What are ThreadPoolExecutors jobs in web UI's Spark Jobs?

After some investigation I found out that run at ThreadPoolExecutor.java:1142 Spark jobs are related to queries with join operators that fit the definition of BroadcastHashJoin where one join side is broadcast to executors for join.

That BroadcastHashJoin operator uses a ThreadPool for this asynchronous broadcasting (see this and this).

scala> spark.version
res16: String = 2.1.0-SNAPSHOT

scala> val left = spark.range(1)
left: org.apache.spark.sql.Dataset[Long] = [id: bigint]

scala> val right = spark.range(1)
right: org.apache.spark.sql.Dataset[Long] = [id: bigint]

scala> left.join(right, Seq("id")).show
+---+
| id|
+---+
|  0|
+---+

When you switch to the SQL tab you should see Completed Queries section and their Jobs (on the right).

In my case the Spark job(s) running on "run at ThreadPoolExecutor.java:1142" where ids 12 and 16.

They both correspond to join queries.

If you wonder "that makes sense that one of my joins is causing this job to appear but as far as I know join is a shuffle transformation and not an action, so why is the job described with the ThreadPoolExecutor and not with my action (as is the case with the rest of my jobs)?", then my answer is usually along the lines:

Spark SQL is an extension of Spark with its own abstractions (Datasets to name just the one that quickly springs to mind) that have their own operators for execution. One "simple" SQL operation can run one or more Spark jobs. It's at the discretion of Spark SQL's execution engine how many Spark jobs to run or submit (but they do use RDDs under the covers) -- you don't have to know such a low-leve details as it's...well...too low-level...given you are so high-level by using Spark SQL's SQL or Query DSL.