Bucketing is a partitioning technique that can improve performance in certain data transformations by avoiding data shuffling and sorting. The general idea of bucketing is to partition, and optionally sort, the data based on a subset of columns while it is written out (a one-time cost), while making successive reads of the data more performant for downstream jobs if the SQL operators can make use of this property. Bucketing can enable faster joins (i.e. single stage sort merge join), the ability to short circuit in FILTER operation if the file is pre-sorted over the column in a filter predicate, and it supports quick data sampling. In this session, you’ll learn how bucketing is implemented in both Hive and Spark. In particular, Patil will describe the changes in the Catalyst optimizer that enable these optimizations in Spark for various bucketing scenarios. Facebook’s performance tests have shown bucketing to improve Spark performance from 3-5x faster when the optimization is enabled. Many tables at Facebook are sorted and bucketed, and migrating these workloads to Spark have resulted in a 2-3x savings when compared to Hive. You’ll also hear about real-world applications of bucketing, like loading of cumulative tables with daily delta, and the characteristics that can help identify suitable candidate jobs that can benefit from bucketing.

1. Hive Bucketing in
Apache Spark
Tejas Patil
Facebook

2. Agenda
• Why bucketing ?
• Why is shuffle bad ?
• How to avoid shuffle ?
• When to use bucketing ?
• Spark’s bucketing support
• Bucketing semantics of Spark vs Hive
• Hive bucketing support in Spark
• SQL Planner improvements

3. Why bucketing ?

4. Table A Table B
. . . . . . . . . . . . JOIN

5. Table A Table B
. . . . . . . . . . . . JOIN
Broadcast hash join
- Ship smaller table to all nodes
- Stream the other table

6. Table A Table B
. . . . . . . . . . . . JOIN
Broadcast hash join Shuffle hash join
- Ship smaller table to all nodes
- Stream the other table
- Shuffle both tables,
- Hash smaller one, stream the
bigger one

7. Table A Table B
. . . . . . . . . . . . JOIN
Broadcast hash join Shuffle hash join
- Ship smaller table to all nodes
- Stream the other table
- Shuffle both tables,
- Hash smaller one, stream the
bigger one
Sort merge join
- Shuffle both tables,
- Sort both tables, buffer one,
stream the bigger one

8. Table A Table B
. . . . . . . . . . . . JOIN
Broadcast hash join Shuffle hash join Sort merge join
- Ship smaller table to all nodes
- Stream the other table
- Shuffle both tables,
- Hash smaller one, stream the
bigger one
- Shuffle both tables,
- Sort both tables, buffer one,
stream the bigger one

9. Table A Table B
. . . . . . . . . . . .
Sort Merge Join

10. Table A Table B
. . . . . . . . . . . .
Shuffle ShuffleShuffleShuffle
Sort Merge Join

11. Table A Table B
. . . . . . . . . . . .
Sort Sort Sort Sort
Shuffle ShuffleShuffleShuffle
Sort Merge Join

12. Table A Table B
. . . . . . . . . . . .
Sort
Join
Sort
Join
Sort
Join
Sort
Join
Shuffle ShuffleShuffleShuffle
Sort Merge Join

13. Sort Merge Join

14. Why is shuffle bad ?

15. Table A Table B
. . . . . . . . . . . .
Disk Disk
Disk IO for shuffle outputs
Why is shuffle bad ?

16. Table A Table B
. . . . . . . . . . . .
Shuffle Shuffle Shuffle Shuffle
M * R network connections
Why is shuffle bad ?

17. How to avoid shuffle ?

19. student s JOIN attendance a
ON s.id = a.student_id
student s JOIN results r
ON s.id = r.student_id
student s JOIN course_registrationc
ON s.id = c.student_id
……..
……..

20. student s JOIN attendance a
ON s.id = a.student_id
student s JOIN results r
ON s.id = r.student_id

21. student s JOIN attendance a
ON s.id = a.student_id
student s JOIN results r
ON s.id = r.student_id
Pre-compute at
table creation

22. Bucketing =
pre-(shuffle + sort) inputs
on join keys

23. Without
bucketing
Job(s) populating input table

24. Without
bucketing
With
bucketing
Job(s) populating input table

25. Performance comparison
0
100
200
300
400
500
600
700
800
900
Before After Bucketing
Reserved CPU time (days)
0
1
2
3
4
5
6
7
8
Before After Bucketing
Latency (hours)

26. When to use bucketing ?
• Tables used frequently in JOINs with same key
• Student -> student_id
• Employee -> employee_id
• Users -> user_id

