Hi Jacek,
Just replied to the SO thread as well, but…
Yes, your first statement is correct. The DFs in the union are read in the same
stage, so in your example where each DF has 8 partitions then you have a stage
with 16 tasks to read the 2 DFs. There's no need to define the DF in a separate
thread. You can verify this also in the Stage UI and looking at the Event
Timeline. You should see the tasks across the DFs executing in parallel as
expected.
Here’s the UI for the following example, in which case each DF only has 1
partition (so we get a stage with 2 tasks):
spark.range(1, 100, 1, 1).write.save("/tmp/df1")
spark.range(101, 200, 1, 1).write.save("/tmp/df2")
spark.read.load("/tmp/df1").union(spark.read.load("/tmp/df2")).foreach { _ => }
[cid:image001.png@01D3766F.77848FA0]
From: Jacek Laskowski <ja...@japila.pl>
Date: Saturday, December 16, 2017 at 6:40 AM
To: "user @spark" <user@spark.apache.org>
Subject: How to...UNION ALL of two SELECTs over different data sources in
parallel?
Hi,
I've been trying to find out the answer to the question about UNION ALL and
SELECTs @ https://stackoverflow.com/q/47837955/1305344
> If I have Spark SQL statement of the form SELECT [...] UNION ALL SELECT
> [...], will the two SELECT statements be executed in parallel? In my specific
> use case the two SELECTs are querying two different database tables. In
> contrast to what I would have expected, the Spark UI seems to suggest that
> the two SELECT statements are performed sequentially.
How to know if the two separate SELECTs are executed in parallel or not? What
are the tools to know it?
My answer was to use explain operator that would show...well...physical plan,
but am not sure how to read it to know whether a query plan is going to be
executed in parallel or not.
I then used the underlying RDD lineage (using rdd.toDebugString) hoping that
gives me the answer, but...I'm not so sure.
For a query like the following:
val q = spark.range(1).union(spark.range(2))
I thought that since both SELECTs are codegen'ed they could be executed in
parallel, but when switched to the RDD lineage I lost my confidence given
there's just one single stage (!)
scala> q.rdd.toDebugString
res4: String =
(16) MapPartitionsRDD[17] at rdd at <console>:26 []
| MapPartitionsRDD[16] at rdd at <console>:26 []
| UnionRDD[15] at rdd at <console>:26 []
| MapPartitionsRDD[11] at rdd at <console>:26 []
| MapPartitionsRDD[10] at rdd at <console>:26 []
| ParallelCollectionRDD[9] at rdd at <console>:26 []
| MapPartitionsRDD[14] at rdd at <console>:26 []
| MapPartitionsRDD[13] at rdd at <console>:26 []
| ParallelCollectionRDD[12] at rdd at <console>:26 []
What am I missing and how to be certain whether and what parts of a query are
going to be executed in parallel?
Please help...
Pozdrawiam,
Jacek Laskowski
----
https://about.me/JacekLaskowski
Spark Structured Streaming https://bit.ly/spark-structured-streaming
Mastering Apache Spark 2 https://bit.ly/mastering-apache-spark
Follow me at https://twitter.com/jaceklaskowski
