Cost-Based Optimizer in Apache Spark 2.2 Ron Hu, Sameer Agarwal, Wenchen Fan and Zhenhua Wang

Ron Hu, Zhenhua Wang
Huawei Technologies, Inc.
Sameer Agarwal, Wenchen Fan
Databricks Inc.
Cost-Based Optimizer in
Apache Spark 2.2

Session 1 Topics
• Motivation
• Statistics Collection Framework
• Cost Based Optimizations
• TPC-DS Benchmark and Query Analysis
• Demo
2

How Spark Executes a Query?
Logical
Plan
Physical
Plan
Catalog
Optimizer
RDDs
…
SQL
Code
Generator
Data
Frames

How Spark Executes a Query?
Logical
Plan
Physical
Plan
Catalog
Optimizer
RDDs
…
SQL
Code
Generator
Data
Frames
Focus of Today’s Talk

Catalyst Optimizer: An Overview
5
events =
sc.read.json(“/logs”)
stats =
events.join(users)
.groupBy(“loc”,“status”)
.avg(“duration”)
errors = stats.where(
stats.status == “ERR”)
Query Plan is an
internal representation
of a user’s program
Series of Transformations
that convert the initial query
plan into an optimized plan
SCAN logs
JOIN
FILTER
AGG
SCAN users SCAN logsSCAN users
JOIN
FILTER
AGG
SCAN users

Catalyst Optimizer: An Overview
6
In Spark, the optimizer’s goal is to
minimize end-to-end query response time.
Two key ideas:
- Prune unnecessary data as early as possible
- e.g., filter pushdown, column pruning
- Minimize per-operator cost
- e.g., broadcast vs shuffle, join re-ordering
SCAN logsSCAN users
JOIN
FILTER
AGG
SCAN users

Rule-based Optimizer in Spark 2.1
• Most of Spark SQL optimizer’s rules are heuristics rules.
– PushDownPredicate, ColumnPruning,
ConstantFolding,…
• Does NOT consider the cost of each operator
• Does NOT consider selectivity when estimating join relation size
• Join order is mostly decided by its position in the SQL queries
• Physical Join implementation is decided based on heuristics
7

An Example (TPC-DS q11 variant)
8
SCAN: store_sales SCAN: customer
SCAN: date_dim
FILTER
JOIN
JOIN
SELECT customer_id
FROM customer, store_sales, date_dim
WHERE c_customer_sk = ss_customer_sk AND
ss_sold_date_sk = d_date_sk AND
c_customer_sk > 1000

9
SCAN: store_sales SCAN: customer
SCAN: date_dim
FILTER
JOIN
JOIN
3 billion 12 million
2.5 billion
10 million
500 million
0.1 million

10
SCAN: store_sales
SCAN: customer
SCAN: date_dim
FILTERJOIN
JOIN
3 billion
12 million
2.5 billion 500 million 10 million
500 million
0.1 million
40% faster
80% less data

11
SCAN: store_sales
SCAN: customer
SCAN: date_dim
FILTERJOIN
JOIN
3 billion
12 million
2.5 billion 500 million 10 million
500 million
0.1 million
How do we automatically optimize queries like these?

Cost Based Optimizer (CBO)
• Collect, infer and propagate table/column
statistics on source/intermediate data
• Calculate the cost for each operator in terms of
number of output rows, size of output, etc.
• Based on the cost calculation, pick the most
optimal query execution plan
12

Rest of the Talk
– Table/Column Level Statistics Collected
– Cardinality Estimation (Filters, Joins, Aggregates etc.)
• Cost-based Optimizations
– Build Side Selection
– Multi-way Join Re-ordering
• TPC-DS Benchmarks
• Demo
13

Statistics Collection Framework
and Cost Based Optimizations
Ron Hu
Huawei Technologies

Step 1: Collect, infer and propagate table
and column statistics on source and
intermediate data

Table Statistics Collected
• Command to collect statistics of a table.
– Ex: ANALYZE TABLE table-name COMPUTE
STATISTICS
• It collects table level statistics and saves into
metastore.
– Number of rows
– Table size in bytes
16

Column Statistics Collected
• Command to collect column level statistics of individual columns.
– Ex: ANALYZE TABLE table-name COMPUTE STATISTICS
FOR COLUMNS column-name1, column-name2, ….
• It collects column level statistics and saves into meta-store.
String/Binary type
✓ Distinct count
✓ Null count
✓ Average length
✓ Max length
Numeric/Date/Timestamp type
✓ Distinct count
✓ Max
✓ Min
✓ Null count
✓ Average length (fixed length)
✓ Max length (fixed length)
17

