Github user HyukjinKwon commented on a diff in the pull request:
https://github.com/apache/spark/pull/18659#discussion_r140121928
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/python/EvalPythonExec.scala
---
@@ -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.spark.sql.execution.python
+
+import java.io.File
+
+import scala.collection.mutable.ArrayBuffer
+
+import org.apache.spark.{SparkEnv, TaskContext}
+import org.apache.spark.api.python.ChainedPythonFunctions
+import org.apache.spark.rdd.RDD
+import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.catalyst.expressions._
+import org.apache.spark.sql.execution.SparkPlan
+import org.apache.spark.sql.types.{DataType, StructField, StructType}
+import org.apache.spark.util.Utils
+
+
+/**
+ * A physical plan that evaluates a [[PythonUDF]], one partition of tuples
at a time.
+ *
+ * Python evaluation works by sending the necessary (projected) input data
via a socket to an
+ * external Python process, and combine the result from the Python process
with the original row.
+ *
+ * For each row we send to Python, we also put it in a queue first. For
each output row from Python,
+ * we drain the queue to find the original input row. Note that if the
Python process is way too
+ * slow, this could lead to the queue growing unbounded and spill into
disk when run out of memory.
+ *
+ * Here is a diagram to show how this works:
+ *
+ * Downstream (for parent)
+ * / \
+ * / socket (output of UDF)
+ * / \
+ * RowQueue Python
+ * \ /
+ * \ socket (input of UDF)
+ * \ /
+ * upstream (from child)
--- End diff --
that's fine but either looks fine and not a big deal.
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