Many to many join on large datasets in Spark - scala

Question

I have two large datasets, A and B, which I wish to join on key K.
Each dataset contains many rows with the same value of K, so this is a many-to-many join.
This join fails with memory related errors if I just try it naively.
Let's also say grouping both datasets by K, doing the join and then exploding back out with some trickery to get the correct result isn't a viable option, again due to memory issues
Are there any clever tricks people have found which improves the chance of this working?
Update:
Adding a very, very contrived concrete example:
spark-shell --master local[4] --driver-memory 5G --conf spark.sql.autoBroadcastJoinThreshold=-1 --conf spark.sql.autoBroadcastJoinThreshold=-1 --conf spark.sql.shuffle.partitions=10000 --conf spark.default.parallelism=10000
val numbersA = (1 to 100000).toList.toDS
val numbersWithDataA = numbersA.repartition(10000).map(n => (n, 1, Array.fill[Byte](1000*1000)(0)))
numbersWithDataA.write.mode("overwrite").parquet("numbersWithDataA.parquet")
val numbersB = (1 to 100).toList.toDS
val numbersWithDataB = numbersB.repartition(100).map(n => (n, 1, Array.fill[Byte](1000*1000)(0)))
numbersWithDataB.write.mode("overwrite").parquet("numbersWithDataB.parquet")
val numbersWithDataInA = spark.read.parquet("numbersWithDataA.parquet").toDF("numberA", "one", "dataA")
val numbersWithDataInB = spark.read.parquet("numbersWithDataB.parquet").toDF("numberB", "one", "dataB")
numbersWithDataInA.join(numbersWithDataInB, Seq("one")).write.mode("overwrite").parquet("joined.parquet")
Fails with Caused by: java.lang.OutOfMemoryError: Java heap space

Answer 1

--conf spark.sql.autoBroadcastJoinThreshold=-1
means you are disabling the broadcast feature.
You can change it to any suitable value <2gb (since 2gb limit is there). spark.sql.autoBroadcastJoinThreshold is default 10mb
as per spark documentation. I dont know the reason you have disabled it. if you disbale it SparkStregies will switch the path to sortmerge join or shuffle hash join. see my article for details
Remaining I dont think there is any need to change as its common pattern of joining 2 datasets.
Further reading DataFrame join optimization - Broadcast Hash Join
UPDATE : Alternatively In your real example (not contrieved :-)) you can do these steps
Steps :
1) Each dataset find out join key (may be for example, pickup a unique/distinct category or country or state field) and collect them as an array since its small data you can collect.
2) For each category element in an array join the 2 datasets (playing with small dataset joins) with category as where condition add to a sequence of dataframes.
3) reduce and union these dataframes.
scala example :
val dfCatgories = Seq(df1Category1, df2Category2, df3Category3)
dfCatgories.reduce(_ union _)
Note : for each join I still prefer BHJ since it will be less/no shuffle