Download Data Warehousing Overview: Issues, Terminology

Data Warehousing Overview
CS245 Notes 11
Hector Garcia-Molina
Stanford University
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Notes11
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Warehousing
Growing industry: $8 billion in 1998
 Range from desktop to huge:

 Walmart:
900-CPU, 2,700 disk, 23TB
Teradata system

Lots of buzzwords, hype
 slice
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& dice, rollup, MOLAP, pivot, ...
Notes11
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Outline
What is a data warehouse?
 Why a warehouse?
 Models & operations
 Implementing a warehouse
 Future directions

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What is a Warehouse?

Collection of diverse data
 subject
oriented
 aimed at executive, decision maker
 often a copy of operational data
 with value-added data (e.g., summaries, history)
 integrated
 time-varying
 non-volatile
more
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What is a Warehouse?

Collection of tools
 gathering
data
 cleansing, integrating, ...
 querying, reporting, analysis
 data mining
 monitoring, administering warehouse
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Warehouse Architecture
Client
Client
Query & Analysis
Metadata
Warehouse
Integration
Source
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Source
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Source
6
Motivating Examples
Forecasting
 Comparing performance of units
 Monitoring, detecting fraud
 Visualization

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Why a Warehouse?

Two Approaches:
 Query-Driven
(Lazy)
 Warehouse (Eager)
?
Source
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Source
8
Query-Driven Approach
Client
Client
Mediator
Wrapper
Source
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Wrapper
Wrapper
Source
Source
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Advantages of Warehousing
High query performance
 Queries not visible outside warehouse
 Local processing at sources unaffected
 Can operate when sources unavailable
 Can query data not stored in a DBMS
 Extra information at warehouse

 Modify,
summarize (store aggregates)
 Add historical information
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Advantages of Query-Driven

No need to copy data
 less
storage
 no need to purchase data
More up-to-date data
 Query needs can be unknown
 Only query interface needed at sources
 May be less draining on sources

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OLTP vs. OLAP

OLTP: On Line Transaction Processing
 Describes

processing at operational sites
OLAP: On Line Analytical Processing
 Describes
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processing at warehouse
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OLTP vs. OLAP
OLTP







OLAP
Mostly updates
Many small transactions
Mb-Tb of data
Raw data
Clerical users
Up-to-date data
Consistency,
recoverability critical
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




Notes11
Mostly reads
Queries long, complex
Gb-Tb of data
Summarized,
consolidated data
Decision-makers,
analysts as users
13
Data Marts
Smaller warehouses
 Spans part of organization

 e.g.,

marketing (customers, products, sales)
Do not require enterprise-wide consensus
 but
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long term integration problems?
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Warehouse Models & Operators

Data Models
 relations
 stars
& snowflakes
 cubes

Operators
 slice
& dice
 roll-up, drill down
 pivoting
 other
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Star
product
prodId
p1
p2
name price
bolt
10
nut
5
sale oderId date
o100 1/7/97
o102 2/7/97
105 3/8/97
customer
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custId
53
81
111
custId
53
53
111
prodId
p1
p2
p1
name
joe
fred
sally
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storeId
c1
c1
c3
address
10 main
12 main
80 willow
store
storeId
c1
c2
c3
qty
1
2
5
amt
12
11
50
city
nyc
sfo
la
city
sfo
sfo
la
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Star Schema
product
prodId
name
price
sale
orderId
date
custId
prodId
storeId
qty
amt
customer
custId
name
address
city
store
storeId
city
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Terms
Fact table
 Dimension tables
 Measures

