Apache Spark：广播联接行为：联接表和临时表的过滤

Question

I need to join 2 tables in spark. But instead of joining 2 tables completely, I first filter out a part of second table:

spark.sql("select * from a join b on a.key=b.key where b.value='xxx' ")

I want to use broadcast join in this case.

Spark has a parameter which defines max table size for broadcast join: spark.sql.autoBroadcastJoinThreshold :

Configures the maximum size in bytes for a table that will be broadcast to all worker nodes when performing a join. By setting this value to -1 broadcasting can be disabled. Note that currently statistics are only supported for Hive Metastore tables where the command ANALYZE TABLE COMPUTE STATISTICS noscan has been run. http://spark.apache.org/docs/2.4.0/sql-performance-tuning.html

I have following questions about this setup:

1. which table size spark will compare with autoBroadcastJoinThreshold's value: FULL size, or size AFTER applying where clause?
2. I am assuming that spark will apply where clause BEFORE broadcasting, correct?
3. the doc says I need to run Hive's Analyze Table command beforehand. How it will work in a case when I am using temp view as a table? As far as I understand I cannot run Analyze Table command against spark's temp view created via dataFrame.createorReplaceTempView("b"). Can I broadcast temp view contents?

Answer 1

Understanding for option 2 is correct. You can not analyze a TEMP table in spark . Read here

In case you want to take the lead and want to specify the dataframe which you want to broadcast, instead spark decides, can use below snippet-

df = df1.join(F.broadcast(df2),df1.some_col == df2.some_col, "left")

Answer 2

I went ahead and did some small experiments to answer your 1st question.

Question 1 :

• created a dataframe a with 3 rows [key,df_a_column]
• created a dataframe b with 10 rows [key,value]
• ran: spark.sql("SELECT * FROM a JOIN b ON a.key = b.key").explain()

== Physical Plan ==
*(1) BroadcastHashJoin [key#122], [key#111], Inner, BuildLeft, false
:- BroadcastExchange HashedRelationBroadcastMode(List(cast(input[0, int, false] as bigint)),false), [id=#168]
:  +- LocalTableScan [key#122, df_a_column#123]
+- *(1) LocalTableScan [key#111, value#112]

As expected the Smaller df a with 3 rows is broadcasted.

• Ran : spark.sql("SELECT * FROM a JOIN b ON a.key = b.key where b.value=\\"bat\\"").explain()

== Physical Plan ==
*(1) BroadcastHashJoin [key#122], [key#111], Inner, BuildRight, false
:- *(1) LocalTableScan [key#122, df_a_column#123]
+- BroadcastExchange HashedRelationBroadcastMode(List(cast(input[0, int, false] as bigint)),false), [id=#152]
   +- LocalTableScan [key#111, value#112]

Here you can notice the dataframe b is Broadcasted ! meaning spark evaluates the size AFTER applying where for choosing which one to broadcast.

Question 2 :

Yes you are right. It's evident from the previous output it applies where first.

Question 3 : No you cannot analyse but you can broadcast tempView table by hinting spark about it even in SQL. ref

Example : spark.sql("SELECT /*+ BROADCAST(b) */ * FROM a JOIN b ON a.key = b.key")

And if you see explain now :

== Physical Plan ==
*(1) BroadcastHashJoin [key#122], [key#111], Inner, BuildRight, false
:- *(1) LocalTableScan [key#122, df_a_column#123]
+- BroadcastExchange HashedRelationBroadcastMode(List(cast(input[0, int, false] as bigint)),false), [id=#184]
   +- LocalTableScan [key#111, value#112]

Now if you see, dataframe b is broadcasted even though it has 10 rows. In question 1, without the hint , a was broadcasted .

Note: Broadcast hint in SQL spark is available for 2.2

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Tips to understand the physical plan :

• Figure out the dataframe from the LocalTableScan[ list of columns ]
• The dataframe present under the sub tree/list of BroadcastExchange is being broadcasted.