27. When to use bucketing ?
• Tables used frequently in JOINs with same key
• Student -> student_id
• Employee -> employee_id
• Users -> user_id
…. => Dimension tables

28. When to use bucketing ?
• Tables used frequently in JOINs with same key
• Student -> student_id
• Employee -> employee_id
• Users -> user_id
• Loading daily cumulative tables
• Both base and delta tables could be bucketed on a common column

29. When to use bucketing ?
• Tables used frequently in JOINs with same key
• Student -> student_id
• Employee -> employee_id
• Users -> user_id
• Loading daily cumulative tables
• Both base and delta tables could be bucketed on a common column
• Indexing capability

30. Spark’s bucketing support

32. Creation of bucketed tables
df.write
.bucketBy(numBuckets, "col1", ...)
.sortBy("col1", ...)
.saveAsTable("bucketed_table")
via Dataframe API

33. Creation of bucketed tables
CREATE TABLE bucketed_table(
column1 INT,
...
) USING parquet
CLUSTERED BY (column1, …)
SORTED BY (column1, …)
INTO `n` BUCKETS
via SQL statement

34. Check bucketing spec
scala> sparkContext.sql("DESC FORMATTED student").collect.foreach(println)
[# col_name,data_type,comment]
[student_id,int,null]
[name,int,null]
[# Detailed Table Information,,]
[Database,default,]
[Table,table1,]
[Owner,tejas,]
[Created,Fri May 12 08:06:33 PDT 2017,]
[Type,MANAGED,]
[Num Buckets,64,]
[Bucket Columns,[`student_id`],]
[Sort Columns,[`student_id`],]
[Properties,[serialization.format=1],]
[Serde Library,org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe,]
[InputFormat,org.apache.hadoop.mapred.SequenceFileInputFormat,]

35. Bucketing config
SET spark.sql.sources.bucketing.enabled=true

36. [SPARK-15453] Extract bucketing info in
FileSourceScanExec
SELECT * FROM tableA JOIN tableB ON tableA.i= tableB.i AND tableA.j= tableB.j

37. [SPARK-15453] Extract bucketing info in
FileSourceScanExec
SELECT * FROM tableA JOIN tableB ON tableA.i= tableB.i AND tableA.j= tableB.j
Sort Merge Join
Sort
Exchange
TableScan
Sort
Exchange
TableScan Already bucketed on columns (i, j)
Shuffle on columns (i, j)
Sort on columns (i, j)
Join on [i,j], [i,j] respectively of relations

38. [SPARK-15453] Extract bucketing info in
FileSourceScanExec
SELECT * FROM tableA JOIN tableB ON tableA.i= tableB.i AND tableA.j= tableB.j
Sort Merge Join
Sort
Exchange
TableScan
Sort
Exchange
TableScan Already bucketed on columns (i, j)
Shuffle on columns (i, j)
Sort on columns (i, j)
Join on [i,j], [i,j] respectively of relations

39. [SPARK-15453] Extract bucketing info in
FileSourceScanExec
SELECT * FROM tableA JOIN tableB ON tableA.i= tableB.i AND tableA.j= tableB.j
Sort Merge Join
TableScan TableScan
Sort Merge Join
TableScan TableScanSort
Exchange
Sort
Exchange

40. Bucketing semantics of
Spark vs Hive

41. Bucketing semantics of Spark vs Hive
bucket 0 bucket 1
bucket
(n-1)
…..
Hive
my_bucketed_table

42. Bucketing semantics of Spark vs Hive
bucket 0 bucket 1
bucket
(n-1)
…..
Hive Spark
bucket 0 bucket 1
bucket
(n-1)
…..
bucket 0
bucket 0
bucket 1
bucket
(n-1)bucket
(n-1)bucket
(n-1)
my_bucketed_tablemy_bucketed_table

43. Bucketing semantics of Spark vs Hive
bucket 0 bucket 1
bucket
(n-1)
…..
Hive
M M M M M
R R R
…..

44. Bucketing semantics of Spark vs Hive
bucket 0 bucket 1
bucket
(n-1)
…..
Hive Spark
M M M M M
R R R
…..
bucket 0 bucket 1
bucket
(n-1)
…..
M M M M M…..
bucket 0
bucket 0
bucket 1
bucket
(n-1)bucket
(n-1)bucket
(n-1)

45. Bucketing semantics of Spark vs Hive
Hive Spark
Model Optimizes reads, writes are costly Writes are cheaper, reads are
costlier

