Challenges of Building a First Class SQL-on-Hadoop Engine: Why and what is Big SQL 3.0? Overview of the challenges How we solved (some of) them Architecture and interaction with Hadoop Query rewrite Query optimization Future challenges

1. © 2014 IBM Corporation
Challenges of Building a First
Class SQL-on-Hadoop Engine
Scott C. Gray sgray@us.ibm.com @ScottCGrayIBM
Adriana Zubiri zubiri@ca.ibm.com @adrianaZubiri

2. Agenda
► Why and what is Big SQL 3.0?
• Not a sales pitch, I promise!
► Overview of the challenges
► How we solved (some of) them
• Architecture and interaction with Hadoop
• Query rewrite
• Query optimization
► Future challenges

3. The Perfect Storm
► Increase business interest on SQL on Hadoop to
improve the pace and efficiency of adopting Hadoop
► SQL engines on Hadoop moving away from MR
towards MPP architectures
► SQL users expect same level of language expressiveness,
features and (somewhat) performance as RDMSs
► IBM has decades of experience and assets on building
SQL engines… Why not leverage it?

4. The Result? Big SQL 3.0
► MapReduce replaced with a modern
MPP shared-nothing architecture
► Architected from the ground up
for low latency and high throughput
► Same SQL expressiveness as relational
RDBMs, which allows application portability
► Rich enterprise capabilities…

5. Big SQL 3.0 At a Glance

6. How did we do it?
► Big SQL is derived from an existing IBM shared-nothing RDBMS
• A very mature MPP architecture
• Already understands distributed joins and optimization
► Behavior is sufficiently different that
it is considered a separate product
• Certain SQL constructs are disabled
• Traditional data warehouse partitioning
is unavailable
• New SQL constructs introduced
► On the surface, porting a shared
nothing RDBMS to a shared nothing
cluster (Hadoop) seems easy, but …
database
partition
database
partition
database
partition
database
partition
Traditional Distributed RBMS Architecture

7. Challenges for a traditional RDBMS on Hadoop
► Data placement
• Traditional databases expect to have full control over data placement
• Data placement plays an important role in performance (e.g. co-located
joins)
• Hadoop’s randomly scattered data plays against the grain of this
► Reading and writing Hadoop files
• Normally an RDBMS has its own storage format
• Format is highly optimized to minimize cost of moving data into memory
• Hadoop has a practically unbounded number of storage formats all with
different capabilities

8. Challenges for a traditional RDBMS on Hadoop
► Query optimization
• Statistics on Hadoop are a relatively new concept
• The are frequently not available
• The database optimizer can use statistics not traditionally available in Hive
• Hive-style partitioning (grouping data into different files/directories) is a new
concept
► Resource management
• A database server almost always runs in isolation
• In Hadoop the nodes must be shared with many other tasks
– Data nodes
– MR task tracker and tasks
– HBase region servers, etc.
• We needed to learn to play nice with others

9. Architecture Overview
Management Node
Big SQL
Master Node
Management Node
Big SQL
Scheduler
Big SQL
Worker Node
Java
I/O
FMP
Native
I/O
FMP
HDFS
Data
Node
MRTask
Tracker
Other
ServiceHDFS
Data HDFS
Data HDFS
Data
Temp
Data
UDF
FMP
Compute Node
Database
Service
Hive
Metastore
Hive
Server
Big SQL
Worker Node
Java
I/O
FMP
Native
I/O
FMP
HDFS
Data
Node
MRTask
Tracker
Other
ServiceHDFS
Data HDFS
Data HDFS
Data
Temp
Data
UDF
FMP
Compute Node
Big SQL
Worker Node
Java
I/O
FMP
Native
I/O
FMP
HDFS
Data
Node
MRTask
Tracker
Other
ServiceHDFS
Data HDFS
Data HDFS
Data
Temp
Data
UDF
FMP
Compute Node
DDL
FMP
UDF
FMP *FMP = Fenced mode process

10. Big SQL Scheduler
► The Scheduler is the main RDBMS↔Hadoop service interface
• Interfaces with Hive metastore for table metadata
• Acts like the MapReduce job tracker for Big SQL
– Big SQL provides query predicates for
scheduler to perform partition elimination
– Determines splits for each “table” involved in the query
– Schedules splits on available Big SQL nodes
(favoring scheduling locally to the data)
– Serves work (splits) to I/O engines
– Coordinates “commits” after INSERTs
► Scheduler allows the database engine to
be largely unaware of the Hadoop world
Management Node
Big SQL
Master Node
Big SQL
Scheduler
DDL
FMP
UDF
FMP
Mgmt Node
Database
Service
Hive
Metastore
Big SQL
Worker Node
Java
I/O
FMP
Native
I/O
FMP
HDFS
Data
Node
MRTask
TrackerUDF
FMP