product
prodId
name
price
sale
orderId
date
custId
prodId
storeId
qty
amt
customer
custId
name
address
city
store
storeId
city
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Dimension Hierarchies
sType
store
store storeId
s5
s7
s9
city
cityId
sfo
sfo
la
tId
t1
t2
t1
mgr
joe
fred
nancy
region
sType tId
t1
t2
city
 snowflake schema
 constellations
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size
small
large
cityId pop
sfo
1M
la
5M
location
downtown
suburbs
regId
north
south
region regId
name
north cold region
south warm region
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Cube
Fact table view:
sale
prodId storeId
p1
c1
p2
c1
p1
c3
p2
c2
Multi-dimensional cube:
amt
12
11
50
8
p1
p2
c1
12
11
c2
c3
50
8
dimensions = 2
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3-D Cube
Fact table view:
sale
prodId
p1
p2
p1
p2
p1
p1
storeId
c1
c1
c3
c2
c1
c2
Multi-dimensional cube:
date
1
1
1
1
2
2
amt
12
11
50
8
44
4
day 2
day 1
p1
p2 c1
p1
12
p2
11
c1
44
c2
4
c2
c3
c3
50
8
dimensions = 3
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ROLAP vs. MOLAP
ROLAP:
Relational On-Line Analytical Processing
 MOLAP:
Multi-Dimensional On-Line Analytical
Processing

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Aggregates
• Add up amounts for day 1
• In SQL: SELECT sum(amt) FROM SALE
WHERE date = 1
sale
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prodId storeId
p1
c1
p2
c1
p1
c3
p2
c2
p1
c1
p1
c2
date
1
1
1
1
2
2
amt
12
11
50
8
44
4
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81
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Aggregates
• Add up amounts by day
• In SQL: SELECT date, sum(amt) FROM SALE
GROUP BY date
sale
prodId storeId
p1
c1
p2
c1
p1
c3
p2
c2
p1
c1
p1
c2
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date
1
1
1
1
2
2
amt
12
11
50
8
44
4
Notes11
ans
date
1
2
sum
81
48
24
Another Example
• Add up amounts by day, product
• In SQL: SELECT date, sum(amt) FROM SALE
GROUP BY date, prodId
sale
prodId storeId
p1
c1
p2
c1
p1
c3
p2
c2
p1
c1
p1
c2
date
1
1
1
1
2
2
amt
12
11
50
8
44
4
sale
prodId
p1
p2
p1
date
1
1
2
amt
62
19
48
rollup
drill-down
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Aggregates
Operators: sum, count, max, min,
median, ave
 “Having” clause
 Using dimension hierarchy

 average
by region (within store)
 maximum by month (within date)
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Cube Aggregation
day 2
day 1
p1
p2 c1
p1
12
p2
11
p1
p2
c1
56
11
c1
44
c2
4
c2
c3
c3
50
8
c2
4
8
rollup
drill-down
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Example: computing sums
...
c3
50
sum
c1
67
c2
12
c3
50
129
p1
p2
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sum
110
19
27
Cube Operators
day 2
day 1
p1
p2 c1
p1
12
p2
11
c1
56
11
p1
p2
c1
44
c2
4
c2
...
c3
50
sale(c1,*,*)
8
c2
4
8
c3
50
sale(c2,p2,*)
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c3
sum
c1
67
c2
12
c3
50
129
p1
p2
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sum
110
19
sale(*,*,*)
28
Extended Cube
c2
4
8
c312
p1
p2
c1
*
12
p1
p2
c1*
44
c1
56
11
c267
4
c2
44
c3
4
50
11
23
8
8
50
*
62
19
81
*
day 2
day 1
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p1
p2
*
Notes11
c3
50
* 50
48
48
*
110
19
129
sale(*,p2,*)
29
Aggregation Using Hierarchies
day 2
day 1
p1
p2 c1
p1
12
p2
11
c1
44
c2
4
c2
c3
c3
50
customer
region
8
country
p1
p2
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region A region B
56
54
11
8
(customer c1 in Region A;
customers c2, c3 in Region B)
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Pivoting
Fact table view:
sale
prodId storeId
p1
c1
p2
c1
p1
c3
p2
c2
p1
c1
p1
c2
Multi-dimensional cube:
date
1
1
1
1
2
2
amt
12
11
50
8
44
4
day 2
day 1
p1
p2 c1
p1
12
p2
11
p1
p2
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Notes11
c1
56
11
c1
44
c2
4
c2
c3
c3
50
8
c2
4
8
c3
50
31
Query & Analysis Tools
Query Building
 Report Writers (comparisons, growth, graphs,…)
 Spreadsheet Systems
 Web Interfaces
 Data Mining