Filter Cardinality Estimation
• Between Logical expressions: AND, OR, NOT
• In each logical expression: =, <, <=, >, >=, in, etc
• Current support type in Expression
– For <, <=, >, >=, <=>: Integer, Double, Date, Timestamp, etc
– For = , <=>: String, Integer, Double, Date, Timestamp, etc.
• Example:A <= B
– Based on A, B’s min/max/distinct count/null count values, decide
the relationships between A and B. After completing this
expression, we set the new min/max/distinct count/null count
– Assume all the data is evenly distributed if no histogram
information.
18

Filter Operator Example
• Column A (op) literal B
– (op) can be “=“, “<”, “<=”, “>”, “>=”, “like”
– Like the styles as “l_orderkey = 3”, “l_shipdate <= “1995-03-21”
– Column’s max/min/distinct count/null count should be updated
– Example: Column A < value B
Column AB B
A.min A.max
Filtering Factor =0%
need to change A’s statistics
Filtering Factor =100%
no need to change A’s statistics
Without histograms, suppose data is evenly distributed
Filtering Factor =(B.value – A.min) / (A.max – A.min)
A.min = no change
A.max = B.value
A.ndv = A.ndv * Filtering Factor
19

Filter Operator Example
• Column A (op) Column B
– (op) can be “<”, “<=”, “>”, “>=”
– We cannot suppose the data is evenly distributed, so the empirical filtering factor is set to 1/3
– Example: Column A < Column B
B
A
AA
A
B
B B
selectivity =100% selectivity =0%
selectivity =33.3%
20
selectivity =33.3%

Join Cardinality Estimation
• Inner-Join: The number of rows of “A join B on A.k1 = B.k1” is
estimated as: num(A B) = num(A) * num(B) / max(distinct(A.k1),
distinct(B.k1)),
– where num(A) is the number of records in table A, distinct is the number of
distinct values of that column.
– The underlying assumption for this formula is that each value of the smaller
domain is included in the larger domain.
• We similarly estimate cardinalities for Left-Outer Join, Right-Outer
Join and Full-Outer Join
21

Other Operator Estimation
• Project: does not change row count
• Aggregate: consider uniqueness of group-by
columns
• Limit, Sample, etc.
22

Step 2: Cost Estimation and Optimal
Plan Selection

Build Side Selection
• For two-way hash joins, we need to choose one operand as build side and the
other as probe side.
• Choose lower-cost child as build side of hash join.
– Before: build side was selected based on original table sizes. ➔ BuildRight
– Now with CBO: build side is selected based on
estimated cost of various operators before join. ➔ BuildLeft
Join
Scan t2Filter
Scan t15 billion records,
500 GB
t1.value =200
1 million records,
100 MB
100 million records,
20 GB
24

Hash Join Implementation: Broadcast vs. Shuffle
Physical Plan
➢ SortMergeJoinExec/
BroadcastHashJoinExec/
ShuffledHashJoinExec
➢ CartesianProductExec/
BroadcastNestedLoopJoinExec
Logical Plan
➢ Equi-join
• Inner Join
• LeftSemi/LeftAnti Join
• LeftOuter/RightOuter Join
➢ Theta-join
• Broadcast Criterion: whether the join side’s output size is small (default 10MB).
Join
Scan t2Filter
Scan t15 billion records,
500 GB
t1.value = 100
Only 1000 records,
100 KB
100 million records,
20 GB
Join
Scan t2Aggregate
…
Join
Scan t2Join
… …
25

Multi-way Join Reorder
• Reorder the joins using a dynamic programming algorithm.
1. First we put all items (basic joined nodes) into level 0.
2. Build all two-way joins at level 1 from plans at level 0 (single items).
3. Build all 3-way joins from plans at previous levels (two-way joins and single items).
4. Build all 4-way joins etc, until we build all n-way joins and pick the best plan among
them.
• When building m-way joins, only keep the best plan (optimal sub-solution)
for the same set of m items.
– E.g., for 3-way joins of items {A, B, C}, we keep only the best plan
among: (A J B) J C, (A J C) J B and (B J C) J A
26