11. I/O Fence Mode Processes
► Native I/O FMP
• The high-speed interface for a limited number of common file formats
► Java I/O FMP
• Handles all other formats via standard Hadoop/Hive API’s
► Both perform multi-threaded direct I/O on local data
► The database engine had to be taught storage format capabilities
• Projection list is pushed into I/O format
• Predicates are pushed as close to the data as
possible (into storage format, if possible)
• Predicates that cannot be pushed down are
evaluated within the database engine
► The database engine is only aware of which nodes
need to read
• Scheduler directs the readers to their portion of work
Big SQL
Worker Node
Java
I/O
FMP
Native
I/O
FMP
HDFS
Data
Node
MRTask
Tracker
Other
ServiceHDFS
Data HDFS
Data HDFS
Data
Temp
Data
UDF
FMP
Compute Node

12. Mgmt Node
Big SQL
Master Node
Big SQL
Scheduler
DDL
FMP
UDF
FMP
Query Compilation There is a lot involved in SQL compilation
►Parsing
• Catch syntax errors
• Generate internal representation of query
►Semantic checking
• Determine if query makes sense
• Incorporate view definitions
• Add logic for constraint checking
►Query optimization
• Modify query to improve performance (Query Rewrite)
• Choose the most efficient “access plan”
►Pushdown Analysis
• Federation “optimization”
►Threaded code generation
• Generate efficient “executable” code

13. Query Rewrite
► Why is query re-write important?
• There are many ways to express the same query
• Query generators often produce suboptimal queries and don’t permit “hand optimization”
• Complex queries often result in redundancy, especially with views
• For Large data volumes optimal access plans more crucial as penalty for poor planning is
greater
select sum(l_extendedprice) / 7.0
avg_yearly
from tpcd.lineitem, tpcd.part
where p_partkey = l_partkey
and p_brand = 'Brand#23'
and p_container = 'MED BOX'
and l_quantity < ( select 0.2 *
avg(l_quantity) from tpcd.lineitem
where l_partkey = p_partkey);
select sum(l_extendedprice) / 7.0 as avg_yearly
from temp (l_quantity, avgquantity, l_extendeprice)
as
(select l_quantity, avg(l_quantity) over
(partition by l_partkey)
as avgquantity, l_extenedprice
from tpcd.lineitem, tpcd.part
where p_partkey = l_partkey
and p_brand = 'BRAND#23'
and p_container = 'MED BOX')
where l_quantity < 0.2 * avgquantity
• Query correlation eliminated
• Line item table accessed only once
• Execution time reduced in half!
• Query correlation eliminated
• Line item table accessed only once
• Execution time reduced in half!

14. Query Rewrite
► Most existing query rewrite rules remain unchanged
• 140+ existing query re-writes are leveraged
• Almost none are impacted by “the Hadoop world”
► There were however a few modifications that were required…

15. Query Rewrite and Indexes
► Column nullability and indexes can help drive query optimization
• Can produce more efficiently decorrelated subqueries and joins
• Used to prove uniqueness of joined rows (“early-out” join)
► Very few Hadoop data sources support
the concept of an index
► In the Hive metastore all columns
are implicitly nullable
► Big SQL introduces advisory
constraints and nullability indicators
• User can specify whether or not
constraints can be “trusted” for
query rewrites
create hadoop table users
(
id int not null primary key,
office_id int null,
fname varchar(30) not null,
lname varchar(30) not null,
salary timestamp(3) null,
constraint fk_ofc foreign key (office_id)
references office (office_id)
)
row format delimited
fields terminated by '|'
stored as textfile;
create hadoop table users
(
id int not null primary key,
office_id int null,
fname varchar(30) not null,
lname varchar(30) not null,
salary timestamp(3) null,
constraint fk_ofc foreign key (office_id)
references office (office_id)
)
row format delimited
fields terminated by '|'
stored as textfile;
Nullability IndicatorsNullability Indicators
ConstraintsConstraints

16. Query Pushdown
► Pushdown moves processing down as
close to the data as possible
• Projection pushdown – retrieve only
necessary columns
• Selection pushdown – push search criteria
► Big SQL understands the capabilities of
readers and storage formats involved
• As much as possible is pushed down
• Residual processing done in the server
• Optimizer costs queries based upon how
much can be pushed down
3) External Sarg Predicate,
Comparison Operator: Equal (=)
Subquery Input Required: No
Filter Factor: 0.04
Predicate Text:
--------------
(Q1.P_BRAND = 'Brand#23')
4) External Sarg Predicate,
Comparison Operator: Equal (=)
Subquery Input Required: No
Filter Factor: 0.025
Predicate Text:
--------------
(Q1.P_CONTAINER = 'MED BOX')
select sum(l_extendedprice) / 7.0 as avg_yearly
from temp (l_quantity, avgquantity, l_extendeprice) as
(select l_quantity, avg(l_quantity) over
(partition by l_partkey)
as avgquantity, l_extenedprice
from tpcd.lineitem, tpcd.part
where p_partkey = l_partkey
and p_brand = 'BRAND#23'
and p_container = 'MED BOX')
where l_quantity < 0.2 * avgquantity
select sum(l_extendedprice) / 7.0 as avg_yearly
from temp (l_quantity, avgquantity, l_extendeprice) as
(select l_quantity, avg(l_quantity) over
(partition by l_partkey)
as avgquantity, l_extenedprice
from tpcd.lineitem, tpcd.part
where p_partkey = l_partkey
and p_brand = 'BRAND#23'
and p_container = 'MED BOX')
where l_quantity < 0.2 * avgquantity