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Other Operations

Time functions
 e.g.,

Computed Attributes
 e.g.,

time average
commission = sales * rate
Text Queries
 e.g.,
find documents with words X AND B
 e.g., rank documents by frequency of
words X, Y, Z
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Data Mining
Decision Trees
 Clustering
 Association Rules

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Decision Trees
Example:
• Conducted survey to see what customers were
interested in new model car
• Want to select customers for advertising campaign
sale
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custId
c1
c2
c3
c4
c5
c6
car
taurus
van
van
taurus
merc
taurus
age
27
35
40
22
50
25
Notes11
city newCar
sf
yes
la
yes
sf
yes
sf
yes
la
no
la
no
training
set
35
One Possibility
sale
custId
c1
c2
c3
c4
c5
c6
age<30
Y
N
city=sf
Y
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age
27
35
40
22
50
25
city newCar
sf
yes
la
yes
sf
yes
sf
yes
la
no
la
no
car=van
N
likely
car
taurus
van
van
taurus
merc
taurus
unlikely
Y
likely
N
unlikely
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Another Possibility
sale
custId
c1
c2
c3
c4
c5
c6
car=taurus
Y
N
city=sf
Y
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age
27
35
40
22
50
25
city newCar
sf
yes
la
yes
sf
yes
sf
yes
la
no
la
no
age<45
N
likely
car
taurus
van
van
taurus
merc
taurus
unlikely
Y
likely
N
unlikely
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Issues

Decision tree cannot be “too deep”


would not have statistically significant amounts of
data for lower decisions
Need to select tree that most reliably
predicts outcomes
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Clustering
income
education
age
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Another Example: Text

Each document is a vector
 e.g.,
<100110...> contains words 1,4,5,...
Clusters contain “similar” documents
 Useful for understanding, searching
documents

sports
international
news
business
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Issues
Given desired number of clusters?
 Finding “best” clusters
 Are clusters semantically meaningful?

 e.g.,

“yuppies’’ cluster?
Using clusters for disk storage
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Association Rule Mining
sales
records:
tran1
tran2
tran3
tran4
tran5
tran6
cust33
cust45
cust12
cust40
cust12
cust12
p2,
p5,
p1,
p5,
p2,
p9
p5, p8
p8, p11
p9
p8, p11
p9
market-basket
data
• Trend: Products p5, p8 often bough together
• Trend: Customer 12 likes product p9
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Association Rule
Rule: {p1, p3, p8}
 Support: number of baskets where these
products appear
 High-support set: support  threshold s
 Problem: find all high support sets

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Finding High-Support Pairs
Baskets(basket, item)
 SELECT I.item, J.item, COUNT(I.basket)
FROM Baskets I, Baskets J
WHERE I.basket = J.basket AND
I.item < J.item
WHY?
GROUP BY I.item, J.item
HAVING COUNT(I.basket) >= s;

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Example
basket item
t1
p2
t1
p5
t1
p8
t2
p5
t2
p8
t2
p11
...
...
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basket item1 item2
t1
p2
p5
t1
p2
p8
t1
p5
p8
t2
p5
p8
t2
p5
p11
t2
p8
p11
...
...
...
Notes11
check if
count  s
45
Issues

Performance for size 2 rules
big

basket
t1
t1
t1
t2
t2
t2
...
item
p2
p5
p8
p5
p8
p11
...
basket
t1
t1
t1
t2
t2
t2
...
item1
p2
p2
p5
p5
p5
p8
...
item2
p5
p8
p8
p8
p11
p11
...
even
bigger!
Performance for size k rules
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Implementing a Warehouse
Monitoring: Sending data from sources
 Integrating: Loading, cleansing,...
 Processing: Query processing, indexing, ...
 Managing: Metadata, Design, ...