Multi-way Join Reorder
27
Selinger et al. Access Path Selection in a Relational Database Management System. In SIGMOD 1979

Join Cost Formula
• The cost of a plan is the sum of costs of all intermediate tables.
• Cost = weight X Costcpu + CostIO X (1 - weight)
– In Spark, we use
weight * cardinality + size * (1 – weight)
– weight is a tuning parameter configured via
spark.sql.cbo.joinReorder.card.weight (0.7 as
default)
28

TPC-DS Benchmarks and
Query Analysis
Zhenhua Wang
Huawei Technologies

Session 2 Topics
• Motivation
• Cost Based Optimizations
• TPC-DS Benchmark and Query Analysis
• Demo
30

Preliminary Performance Test
• Setup:
− TPC-DS size at 1 TB (scale factor 1000)
− 4 node cluster (Huawei FusionServer RH2288: 40 cores, 384GB mem)
− Apache Spark 2.2 RC (dated 5/12/2017)
• Statistics collection
– A total of 24 tables and 425 columns
➢ Take 14 minutes to collect statistics for all tables and all columns.
– Fast because all statistics are computed by integrating with Spark’s built-in
aggregate functions.
– Should take much less time if we collect statistics for columns used in predicate,
join, and group-by only.
31

TPC-DS Query Q11
32
WITH year_total AS (
SELECT
c_customer_id customer_id,
c_first_name customer_first_name,
c_last_name customer_last_name,
c_preferred_cust_flag customer_preferred_cust_flag,
c_birth_country customer_birth_country,
c_login customer_login,
c_email_address customer_email_address,
d_year dyear,
sum(ss_ext_list_price - ss_ext_discount_amt) year_total,
's' sale_type
FROM customer, store_sales, date_dim
WHERE c_customer_sk =ss_customer_sk
AND ss_sold_date_sk = d_date_sk
GROUP BY c_customer_id, c_first_name, c_last_name, d_year
, c_preferred_cust_flag, c_birth_country, c_login, c_email_address, d_year
UNION ALL
SELECT
c_customer_id customer_id,
c_first_name customer_first_name,
c_last_name customer_last_name,
c_preferred_cust_flag customer_preferred_cust_flag,
c_birth_country customer_birth_country,
c_login customer_login,
c_email_address customer_email_address,
d_year dyear,
sum(ws_ext_list_price - ws_ext_discount_amt)year_total,
'w' sale_type
FROM customer, web_sales, date_dim
WHERE c_customer_sk =ws_bill_customer_sk AND ws_sold_date_sk = d_date_sk
GROUP BY c_customer_id, c_first_name, c_last_name, c_preferred_cust_flag,
c_birth_country, c_login, c_email_address, d_year)
SELECT t_s_secyear.customer_preferred_cust_flag
FROM year_total t_s_firstyear
, year_total t_s_secyear
, year_total t_w_firstyear
, year_total t_w_secyear
WHERE t_s_secyear.customer_id = t_s_firstyear.customer_id
AND t_s_firstyear.customer_id = t_w_secyear.customer_id
AND t_s_firstyear.customer_id = t_w_firstyear.customer_id
AND t_s_firstyear.sale_type = 's'
AND t_w_firstyear.sale_type = 'w'
AND t_s_secyear.sale_type = 's'
AND t_w_secyear.sale_type = 'w'
AND t_s_firstyear.dyear= 2001
AND t_s_secyear.dyear = 2001 + 1
AND t_w_firstyear.dyear = 2001
AND t_w_secyear.dyear= 2001 + 1
AND t_s_firstyear.year_total > 0
AND t_w_firstyear.year_total >0
AND CASE WHEN t_w_firstyear.year_total > 0
THEN t_w_secyear.year_total / t_w_firstyear.year_total
ELSE NULL END
> CASE WHEN t_s_firstyear.year_total > 0
THEN t_s_secyear.year_total / t_s_firstyear.year_total
ELSE NULL END
ORDER BY t_s_secyear.customer_preferred_cust_flag
LIMIT 100

Query Analysis – Q11 CBO OFF
Large join result
Join #1
store_sales customer
date_dim
2.9 billion
…
…
Join #2
web_sales customer
date_dim
Join #4
…
Join #3
12 million
2.7 billion 73,049 73,049
12 million720 million
534 million
719 million
144 million
33