46. Bucketing semantics of Spark vs Hive
Hive Spark
Model Optimizes reads, writes are costly Writes are cheaper, reads are
costlier
Unit of bucketing A single file represents a single
bucket
A collection of files together
comprise a single bucket

47. Bucketing semantics of Spark vs Hive
Hive Spark
Model Optimizes reads, writes are costly Writes are cheaper, reads are
costlier
Unit of bucketing A single file represents a single
bucket
A collection of files together
comprise a single bucket
Sort ordering Each bucket is sorted globally. This
makes it easy to optimize queries
reading this data
Each file is individually sorted but
the “bucket” as a whole is not
globally sorted

48. Bucketing semantics of Spark vs Hive
Hive Spark
Model Optimizes reads, writes are costly Writes are cheaper, reads are
costlier
Unit of bucketing A single file represents a single
bucket
A collection of files together
comprise a single bucket
Sort ordering Each bucket is sorted globally. This
makes it easy to optimize queries
reading this data
Each file is individually sorted but
the “bucket” as a whole is not
globally sorted
Hashing function Hive’s inbuilt hash Murmur3Hash

49. [SPARK-19256] Hive bucketing support
• Introduce Hive’s hashing function [SPARK-17495]
• Enable creating hive bucketed tables [SPARK-17729]
• Support Hive’s `Bucketed file format`
• Propagate Hive bucketing information to planner [SPARK-17654]
• Expressing outputPartitioning and requiredChildDistribution
• Creating empty bucket files when no data for a bucket
• Allow Sort Merge join over tables with number of buckets
multiple of each other
• Support N-way Sort Merge Join

50. [SPARK-19256] Hive bucketing support
• Introduce Hive’s hashing function [SPARK-17495]
• Enable creating hive bucketed tables [SPARK-17729]
• Support Hive’s `Bucketed file format`
• Propagate Hive bucketing information to planner [SPARK-17654]
• Expressing outputPartitioning and requiredChildDistribution
• Creating empty bucket files when no data for a bucket
• Allow Sort Merge join over tables with number of buckets
multiple of each other
• Support N-way Sort Merge Join
Merged in
upstream

51. [SPARK-19256] Hive bucketing support
• Introduce Hive’s hashing function [SPARK-17495]
• Enable creating hive bucketed tables [SPARK-17729]
• Support Hive’s `Bucketed file format`
• Propagate Hive bucketing information to planner [SPARK-17654]
• Expressing outputPartitioning and requiredChildDistribution
• Creating empty bucket files when no data for a bucket
• Allow Sort Merge join over tables with number of buckets
multiple of each other
• Support N-way Sort Merge Join
FB-prod (6 months)

52. SQL Planner improvements

53. [SPARK-19122] Unnecessary shuffle+sort added if
join predicates ordering differ from bucketing and
sorting order
SELECT * FROM a JOIN b ON a.x=b.xAND a.y=b.y

54. [SPARK-19122] Unnecessary shuffle+sort added if
join predicates ordering differ from bucketing and
sorting order
Sort Merge Join
TableScan TableScan
SELECT * FROM a JOIN b ON a.x=b.xAND a.y=b.y

55. [SPARK-19122] Unnecessary shuffle+sort added if
join predicates ordering differ from bucketing and
sorting order
SELECT * FROM a JOIN b ON a.x=b.xAND a.y=b.y
Sort Merge Join
TableScan TableScan
SELECT * FROM a JOIN b ON a.y=b.y AND a.x=b.x

56. [SPARK-19122] Unnecessary shuffle+sort added if
join predicates ordering differ from bucketing and
sorting order
Sort Merge Join
Exchange
TableScan
Sort
Exchange
TableScan
Sort
Sort Merge Join
TableScan TableScan
SELECT * FROM a JOIN b ON a.x=b.xAND a.y=b.y SELECT * FROM a JOIN b ON a.y=b.y AND a.x=b.x

57. [SPARK-19122] Unnecessary shuffle+sort added if
join predicates ordering differ from bucketing and
sorting order
Sort Merge Join
Exchange
TableScan
Sort
Exchange
TableScan
Sort
Sort Merge Join
TableScan TableScan
SELECT * FROM a JOIN b ON a.x=b.xAND a.y=b.y SELECT * FROM a JOIN b ON a.y=b.y AND a.x=b.x

58. [SPARK-19122] Unnecessary shuffle+sort added if
join predicates ordering differ from bucketing and
sorting order
Sort Merge Join
TableScan TableScan
SELECT * FROM a JOIN b ON a.x=b.xAND a.y=b.y SELECT * FROM a JOIN b ON a.y=b.y AND a.x=b.x
Sort Merge Join
TableScan TableScan