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Monitoring
Source Types: relational, flat file, IMS,
VSAM, IDMS, WWW, news-wire, …
 Incremental vs. Refresh

customer
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id
53
81
111
name
joe
fred
sally
address
10 main
12 main
80 willow
Notes11
city
sfo
sfo
la
new
48
Periodic snapshots
 Database triggers
 Log shipping
 Data shipping (replication service)
 Transaction shipping
 Polling (queries to source)
 Screen scraping
 Application level monitoring

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Notes11
Advantages & Disadvantages!!
Monitoring Techniques
49
Monitoring Issues

Frequency
 periodic:
daily, weekly, …
 triggered: on “big” change, lots of changes, ...

Data transformation
 convert
data to uniform format
 remove & add fields (e.g., add date to get history)
Standards (e.g., ODBC)
 Gateways

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Integration
Data Cleaning
 Data Loading
 Derived Data

Client
Client
Query & Analysis
Metadata
Warehouse
Integration
Source
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Source
Source
51
Data Cleaning



Migration (e.g., yen  dollars)
Scrubbing: use domain-specific knowledge (e.g.,
social security numbers)
Fusion (e.g., mail list, customer merging)
billing DB
customer1(Joe)
merged_customer(Joe)
service DB

customer2(Joe)
Auditing: discover rules & relationships
(like data mining)
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Loading Data
Incremental vs. refresh
 Off-line vs. on-line
 Frequency of loading

 At

night, 1x a week/month, continuously
Parallel/Partitioned load
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Derived Data

Derived Warehouse Data
 indexes
 aggregates
 materialized
views (next slide)
When to update derived data?
 Incremental vs. refresh

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Materialized Views

sale
Define new warehouse relations using
SQL expressions
prodId storeId
p1
c1
p2
c1
p1
c3
p2
c2
p1
c1
p1
c2
joinTb
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date
1
1
1
1
2
2
prodId
p1
p2
p1
p2
p1
p1
amt
12
11
50
8
44
4
name
bolt
nut
bolt
nut
bolt
bolt
product
price
10
5
10
5
10
10
Notes11
storeId
c1
c1
c3
c2
c1
c2
date
1
1
1
1
2
2
id
p1
p2
amt
12
11
50
8
44
4
name price
bolt
10
nut
5
does not exist
at any source
55
Processing
ROLAP servers vs. MOLAP servers
 Index Structures
 What to Materialize?
 Algorithms

Client
Client
Query & Analysis
Metadata
Warehouse
Integration
Source
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Notes11
Source
Source
56
ROLAP Server

Relational OLAP Server
sale
prodId
p1
p2
p1
date
1
1
2
sum
62
19
48
tools
utilities
ROLAP
server
Special indices, tuning;
Schema is “denormalized”
relational
DBMS
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MOLAP Server

Multi-Dimensional OLAP Server
Sales
M.D. tools
Product
B
A
milk
soda
eggs
soap
1
utilities
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multidimensional
server
Notes11
2 3 4
Date
could also
sit on
relational
DBMS
58
Index Structures

Traditional Access Methods
 B-trees,

hash tables, R-trees, grids, …
Popular in Warehouses
 inverted
lists
 bit map indexes
 join indexes
 text indexes
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Inverted Lists
18
19
20
21
22
23
25
26
age
index
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r5
r19
r37
r40
name age
joe
20
fred
20
sally
21
nancy 20
tom
20
pat
25
dave
21
jeff
26
...
20
23
rId
r4
r18
r19
r34
r35
r36
r5
r41
r4
r18
r34
r35
inverted
lists
Notes11
data
records
60
Using Inverted Lists

Query:
 Get
people with age = 20 and name = “fred”
List for age = 20: r4, r18, r34, r35
 List for name = “fred”: r18, r52
 Answer is intersection: r18

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Bit Maps
20
23
20
21
22
1
1
0
1
1
0
0
0
0
0
0
1
0
0
0
1
0
1
1
23
25
26
age
index
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bit
maps
Notes11
id
1
2
3
4
5
6
7
8
name age
joe
20
fred
20
sally
21
nancy 20
tom
20
pat
25
dave
21
jeff
26
...
18
19
data
records
62
Using Bit Maps