Query Analysis – Q11 CBO ON
Small join result
Join #1
store_sales date_dim
customer
2.9 billion
…
…
Join #2
web_sales date_dim
customer
Join #4
…
Join #3
73,049
534 million 12 million 12 million
73,049720 million
534 million
144 million
144 million
1.4x Speedup
34
80% less

TPC-DS Query Q72
35
SELECT
i_item_desc,
w_warehouse_name,
d1.d_week_seq,
count(CASE WHEN p_promo_sk IS NULL
THEN 1
ELSE 0 END) no_promo,
count(CASE WHEN p_promo_sk IS NOT NULL
THEN 1
ELSE 0 END) promo,
count(*) total_cnt
FROM catalog_sales
JOIN inventory ON (cs_item_sk = inv_item_sk)
JOIN warehouse ON (w_warehouse_sk = inv_warehouse_sk)
JOIN item ON (i_item_sk = cs_item_sk)
JOIN customer_demographics ON (cs_bill_cdemo_sk = cd_demo_sk)
JOIN household_demographics ON (cs_bill_hdemo_sk = hd_demo_sk)
JOIN date_dim d1 ON (cs_sold_date_sk = d1.d_date_sk)
JOIN date_dim d2 ON (inv_date_sk = d2.d_date_sk)
JOIN date_dim d3 ON (cs_ship_date_sk = d3.d_date_sk)
LEFT OUTER JOIN promotion ON (cs_promo_sk = p_promo_sk)
LEFT OUTER JOIN catalog_returns ON (cr_item_sk = cs_item_sk AND cr_order_number = cs_order_number)
WHERE d1.d_week_seq = d2.d_week_seq
AND inv_quantity_on_hand < cs_quantity
AND d3.d_date > (cast(d1.d_date AS DATE) + interval 5 days)
AND hd_buy_potential = '>10000'
AND d1.d_year = 1999
AND hd_buy_potential = '>10000'
AND cd_marital_status = 'D'
AND d1.d_year = 1999
GROUP BY i_item_desc, w_warehouse_name, d1.d_week_seq
ORDER BY total_cnt DESC, i_item_desc, w_warehouse_name, d_week_seq
LIMIT 100

Join #1
Join #2
Join #3
Join #4
Join #5
Join #6
Join #7
Join #8
catalog_sales inventory
warehouse
item
customer_demographics
date_dim d1
date_dim d2
date_dim d3
household_demographics
1.4 billion 783 million
223 billion 20
300,000223 billion
223 billion
44.7 million
7.5 million
1.6 million
9 million
1.9 million
7,200
73,049
73,049
73,049
8.7 million
Really large
intermediate
results
36

Join #1
Join #2
Join #3
Join #4
Join #8
Join #5
Join #6
Join #7
catalog_sales
inventory
warehouseitem
customer_demographics
date_dim d1 date_dim d2date_dim d3
household_demographics
1.4 billion
783 million
20300,000
1.9 million
7,200
73,049 73,049 73,049
238 million
238 million
47.6 million
47.6 million
2,555
1 billion
1 billion
8.7 million
Much smaller
intermediate
results !
37
2-3 orders of
magnitude less!
8.1x Speedup

TPC-DS Query Performance
38
0
350
700
1050
1400
1750
q1 q5 q9 q13 q16 q20 q23bq26 q30 q34 q38 q41 q45 q49 q53 q57 q61 q65 q70 q74 q78 q82 q86 q90 q94 q98
Runtime(seconds)
without CBO
with CBO

TPC-DS Query Speedup
• TPC-DS query speedup
ratio with CBO versus
without CBO
• 16 queries show speedup
> 30%
• The max speedup is 8X.
• The geo-mean of
speedup is 2.2X.
39

