Optimizing Apache Spark SQL Joins

Optimizing
Apache Spark SQL Joins
Vida Ha
Solutions Architect

About Me
2005 Mobile Web & Voice Search
2

About Me
3
2012 Reporting & Analytics

About Me
4
2012 Reporting & Analytics
2014 Solutions Architect

Evolution of Spark…
5
2014:
• Spark 1.x
• RDD based API’s.
• Everyday I’m Shufflin’
2017:
• Spark 2.x
• Dataframes & Datasets
• Adv SQL Catalyst
• Optimizing Joins

6
SELECT …
FROM TABLE A
JOIN TABLE B
ON A.KEY1 = B.KEY2
Spark SQL Joins

Topics Covered Today
7
Basic Joins:
• Shuffle Hash Join
• Troubleshooting
• Broadcast Hash Join
• Cartesian Join
Special Cases:
• Theta Join
• One to Many Join

Shuffle Hash Join
8
A Shuffle Hash Join is the most basic type of
join, and goes back to Map Reduce
Fundamentals.
• Map through two different data frames/tables.
• Use the fields in the join condition as the output
key.
• Shuffle both datasets by the output key.
• In the reduce phase, join the two datasets now
any rows of both tables with the same keys are on
the same machine and are sorted.

Shuffle Hash Join
9
Table 1 Table 2MAP
SHUFFLE
REDUCE Output Output Output Output Output

join_rdd = sqlContext.sql(“select *
FROM people_in_the_us
JOIN states
ON people_in_the_us.state = states.name”)
Shuffle Hash Join Performance
Works best when the DF’s:
• Distribute evenly with the key you are joining on.
• Have an adequate number of keys for parallelism.

US DF
Partition 1
Problems:
● Uneven
Sharding
● Limited
parallelism w/
50 output
partitions
US RDD
Partition 2
US RDD
Partition 2**All** the
Data for CA
**All** the
Data for RI
CA
RI
All the data
for the US
will be
shuffled
into only 50
keys for
each of the
states.
Uneven Sharding & Limited Parallelism,
US DF
Partition 2
US DF
Partition N Small State
DF
A larger Spark Cluster will not solve these
problems!

US DF
Partition 1
Problems:
● Uneven
Sharding
● Limited
parallelism w/
50 output
partitions
US RDD
Partition 2
US RDD
Partition 2**All** the
Data for CA
**All** the
Data for RI
CA
RI
All the data
for the US
will be
shuffled
into only 50
keys for
each of the
states.
Uneven Sharding & Limited Parallelism,
US DF
Partition 2
US DF
Partition N Small State
DF
Broadcast Hash Join can address this problem if
one DF is small enough to fit in memory.

FROM people_in_california
LEFT JOIN all_the_people_in_the_world
ON people_in_california.id =
all_the_people_in_the_world.id”)
More Performance Considerations
Final output keys = # of people in CA, so don’t
need a huge Spark cluster, right?

The Size of the Spark Cluster to run this job is limited
by the Large table rather than the Medium Sized Table.
Left Join - Shuffle Step
Not a Problem:
● Even Sharding
● Good Parallelism
Shuffles everything
before dropping keys
All CA DF All World
DF
All the Data from
Both Tables
Final
Joined
Output

A Better Solution
Filter the World DF for only entries that match the CA ID
Filter Transform
Benefits:
● Less Data shuffled
over the network
and less shuffle
space needed.
● More transforms,
but still faster.
Shuffle
All CA DF All World
DF
Final
Joined
Output
Partial
World DF

● Can’t tell you.
● There aren’t always strict rules for optimizing.
● If you were only considering two small
columns from the World RDD in Parquet
format, the filtering step may not be worth it.
You should understand your data and it’s unique properties in
order to best optimize your Spark Job.
What’s the Tipping Point for Huge?

Things to Look for:
● Tasks that take much longer to run than others.
● Speculative tasks that are launching.
● Shards that have a lot more input or shuffle output.
Check the Spark
UI pages for task
level detail about
your Spark job.
In Practice: Detecting Shuffle Problems

Broadcast Hash Join
18
Parallelism of the large DF is maintained (n output
partitions), and shuffle is not even needed.
Broadcast
Large DF
Partition N
Large DF
Partition 1
Large DF
Partition 2
Optimization: When one of the DF’s is small
enough to fit in memory on a single machine.
Small DF
Small DF Small DF Small DF

Broadcast Hash Join
19
• Often optimal over Shuffle Hash Join.
• Use “explain” to determine if the Spark SQL
catalyst hash chosen Broadcast Hash Join.
• Should be automatic for many Spark SQL tables,
may need to provide hints for other types.

Cartesian Join
20
• A cartesian join can easily explode the number of output
rows.
100,000 X 100,000 = 10 Billion
• Alternative to a full blown cartesian join:
• Create an RDD of UID by UID.
• Force a Broadcast of the rows of the table .
• Call a UDF given the UID by UID to look up the table
rows and perform your calculation.
• Time your calculation on a sample set to size your cluster.

One To Many Join
21
• A single row on one table can map to many rows on the
2nd table.
• Can explode the number of output rows.
• Not a problem if you use parquet - the size of the output
files is not that much since the duplicate data encodes
well.

Theta Join
22
• Spark SQL consider each keyA against each keyB in
the example above and loop to see if the theta
condition is met.
• Better Solution - create buckets for keyA and KeyB
can be matched on.
FROM tableA
JOIN tableB
ON (keyA < keyB + 10)”)

Optimizing Apache Spark SQL Joins

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