Github user brkyvz commented on a diff in the pull request:
https://github.com/apache/spark/pull/19271#discussion_r139804087
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/StreamingSymmetricHashJoinExec.scala
---
@@ -0,0 +1,330 @@
+/*
+ * 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.streaming
+
+import java.util.concurrent.TimeUnit.NANOSECONDS
+
+import org.apache.spark.rdd.RDD
+import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.catalyst.expressions.{Attribute, Expression,
JoinedRow, NamedExpression, UnsafeProjection, UnsafeRow}
+import org.apache.spark.sql.catalyst.plans._
+import org.apache.spark.sql.catalyst.plans.logical.EventTimeWatermark._
+import org.apache.spark.sql.catalyst.plans.physical._
+import org.apache.spark.sql.execution.{BinaryExecNode, SparkPlan}
+import
org.apache.spark.sql.execution.streaming.StreamingSymmetricHashJoinExecHelper._
+import org.apache.spark.sql.execution.streaming.state._
+import org.apache.spark.sql.internal.SessionState
+import org.apache.spark.util.{CompletionIterator,
SerializableConfiguration}
+
+
+/**
+ * Performs stream-stream join using symmetric hash join algorithm. It
works as follows.
+ *
+ * /-----------------------\
+ * left side input --------->| left side state |------\
+ * \-----------------------/ |
+ * |-------->
joined output
+ * /-----------------------\ |
+ * right side input -------->| right side state |------/
+ * \-----------------------/
+ *
+ * Each join side buffers past input rows as streaming state so that the
past input can be joined
+ * with future input on the other side. This buffer state is effectively a
multi-map:
+ * equi-join key -> list of past input rows received with the join key
+ *
+ * For each input row in each side, the following operations take place.
+ * - Calculate join key from the row.
+ * - Use the join key to append the row to the buffer state of the side
that the row came from.
+ * - Find past buffered values for the key from the other side. For each
such value, emit the
+ * "joined row" (left-row, right-row)
+ * - Apply the optional condition to filter the joined rows as the final
output.
+ *
+ * If a timestamp column with event time watermark is present in the join
keys or in the input
+ * data, then the it uses the watermark figure out which rows in the
buffer will not join with
+ * and new data, and therefore can be discarded. Depending on the provided
query conditions, we
--- End diff --
nit: `with the new data`
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