TPC-DS Query 64
40
WITH cs_ui AS
(SELECT
cs_item_sk,
sum(cs_ext_list_price) AS sale,
sum(cr_refunded_cash + cr_reversed_charge + cr_store_credit) AS refund
FROM catalog_sales, catalog_returns
WHERE cs_item_sk = cr_item_sk AND cs_order_number = cr_order_number
GROUP BY cs_item_sk
HAVING sum(cs_ext_list_price) > 2 * sum(cr_refunded_cash + cr_reversed_charge + cr_store_credit)),
cross_sales AS
(SELECT
i_product_name product_name, i_item_sk item_sk, s_store_name store_name,
s_zip store_zip, ad1.ca_street_number b_street_number, ad1.ca_street_name b_streen_name,
ad1.ca_city b_city, ad1.ca_zip b_zip, ad2.ca_street_number c_street_number,
ad2.ca_street_name c_street_name, ad2.ca_city c_city, ad2.ca_zip c_zip,
d1.d_year AS syear, d2.d_year AS fsyear, d3.d_year s2year,
count(*) cnt, sum(ss_wholesale_cost) s1, sum(ss_list_price) s2, sum(ss_coupon_amt) s3
FROM store_sales, store_returns, cs_ui, date_dim d1, date_dim d2, date_dim d3,
store, customer, customer_demographics cd1, customer_demographics cd2,
promotion, household_demographics hd1, household_demographics hd2,
customer_address ad1, customer_address ad2, income_band ib1, income_band ib2, item
WHERE ss_store_sk = s_store_sk AND ss_sold_date_sk = d1.d_date_sk AND
ss_customer_sk = c_customer_sk AND ss_cdemo_sk = cd1.cd_demo_sk AND
ss_hdemo_sk = hd1.hd_demo_sk AND ss_addr_sk = ad1.ca_address_sk AND
ss_item_sk = i_item_sk AND ss_item_sk = sr_item_sk AND
ss_ticket_number = sr_ticket_number AND ss_item_sk = cs_ui.cs_item_sk AND
c_current_cdemo_sk = cd2.cd_demo_sk AND c_current_hdemo_sk = hd2.hd_demo_sk AND
c_current_addr_sk = ad2.ca_address_sk AND c_first_sales_date_sk = d2.d_date_sk AND
c_first_shipto_date_sk = d3.d_date_sk AND ss_promo_sk = p_promo_sk AND
hd1.hd_income_band_sk = ib1.ib_income_band_sk AND
hd2.hd_income_band_sk = ib2.ib_income_band_sk AND
cd1.cd_marital_status <> cd2.cd_marital_status AND
i_color IN ('purple', 'burlywood', 'indian', 'spring', 'floral', 'medium') AND
i_current_price BETWEEN 64 AND 64 + 10 AND i_current_price BETWEEN 64 + 1 AND 64 + 15
GROUP BY i_product_name, i_item_sk, s_store_name, s_zip, ad1.ca_street_number,
ad1.ca_street_name, ad1.ca_city, ad1.ca_zip, ad2.ca_street_number,
ad2.ca_street_name, ad2.ca_city, ad2.ca_zip, d1.d_year, d2.d_year, d3.d_year)
SELECT
cs1.product_name,
cs1.store_name,
cs1.store_zip,
cs1.b_street_number,
cs1.b_streen_name,
cs1.b_city,
cs1.b_zip,
cs1.c_street_number,
cs1.c_street_name,
cs1.c_city,
cs1.c_zip,
cs1.syear,
cs1.cnt,
cs1.s1,
cs1.s2,
cs1.s3,
cs2.s1,
cs2.s2,
cs2.s3,
cs2.syear,
cs2.cnt
FROM cross_sales cs1, cross_sales cs2
WHERE cs1.item_sk = cs2.item_sk AND
cs1.syear = 1999 AND
cs2.syear = 1999 + 1 AND
cs2.cnt <= cs1.cnt AND
cs1.store_name = cs2.store_name AND
cs1.store_zip = cs2.store_zip
ORDER BY cs1.product_name, cs1.store_name, cs2.cnt

41
10% slower
FileScan (store_sales)
Exchange
ExchangeSort
Aggregate
SortMergeJoin
BroadcastHashJoin
FileScan (store_returns)
Exchange
Sort
BroadcastExchange (cs_ui)
ReusedExchange
Sort Sort
BroadcastHashJoin
ReusedExchange
ReusedExchange
Sort
SortMergeJoin
Sort
Fragment 1
Fragment 2

42
FileScan (store_sales)
Exchange
Exchange
Sort
Aggregate
SortMergeJoin
SortMergeJoin
FileScan (store_returns)
Exchange
Sort
Sort (cs_ui)
SortMergeJoin
Sort
ReusedExchange
Aggregate
Sort (cs_ui)
ReusedExchange
Sort
Exchange
Fragment 1
Fragment 2

CBO Demo
Wenchen Fan
Databricks

Current Status, Credits and
Future Work
Ron Hu
Huawei Technologies

Available in Apache Spark 2.2
• Configured via spark.sql.cbo.enabled
• ‘Off By Default’. Why?
– Spark is used in production
– Many Spark users may already rely on “human
intelligence” to write queries in best order
– Plan on enabling this by default in Spark 2.3
• We encourage you test CBO with Spark 2.2!
45