Query:
 Get
people with age = 20 and name = “fred”
List for age = 20: 1101100000
 List for name = “fred”: 0100000001
 Answer is intersection: 010000000000

Good if domain cardinality small
 Bit vectors can be compressed

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63
Join
• “Combine” SALE, PRODUCT relations
• In SQL: SELECT * FROM SALE, PRODUCT
sale
prodId storeId
p1
c1
p2
c1
p1
c3
p2
c2
p1
c1
p1
c2
joinTb
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date
1
1
1
1
2
2
prodId
p1
p2
p1
p2
p1
p1
amt
12
11
50
8
44
4
name
bolt
nut
bolt
nut
bolt
bolt
product
price
10
5
10
5
10
10
Notes11
storeId
c1
c1
c3
c2
c1
c2
date
1
1
1
1
2
2
id
p1
p2
name price
bolt
10
nut
5
amt
12
11
50
8
44
4
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Join Indexes
join index
product
sale
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id
p1
p2
rId
r1
r2
r3
r4
r5
r6
name price
bolt
10
nut
5
jIndex
r1,r3,r5,r6
r2,r4
prodId storeId
p1
c1
p2
c1
p1
c3
p2
c2
p1
c1
p1
c2
Notes11
date
1
1
1
1
2
2
amt
12
11
50
8
44
4
65
What to Materialize?
Store in warehouse results useful for
common queries
 Example:
total sales

day 2
day 1
c1
c2
c3
p1
44
4
p2 c1
c2
c3
p1
12
50
p2
11
8
p1
p2
c1
56
11
c2
4
8
c3
50
materialize
CS 245
...
p1
c2
12
c3
50
129
p1
p2
Notes11
c1
67
c1
110
19
66
Materialization Factors
Type/frequency of queries
 Query response time
 Storage cost
 Update cost

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Notes11
67
Cube Aggregates Lattice
129
all
c1
67
p1
c2
12
c3
50
city
city, product
p1
p2
c1
56
11
c2
4
8
product
city, date
date
product, date
c3
50
day 2
day 1
CS 245
c1
c2
c3
p1
44
4
p2 c1
c2
c3
p1
12
50
p2
11
8
city, product, date
Notes11
use greedy
algorithm to
decide what
to materialize
68
Dimension Hierarchies
all
cities
state
city
c1
c2
state
CA
NY
city
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69
Dimension Hierarchies
all
city
city, product
product
city, date
date
product, date
state
city, product, date
state, date
state, product
state, product, date
not all arcs shown...
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70
Interesting Hierarchy
time
all
years
weeks
quarters
day
1
2
3
4
5
6
7
8
week
1
1
1
1
1
1
1
2
month
1
1
1
1
1
1
1
1
quarter
1
1
1
1
1
1
1
1
year
2000
2000
2000
2000
2000
2000
2000
2000
conceptual
dimension table
months
days
CS 245
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71
Algorithms
Query Optimization
 Parallel Processing
 Data Mining

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72
Example: Association Rules
How do we perform rule mining efficiently?
 Observation: If set X has support t, then
each X subset must have at least support t
 For 2-sets:

 if
we need support s for {i, j}
 then each i, j must appear in at least s baskets
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73
Algorithm for 2-Sets
(1) Find OK products
 those
appearing in s or more baskets
(2) Find high-support pairs
using only OK products
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74
Algorithm for 2-Sets


INSERT INTO okBaskets(basket, item)
SELECT basket, item
FROM Baskets
GROUP BY item
HAVING COUNT(basket) >= s;
Perform mining on okBaskets
SELECT I.item, J.item, COUNT(I.basket)
FROM okBaskets I, okBaskets J
WHERE I.basket = J.basket AND
I.item < J.item
GROUP BY I.item, J.item
HAVING COUNT(I.basket) >= s;
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75
Counting Efficiently

