[GitHub] [iceberg] rdblue commented on a diff in pull request #6252: push down min/max/count to iceberg

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

rdblue commented on code in PR #6252:
URL: https://github.com/apache/iceberg/pull/6252#discussion_r1045324550


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api/src/main/java/org/apache/iceberg/expressions/AggregateUtil.java:
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@@ -0,0 +1,142 @@
+/*
+ * 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.expressions;
+
+import java.util.List;
+import org.apache.iceberg.Table;
+import org.apache.iceberg.exceptions.ValidationException;
+import org.apache.iceberg.types.Type;
+import org.apache.iceberg.types.Types;
+
+/** Aggregate utility methods. */
+public class AggregateUtil {
+  private AggregateUtil() {}
+
+  /**
+   * Create a NestedField for this Aggregate Expression. This NestedField is used to build the
+   * pushed down aggregate schema.
+   *
+   * <p>e.g. SELECT COUNT(*), MAX(col1), MIN(col1), MAX(col2), MIN(col3) FROM table;
+   *
+   * <p>Suppose the table schema is Schema( Types.NestedField.required(1, "col1",
+   * Types.IntegerType.get()), Types.NestedField.required(2, "col2", Types.StringType.get()),
+   * Types.NestedField.required(3, "col3", Types.StringType.get()) );
+   *
+   * <p>The returned NestedField for the aggregates are Types.NestedField.required(1, COUNT(*),
+   * Types.LongType.get()) Types.NestedField.required(2, MAX(col1), Types.IntegerType.get())
+   * Types.NestedField.required(3, MIN(col1), Types.IntegerType.get()) Types.NestedField.required(4,
+   * MAX(col2), Types.StringType.get()) Types.NestedField.required(5, MIN(col3),
+   * Types.StringType.get())
+   */
+  public static Types.NestedField getAggregateNestedField(Expression expr, int index) {
+    return ExpressionVisitors.visit(expr, new AggregateNestedFieldVisitor(index));
+  }
+
+  /**
+   * Returns the column name this aggregate function is on. e.g. SELECT Max(col3) FROM table; This
+   * method returns col3
+   */
+  public static String getAggregateColumnName(Expression expr) {
+    return ExpressionVisitors.visit(expr, new AggregateColumnNameVisitor());
+  }
+
+  /**
+   * Returns the data type of this Aggregate Expression. The data type for COUNT is always Long. The
+   * data type for MAX and MIX are the same as the data type of the column this aggregate is applied
+   * on.
+   */
+  public static Type getAggregateType(Expression expr) {
+    return ExpressionVisitors.visit(expr, new AggregateTypeVisitor());
+  }
+
+  /**
+   * Returns the index of this Aggregate column in table schema. e.g. SELECT Max(col3) FROM table;
+   * Suppose the table has columns (col1, col2, col3), this method returns 2.
+   */
+  public static int columnIndexInTableSchema(Expression expr, Table table, boolean caseSensitive) {
+    return ExpressionVisitors.visit(expr, new AggregateColumnIndexVisitor(table, caseSensitive));
+  }
+
+  private static class AggregateNestedFieldVisitor

Review Comment:
   The approach I took for this is to use `BoundAggregate.eval` to produce individual values from a row or data file. Then an `Aggregator` is used to aggregate those values. It seems to fit fairly cleanly.



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