitmap> _presenceBitmaps = new HashMap<>();
+  private final Map<String, List<Object>> _values = new HashMap<>();
+  private final Map<String, DataType> _inferredTypes = new HashMap<>();
+  private int _numDocs;
+
+  // Resolved at seal time
+  @Nullable
+  private Set<String> _resolvedDenseKeys;
+  private final Map<String, PropertiesConfiguration> 
_materializedColumnMetadata = new LinkedHashMap<>();
+
+  public OpenStructColumnSplitter(File indexDir, String columnName, FieldSpec 
fieldSpec,
+      OpenStructIndexConfig config) {
+    _indexDir = indexDir;
+    _columnName = columnName;
+    _config = config;
+    _maxDenseKeys = config.getMaxDenseKeys();
+
+    Map<String, FieldSpec> childFieldSpecs = null;
+    if (fieldSpec instanceof ComplexFieldSpec) {
+      ComplexFieldSpec complexSpec = (ComplexFieldSpec) fieldSpec;
+      childFieldSpecs = complexSpec.getChildFieldSpecs();
+    }
+    _childFieldSpecs = childFieldSpecs != null ? new 
HashMap<>(childFieldSpecs) : new HashMap<>();
+  }
+
+  @Override
+  public void add(Object value, int docId)
+      throws IOException {
+    if (value instanceof Map) {
+      @SuppressWarnings("unchecked")
+      Map<String, Object> map = (Map<String, Object>) value;
+      addMap(map);
+    } else {
+      addMap(null);
+    }
+  }
+
+  @Override
+  public void add(Map<String, Object> openStructValue, int docId)
+      throws IOException {
+    addMap(openStructValue);
+  }
+
+  @Override
+  public void add(Object[] values, @Nullable int[] dictIds)
+      throws IOException {
+    throw new UnsupportedOperationException("OPEN_STRUCT index is single-value 
only");
+  }
+
+  /// Returns the resolved dense-key set after [#seal()] or [#classify()].
+  /// Returns an empty set before resolution.
+  public Set<String> getResolvedDenseKeys() {
+    return _resolvedDenseKeys != null ? 
Collections.unmodifiableSet(_resolvedDenseKeys) : Set.of();
+  }
+
+  /// Resolves dense vs sparse keys without writing any files. Exposed for 
testing and for callers
+  /// that need the classification independent of file output. [#seal()] calls 
this internally.
+  public Set<String> classify() {
+    if (_resolvedDenseKeys != null) {
+      return _resolvedDenseKeys;
+    }
+    if (_numDocs == 0 || _presenceBitmaps.isEmpty()) {
+      _resolvedDenseKeys = new LinkedHashSet<>();
+      return _resolvedDenseKeys;
+    }
+    List<String> allKeys = new ArrayList<>(_presenceBitmaps.keySet());
+    allKeys.sort((a, b) -> {
+      double fillA = (double) _presenceBitmaps.get(a).getCardinality() / 
_numDocs;
+      double fillB = (double) _presenceBitmaps.get(b).getCardinality() / 
_numDocs;
+      int cmp = Double.compare(fillB, fillA);
+      return cmp != 0 ? cmp : a.compareTo(b);
+    });
+
+    double minFillRate = _config.getDenseKeyMinFillRate();
+    _resolvedDenseKeys = new LinkedHashSet<>();
+
+    Set<String> configuredDenseKeys = _config.getDenseKeys();
+    for (String key : configuredDenseKeys) {
+      if (_presenceBitmaps.containsKey(key) && (_maxDenseKeys < 0 || 
_resolvedDenseKeys.size() < _maxDenseKeys)) {
+        _resolvedDenseKeys.add(key);
+      }
+    }
+
+    for (String key : allKeys) {
+      if (_resolvedDenseKeys.contains(key)) {
+        continue;
+      }
+      double fillRate = (double) _presenceBitmaps.get(key).getCardinality() / 
_numDocs;

Review Comment:
   A key's type is locked to the first value seen (`computeIfAbsent`/first-row 
inference) here and in `MutableOpenStructIndex.index`. A later value that fails 
coercion to that type is dropped — `bitmap.remove()` + `continue`, only a 
`LOGGER.warn`. For a *dense* key this is quiet correctness loss on 
heterogeneous JSON (`{"x": 5}` then `{"x": "hello"}`). Is that intended for 
dense keys too, and should we surface it via a metric/counter rather than just 
a log line?



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