One way:
basket I.item J.item
t1
p5
p8
t2
p5
p8
t2
p8
p11
t3
p2
p3
t3
p5
p8
t3
p2
p8
...
...
...
CS 245
sort
threshold = 3
basket I.item J.item
t3
p2
p3
t3
p2
p8
t1
p5
p8
t2
p5
p8
t3
p5
p8
t2
p8
p11
...
...
...
Notes11
count &
remove
count I.item J.item
3
p5
p8
5
p12 p18
...
...
...
76
Counting Efficiently

Another way:
basket I.item J.item
t1
p5
p8
t2
p5
p8
t2
p8
p11
t3
p2
p3
t3
p5
p8
t3
p2
p8
...
...
...
scan &
count
threshold = 3
count I.item J.item
1
p2
p3
2
p2
p8
3
p5
p8
5
p12
p18
1
p21
p22
2
p21
p23
...
...
...
remove
count I.item J.item
3
p5
p8
5
p12 p18
...
...
...
keep counter
array in memory
CS 245
Notes11
77
Yet Another Way
basket I.item J.item
t1
p5
p8
t2
p5
p8
t2
p8
p11
t3
p2
p3
t3
p5
p8
t3
p2
p8
...
...
...
(1)
scan &
hash &
count
(2) scan &
remove
false positive
CS 245
count
1
5
2
1
8
1
...
bucket
A
B
C
D
E
F
...
in-memory
hash table
in-memory
counters
basket I.item J.item
t1
p5
p8
t2
p5
p8
t2
p8
p11
t3
p5
p8
t5
p12 p18
t8
p12 p18
...
...
...
Notes11
threshold = 3
count I.item J.item
3
p5
p8
5
p12 p18
...
...
...
(4) remove
count I.item J.item
3
p5
p8
1
p8
p11
5
p12
p18
...
...
...
(3) scan& count
78
Discussion


frequency

Hashing scheme: 2 (or 3) scans of data
Sorting scheme: requires a sort!
Hashing works well if few high-support pairs
and many low-support ones
iceberg queries
threshold
item-pairs ranked by frequency
CS 245
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79
Managing
Metadata
 Warehouse Design
 Tools

Client
Client
Query & Analysis
Metadata
Warehouse
Integration
Source
CS 245
Notes11
Source
Source
80
Metadata

Administrative
 definition
of sources, tools, ...
 schemas, dimension hierarchies, …
 rules for extraction, cleaning, …
 refresh, purging policies
 user profiles, access control, ...
CS 245
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81
Metadata

Business
 business
terms & definition
 data ownership, charging

Operational
 data
lineage
 data currency (e.g., active, archived, purged)
 use stats, error reports, audit trails
CS 245
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82
Design
What data is needed?
 Where does it come from?
 How to clean data?
 How to represent in warehouse (schema)?
 What to summarize?
 What to materialize?
 What to index?

CS 245
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83
Tools

Development


Planning & Analysis


performance monitoring, usage patterns, exception reporting
System & Network Management


what-if scenarios (schema changes, refresh rates), capacity planning
Warehouse Management


design & edit: schemas, views, scripts, rules, queries, reports
measure traffic (sources, warehouse, clients)
Workflow Management

“reliable scripts” for cleaning & analyzing data
CS 245
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84
Current State of Industry

Extraction and integration done off-line
 Usually

in large, time-consuming, batches
Everything copied at warehouse
 Not
selective about what is stored
 Query benefit vs storage & update cost

Query optimization aimed at OLTP
 High
throughput instead of fast response
 Process whole query before displaying
anything
CS 245
Notes11
85
Future Directions
Better performance
 Larger warehouses
 Easier to use
 What are companies & research labs
working on?

CS 245
Notes11
86
Research (1)
Incremental Maintenance
 Data Consistency
 Data Expiration
 Recovery
 Data Quality
 Error Handling (Back Flush)

CS 245
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87
Research (2)
Rapid Monitor Construction
 Temporal Warehouses
 Materialization & Index Selection
 Data Fusion
 Data Mining
 Integration of Text & Relational Data

CS 245
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88
Conclusions
Massive amounts of data and
complexity of queries will push limits
of current warehouses
 Need better systems:

 easier
to use
 provide quality information
CS 245
Notes11
89