Current Status
• SPARK-16026 is the umbrella jira.
– 32 sub-tasks have been resolved
– A big project spanning 8 months
– 10+ Spark contributors involved
– 7000+ lines of Scala code have been contributed
• Good framework to allow integrations
– Use statistics to derive if a join attribute is unique
– Benefit star schema detection and its integration into join
reorder
46

Birth of Spark SQL CBO
• Prototype
– In 2015, Ron Hu, Fang Cao, etc. of Huawei’s research
department prototyped the CBO concept on Spark 1.2.
– After a successful prototype, we shared technology with
Zhenhua Wang, Fei Wang, etc of Huawei’s product
development team.
• We delivered a talk at Spark Summit 2016:
– “Enhancing Spark SQL Optimizer with Reliable Statistics”.
• The talk was well received by the community.
– https://issues.apache.org/jira/browse/SPARK-16026
47

Collaboration
• Good community support
– Developers: Zhenhua Wang, Ron Hu, Reynold Xin,
Wenchen Fan, Xiao Li
– Reviewers: Wenchen, Herman, Reynold, Xiao, Liang-chi,
Ioana, Nattavut, Hyukjin, Shuai, …..
– Extensive discussion in JIRAs and PRs (tens to hundreds
conversations).
– All the comments made the development time longer, but
improved code quality.
• It was a pleasure working with community.
48

Future Work: Cost Based Optimizer
• Current cost formula is coarse.
Cost = cardinality * weight + size * (1 - weight)
• Cannot tell the cost difference between sort-
merge join and hash join
– spark.sql.join.preferSortMergeJoin defaults to true.
• Underestimates (or ignores) shuffle cost.
• Will improve cost formula in next release.
49

Future Work: Statistics Collection Framework
• Advanced statistics: e.g. histograms, sketches.
• Hint mechanism.
• Partition level statistics.
• Speed up statistics collection by sampling data
for large tables.
50

Conclusion
• Motivation
– Table/Column Level Statistics Collected
– Cardinality Estimation (Filters, Joins, Aggregates etc.)
• Cost-based Optimizations
– Build Side Selection
– Multi-way Join Re-ordering
• TPC-DS Benchmarks
• Demo
51

Thank You.
ron.hu@huawei.com
wangzhenhua@huawei.com
sameer@databricks.com
wenchen@databricks.com

Multi-way Join Reorder – Example
• Given A J B J C J D with join conditions A.k1 = B.k1 and
B.k2 = C.k2 and C.k3 = D.k3
level 0: p({A}), p({B}), p({C}), p({D})
level 1: p({A, B}), p({A, C}), p({A, D}), p({B, C}), p({B, D}), p({C, D})
level 2: p({A, B, C}), p({A, B, D}), p({A, C, D}), p({B, C, D})
level 3: p({A, B, C, D}) -- final output plan
53

Multi-way Join Reorder – Example
• Pruning strategy: exclude cartesian product candidates.
This significantly reduces the search space.
level 0: p({A}), p({B}), p({C}), p({D})
level 1: p({A, B}), p({A, C}), p({A, D}), p({B, C}), p({B, D}), p({C, D})
level 2: p({A, B, C}), p({A, B, D}), p({A, C, D}), p({B, C, D})
level 3: p({A, B, C, D}) -- final output plan
54

New Commands in Apache Spark 2.2
• CBO commands
– Collect table-level statistics
• ANALYZE TABLE table_name COMPUTE STATISTICS
– Collect column-level statistics
• ANALYZE TABLE table-name COMPUTE STATISTICS FOR COLUMNS column_name1,
column_name2, …
– Display statistics in the optimized logical plan
> EXPLAIN COST
> SELECTcc_call_center_sk,cc_call_center_id FROM call_center;
…
== Optimized Logical Plan ==
Project[cc_call_center_sk#75,cc_call_center_id#76],Statistics(sizeInBytes=1680.0 B,rowCount=42,hints=none)
+- Relation[…31 fields]parquet,Statistics(sizeInBytes=22.5 KB,rowCount=42,hints=none)
…
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Cost-Based Optimizer in Apache Spark 2.2 Ron Hu, Sameer Agarwal, Wenchen Fan and Zhenhua Wang

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