Github user yhuai commented on a diff in the pull request:
https://github.com/apache/spark/pull/12874#discussion_r63096674
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/collect.scala
---
@@ -0,0 +1,119 @@
+/*
+ * 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.catalyst.expressions.aggregate
+
+import scala.collection.generic.Growable
+import scala.collection.mutable
+
+import org.apache.spark.sql.catalyst.expressions._
+import org.apache.spark.sql.catalyst.util.GenericArrayData
+import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.types._
+
+/**
+ * The Collect aggregate function collects all seen expression values into
a list of values.
+ *
+ * The operator is bound to the slower sort based aggregation path because
the number of
+ * elements (and their memory usage) can not be determined in advance.
This also means that the
+ * collected elements are stored on heap, and that too many elements can
cause GC pauses and
+ * eventually Out of Memory Errors.
+ */
+abstract class Collect extends ImperativeAggregate {
+
+ val child: Expression
+
+ override def children: Seq[Expression] = child :: Nil
+
+ override def nullable: Boolean = true
+
+ override def dataType: DataType = ArrayType(child.dataType)
+
+ override def inputTypes: Seq[AbstractDataType] = Seq(AnyDataType)
+
+ override def supportsPartial: Boolean = false
+
+ override def aggBufferAttributes: Seq[AttributeReference] = Nil
+
+ override def aggBufferSchema: StructType =
StructType.fromAttributes(aggBufferAttributes)
+
+ override def inputAggBufferAttributes: Seq[AttributeReference] = Nil
+
+ protected[this] val buffer: Growable[Any] with Iterable[Any]
+
+ override def initialize(b: MutableRow): Unit = {
+ buffer.clear()
+ }
+
+ override def update(b: MutableRow, input: InternalRow): Unit = {
+ buffer += child.eval(input)
+ }
+
+ override def merge(buffer: MutableRow, input: InternalRow): Unit = {
+ sys.error("Collect cannot be used in partial aggregations.")
+ }
+
+ override def eval(input: InternalRow): Any = {
+ new GenericArrayData(buffer.toArray)
--- End diff --
oh, I see. We have an internal buffer and only generate this array data
when we call eval.
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