17. Statistics
► Big SQL utilizes Hive statistics
collection with some extensions:
• Additional support for column groups,
histograms and frequent values
• Automatic determination of partitions that
require statistics collection vs. explicit
• Partitioned tables: added table-level
versions of NDV, Min, Max, Null count,
Average column length
• Hive catalogs as well as database engine
catalogs are also populated
• We are restructuring the relevant code for
submission back to Hive
► Capability for statistic fabrication
if no stats available at compile time
Table statistics
• Cardinality (count)
• Number of Files
• Total File Size
Column statistics
• Minimum value (all types)
• Maximum value (all types)
• Cardinality (non-nulls)
• Distribution (Number of Distinct Values NDV)
• Number of null values
• Average Length of the column value (all types)
• Histogram - Number of buckets configurable
• Frequent Values (MFV) – Number configurable
Column group statistics

18. Costing Model
► Few extensions required to the Cost Model
► TBSCAN operator cost model extended
to evaluate cost of reading from Hadoop
► New elements taken into account:
# of files, size of files, # of partitions, # of nodes
► Optimizer now knows in which subset of
nodes the data resides
→ Better costing!
|
2.66667e-08
HSJOIN
( 7)
1.1218e+06
8351
/--------+--------
5.30119e+08 3.75e+07
BTQ NLJOIN
( 8) ( 11)
948130 146345
7291 1060
| /----+----
5.76923e+08 1 3.75e+07
LTQ GRPBY FILTER
( 9) ( 12) ( 20)
855793 114241 126068
7291 1060 1060
| | |
5.76923e+08 13 7.5e+07
TBSCAN TBSCAN BTQ
( 10) ( 13) ( 21)
802209 114241 117135
7291 1060 1060
| | |
7.5e+09 13 5.76923e+06
TABLE: TPCH5TB_PARQ TEMP LTQ
ORDERS ( 14) ( 22)
Q1 114241 108879
1060 1060
| |
13 5.76923e+06
DTQ TBSCAN
( 15) ( 23)
114241 108325
1060 1060
| |
1 7.5e+08
GRPBY TABLE: TPCH5TB_PARQ
( 16) CUSTOMER
114241 Q5
1060
|
1
LTQ
( 17)
114241
1060
|
1
GRPBY
( 18)
114241
1060
|
5.24479e+06
TBSCAN
( 19)
113931
1060
|
7.5e+08
TABLE: TPCH5TB_PARQ
CUSTOMER
Q2

19. We can access a Hadoop table as:
►“Scattered” Partitioned:
• Only accesses local data to the node
►Replicated:
• Accesses local and remote data
– Optimizer could also use a broadcast table queue
– HDFS shared file system provides replication
New Access Plans
Data not hash partitioned on a particular columns
(aka “Scattered partitioned”)
New Parallel Join Strategy
introduced
New Parallel Join Strategy
introduced

20. Parallel Join Strategies
Replicated vs. Broadcast join
All tables are “scatter” partitioned
Join predicate:
STORE.STOREKEY = DAILY_SALES.STOREKEY
19
Replicate smaller table to partitions
of the larger table using:
• Broadcast table queue
• Replicated HDFS scan
Table Queue represents
communication between
nodes or subagents
JOIN
Store
Daily Sales
SCAN
SCAN
Broadcast
TQ SCAN
replicated SCAN

21. Parallel Join Strategies
Repartitioned join
All tables are “scatter” partitioned
Join predicate:
DAILY_FORECAST.STOREKEY = DAILY_SALES.STOREKEY
20
• Both tables large
• Too expensive to broadcast or
replicate either
• Repartition both tables on the
join columns
• Use directed table queue (DTQ)
JOIN
Daily
Forecast
Daily Sales
SCAN SCAN
Directed
TQ
Directed
TQ

22. Future Challenges
► The challenges never end!
• That’s what makes this job fun!
• The Hadoop ecosystem continues to expand
• New storage techniques, indexing techniques, etc.
► Here are a few areas we’re exploring….

23. Future Challenges
► Dynamic split allocation
• React to competing workloads
• If one node is slow, hand work you would have handed it to another node
► More pushdown!
• Currently we push projection/selection down
• Should we push more advanced operations? Aggregation? Joins?
► Join co-location
• Perform co-located joins when tables are partitioned on the same join key
► Explicit MapReduce style parallelism (“SQL MR”)
• Expand SQL to explicitly perform partitioned operations

24. Queries?
(Optimized, of course)
Try Big SQL 3.0 Beta on the cloud!
https://bigsql.imdemocloud.com/
Scott C. Gray sgray@us.ibm.com @ScottCGrayIBM
Adriana Zubiri zubiri@ca.ibm.com @adrianaZubiri