Github user rxin commented on a diff in the pull request:
https://github.com/apache/spark/pull/12161#discussion_r59325685
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/aggregate/ColumnarAggMapCodeGenerator.scala
---
@@ -0,0 +1,193 @@
+/*
+ * 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.spark.sql.execution.aggregate
+
+import org.apache.spark.sql.catalyst.expressions.codegen.CodegenContext
+import org.apache.spark.sql.types.StructType
+
+/**
+ * This is a helper object to generate an append-only single-key/single
value aggregate hash
+ * map that can act as a 'cache' for extremely fast key-value lookups
while evaluating aggregates
+ * (and fall back to the `BytesToBytesMap` if a given key isn't found).
This is 'codegened' in
+ * TungstenAggregate to speed up aggregates w/ key.
+ *
+ * It is backed by a power-of-2-sized array for index lookups and a
columnar batch that stores the
+ * key-value pairs. The index lookups in the array rely on linear probing
(with a small number of
+ * maximum tries) and use an inexpensive hash function which makes it
really efficient for a
+ * majority of lookups. However, using linear probing and an inexpensive
hash function also makes it
+ * less robust as compared to the `BytesToBytesMap` (especially for a
large number of keys or even
+ * for certain distribution of keys) and requires us to fall back on the
latter for correctness.
+ */
+class ColumnarAggMapCodeGenerator(
--- End diff --
everything in execution is private
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