deniskuzZ commented on code in PR #11216:
URL: https://github.com/apache/iceberg/pull/11216#discussion_r1907057882


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data/src/main/java/org/apache/iceberg/data/PartitionStatsHandler.java:
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@@ -0,0 +1,332 @@
+/*
+ * 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.iceberg.data;
+
+import static org.apache.iceberg.TableProperties.DEFAULT_FILE_FORMAT;
+import static org.apache.iceberg.TableProperties.DEFAULT_FILE_FORMAT_DEFAULT;
+
+import java.io.Closeable;
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Locale;
+import java.util.function.BiFunction;
+import org.apache.iceberg.FileFormat;
+import org.apache.iceberg.HasTableOperations;
+import org.apache.iceberg.ImmutableGenericPartitionStatisticsFile;
+import org.apache.iceberg.PartitionSpec;
+import org.apache.iceberg.PartitionStatisticsFile;
+import org.apache.iceberg.PartitionStats;
+import org.apache.iceberg.PartitionStatsUtil;
+import org.apache.iceberg.Partitioning;
+import org.apache.iceberg.Schema;
+import org.apache.iceberg.Snapshot;
+import org.apache.iceberg.StructLike;
+import org.apache.iceberg.Table;
+import org.apache.iceberg.avro.Avro;
+import org.apache.iceberg.data.avro.DataReader;
+import org.apache.iceberg.data.orc.GenericOrcReader;
+import org.apache.iceberg.data.parquet.GenericParquetReaders;
+import org.apache.iceberg.encryption.EncryptedFiles;
+import org.apache.iceberg.encryption.EncryptionKeyMetadata;
+import org.apache.iceberg.io.CloseableIterable;
+import org.apache.iceberg.io.DataWriter;
+import org.apache.iceberg.io.FileWriterFactory;
+import org.apache.iceberg.io.InputFile;
+import org.apache.iceberg.io.OutputFile;
+import org.apache.iceberg.orc.ORC;
+import org.apache.iceberg.parquet.Parquet;
+import 
org.apache.iceberg.relocated.com.google.common.annotations.VisibleForTesting;
+import org.apache.iceberg.relocated.com.google.common.base.Preconditions;
+import org.apache.iceberg.types.Types.IntegerType;
+import org.apache.iceberg.types.Types.LongType;
+import org.apache.iceberg.types.Types.NestedField;
+import org.apache.iceberg.types.Types.StructType;
+import org.apache.iceberg.util.SnapshotUtil;
+
+/**
+ * Computes, writes and reads the {@link PartitionStatisticsFile}. Uses 
generic readers and writers
+ * to support writing and reading of the stats in table default format.
+ */
+public final class PartitionStatsHandler {
+
+  private PartitionStatsHandler() {}
+
+  public enum Column {
+    PARTITION(0),
+    SPEC_ID(1),
+    DATA_RECORD_COUNT(2),
+    DATA_FILE_COUNT(3),
+    TOTAL_DATA_FILE_SIZE_IN_BYTES(4),
+    POSITION_DELETE_RECORD_COUNT(5),
+    POSITION_DELETE_FILE_COUNT(6),
+    EQUALITY_DELETE_RECORD_COUNT(7),
+    EQUALITY_DELETE_FILE_COUNT(8),
+    TOTAL_RECORD_COUNT(9),
+    LAST_UPDATED_AT(10),
+    LAST_UPDATED_SNAPSHOT_ID(11);
+
+    private final int id;
+
+    Column(int id) {
+      this.id = id;
+    }
+
+    public int id() {
+      return id;
+    }
+  }
+
+  /**
+   * Generates the partition stats file schema based on a given partition type.
+   *
+   * <p>Note: Provide the unified partition schema type as mentioned in the 
spec.
+   *
+   * @param partitionType unified partition schema type.
+   * @return a schema that corresponds to the provided unified partition type.
+   */
+  public static Schema schema(StructType partitionType) {
+    Preconditions.checkState(!partitionType.fields().isEmpty(), "table must be 
partitioned");
+
+    return new Schema(
+        NestedField.required(1, Column.PARTITION.name(), partitionType),
+        NestedField.required(2, Column.SPEC_ID.name(), IntegerType.get()),
+        NestedField.required(3, Column.DATA_RECORD_COUNT.name(), 
LongType.get()),
+        NestedField.required(4, Column.DATA_FILE_COUNT.name(), 
IntegerType.get()),
+        NestedField.required(5, Column.TOTAL_DATA_FILE_SIZE_IN_BYTES.name(), 
LongType.get()),
+        NestedField.optional(6, Column.POSITION_DELETE_RECORD_COUNT.name(), 
LongType.get()),
+        NestedField.optional(7, Column.POSITION_DELETE_FILE_COUNT.name(), 
IntegerType.get()),
+        NestedField.optional(8, Column.EQUALITY_DELETE_RECORD_COUNT.name(), 
LongType.get()),
+        NestedField.optional(9, Column.EQUALITY_DELETE_FILE_COUNT.name(), 
IntegerType.get()),
+        NestedField.optional(10, Column.TOTAL_RECORD_COUNT.name(), 
LongType.get()),
+        NestedField.optional(11, Column.LAST_UPDATED_AT.name(), 
LongType.get()),
+        NestedField.optional(12, Column.LAST_UPDATED_SNAPSHOT_ID.name(), 
LongType.get()));
+  }
+
+  /**
+   * Computes and writes the {@link PartitionStatisticsFile} for a given 
table's current snapshot.
+   *
+   * @param table The {@link Table} for which the partition statistics is 
computed.
+   * @return {@link PartitionStatisticsFile} for the current snapshot.
+   */
+  public static PartitionStatisticsFile computeAndWriteStatsFile(Table table) {
+    return computeAndWriteStatsFile(table, null);
+  }
+
+  /**
+   * Computes and writes the {@link PartitionStatisticsFile} for a given table 
and branch.
+   *
+   * @param table The {@link Table} for which the partition statistics is 
computed.
+   * @param branch A branch information to select the required snapshot.
+   * @return {@link PartitionStatisticsFile} for the given branch.
+   */
+  public static PartitionStatisticsFile computeAndWriteStatsFile(Table table, 
String branch) {

Review Comment:
   Wouldn't it be better not to have fat methods with multiple 
responsibilities? What if we introduce the write method that gets the stats 
iterator as an argument. 
   We might not need to execute the complete stats rebuild for all the 
registered partitions, but only for those that were changed in the current 
snapshot. 
   Snapshot summary already has a metric for the number of changed partitions, 
maybe we could extend it with the partition list and re-compute stats only for 
them. Then generate a new stats file based on the prev snapshot stats with 
updates to the changed partitions. 



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