59. [SPARK-17271] Planner adds un-necessary Sort
even if child ordering is semantically same as
required ordering
SELECT * FROM table1 a JOIN table2 b ON a.col1=b.col1

60. [SPARK-17271] Planner adds un-necessary Sort
even if child ordering is semantically same as
required ordering
SELECT * FROM table1 a JOIN table2 b ON a.col1=b.col1
Sort Merge Join
Sort
TableScan
Sort
TableScan Already bucketed on column `col1`
Sort on columns (a.col1 and b.col1)
Join on [col1], [col1] respectively of relations

61. [SPARK-17271] Planner adds un-necessary Sort
even if child ordering is semantically same as
required ordering
SELECT * FROM table1 a JOIN table2 b ON a.col1=b.col1
Sort Merge Join
TableScan TableScan Already bucketed on column `col1`
Sort on columns (a.col1 and b.col1)Sort Sort
Join on [col1], [col1] respectively of relations

62. [SPARK-17271] Planner adds un-necessary Sort
even if child ordering is semantically same as
required ordering
SELECT * FROM table1 a JOIN table2 b ON a.col1=b.col1
Sort Merge Join
TableScan TableScan
Sort Sort
Sort Merge Join
TableScan TableScan

63. [SPARK-17698] Join predicates should not
contain filter clauses
SELECT a.id, b.id FROM table1 a FULL OUTER JOIN table2 b ON a.id= b.id AND a.id='1' AND b.id='1'

64. [SPARK-17698] Join predicates should not
contain filter clauses
SELECT a.id, b.id FROM table1 a FULL OUTER JOIN table2 b ON a.id= b.id AND a.id='1' AND b.id='1'
Sort Merge Join
TableScan TableScan
Sort
Exchange
Sort
Exchange
Already bucketed on column [id]
Shuffle on columns [id, 1, id] and [id, id, 1]
Sort on columns [id, id, 1] and [id, 1, id]
Join on [id, id, 1], [id, 1, id] respectively of relations

65. [SPARK-17698] Join predicates should not
contain filter clauses
SELECT a.id, b.id FROM table1 a FULL OUTER JOIN table2 b ON a.id = b.id AND a.id='1' AND b.id='1'
Sort Merge Join
TableScan TableScan
Sort
Exchange
Sort
Exchange
Already bucketed on column [id]
Shuffle on columns [id, 1, id] and [id, id, 1]
Sort on columns [id, id, 1] and [id, 1, id]
Join on [id, id, 1], [id, 1, id] respectively of relations

66. [SPARK-17698] Join predicates should not
contain filter clauses
SELECT a.id, b.id FROM table1 a FULL OUTER JOIN table2 b ON a.id = b.id AND a.id='1' AND b.id='1'
Sort Merge Join
TableScan TableScan
Sort
Exchange
Sort
Exchange
Already bucketed on column [id]
Shuffle on columns [id, 1, id] and [id, id, 1]
Sort on columns [id, id, 1] and [id, 1, id]
Join on [id, id, 1], [id, 1, id] respectively of relations

67. [SPARK-17698] Join predicates should not
contain filter clauses
SELECT a.id, b.id FROM table1 a FULL OUTER JOIN table2 b ON a.id = b.id AND a.id='1' AND b.id='1'
Sort Merge Join
TableScan TableScan
Sort Merge Join
TableScan TableScanSort
Exchange
Sort
Exchange

68. [SPARK-13450] Introduce
ExternalAppendOnlyUnsafeRowArray

69. [SPARK-13450] Introduce
ExternalAppendOnlyUnsafeRowArray
Buffered
relation
Streamed
relation
If there are a lot of rows to be
buffered (eg. Skew),
the query would OOM
row with
key=`foo`
rows with
key=`foo`

70. [SPARK-13450] Introduce
ExternalAppendOnlyUnsafeRowArray
Buffered
relation
Streamed
relation
row with
key=`foo`
rows with
key=`foo`
Disk
buffer would spill to disk
after reaching a threshold
spill

71. Summary
• Shuffle and sort is costly due to disk and network IO
• Bucketing will pre-(shuffle and sort) the inputs
• Expect at least 2-5x gains after bucketing input tables for joins
• Candidates for bucketing:
• Tables used frequently in JOINs with same key
• Loading of data in cumulative tables
• Spark’s bucketing is not compatible with Hive’s bucketing

72. Questions ?