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CS 345:
Topics in Data Warehousing
Thursday, October 14, 2004
Review of Tuesday’s Class
• Customer Relationship Management (CRM)
• Dimension-focused queries
– Drill-across
– Conformed dimensions
• Customer dimension
– Behavioral attributes
– Auxiliary tables
• Techniques for very large dimensions
– Outriggers
– Mini-dimensions
Outline of Today’s Class
• Bridge tables
– Hierarchies
– Multi-Valued Dimensions
• Extraction-Transformation-Load
– Data staging area vs. data warehouse
– Assigning surrogate keys
– Detecting changed rows
– Detecting duplicate dimension rows
More Outriggers / Mini-Dims
• Lots of information about some customers, little info about others
– A common scenario
• Example: web site browsing behavior
– Web User dimension (= Customer dimension)
– Unregistered users
• User identity tracked over time via cookies
• Limited information available
– First active date, Latest active date, Behavioral attributes
– Possibly ZIP code through IP lookup
– Registered users
• Lots of data provided by user during registration
– Many more unregistered users than registered users
– Most attribute values are unknown for unregistered users
• Split registered user attributes into a separate table
– Either an outrigger or a mini-dimension
– For unregistered users, point to special “Unregistered” row
Handling Hierarchies
• Hierarchical relationships among dimension attributes are common
• There are various ways to handle hierarchies
– Store all levels of hierarchy in denormalized dimension table
• The preferred solution in almost all cases!
– Create “snowflake” schema with hierarchy captured in separate
outrigger table
• Only recommended for huge dimension tables
• Storage savings have negligible impact in most cases
• What about variable-depth hierarchies?
– Examples:
• Corporate organization chart
• Parts composed of subparts
– Previous two solutions assumed fixed-depth
– Creating recursive foreign key to parent row is a possibility
•
•
•
•
Employee dimension has “boss” attribute which is FK to Employee
The CEO has NULL value for boss
This approach is not recommended
Cannot be queried effectively using SQL
– Alternative approach: bridge table
Bridge Tables
•
•
Customer dimension has one row
for each customer entity at any
level of the hierarchy
Separate bridge table has
schema:
–
–
–
–
–
•
Parent customer key
Subsidiary customer key
Depth of subsidiary
Bottom flag
Top flag
One row in bridge table for every
(ancestor, descendant) pair
– Customer counts as its own
Depth-0 ancestor
– 16 rows for the hierarchy at right
•
Fact table can join:
– Directly to customer dimension
– Through bridge table to customer
dimension
Customer
1
Customer Customer
2
3
Customer
5
Fact
Customer
6
Customer
7
Bridge
Customer
parent_id
cust_id
Customer
4
child_id
cust_id
Bridge Table Example
parent_id
child_id
depth
top_flag
bottom_flag
1
1
0
Y
N
1
2
1
Y
N
2
2
0
N
N
1
3
1
Y
Y
3
3
0
N
Y
1
4
1
Y
N
4
4
0
N
N
1
5
2
Y
Y
2
5
1
N
Y
5
5
0
N
Y
1
6
2
Y
Y
2
6
1
N
Y
…
…
…
…
…
Using Bridge Tables in Queries
• Two join directions
– Navigate up the hierarchy
• Fact joins to subsidiary customer key
• Dimension joins to parent customer key
– Navigate down the hierarchy
• Fact joins to parent customer key
• Dimension joins to subsidiary customer key
• Safe uses of the bridge table:
– Filter on customer dimension restricts query to a single customer
• Use bridge table to combine data about that customer’s subsidiaries or
parents
– Filter on bridge table restricts query to a single level
• Require Top Flag = Y
• Require Depth = 1
– For immediate parent / child organizations
• Require (Depth = 1 OR (Depth < 1 AND Top Flag = Y))
– Generalizes the previous example to properly treat top-level customers
• Other uses of the bridge table risk over-counting
– Bridge table is many-to-many between fact and dimension
Restricting to One Customer
parent_id
child_id
depth
top_flag
bottom_flag
1
1
0
Y
N
1
2
1
Y
N
2
2
0
N
N
1
3
1
Y
Y
3
3
0
N
Y
1
4
1
Y
N
4
4
0
N
N
1
5
2
Y
Y
2
5
1
N
Y
5
5
0
N
Y
1
6
2
Y
Y
2
6
1
N
Y
…
…
…
…
…
Restricting to One Depth
parent_id
child_id
depth
top_flag
bottom_flag
1
1
0
Y
N
1
2
1
Y
N
2
2
0
N
N
1
3
1
Y
Y
3
3
0
N
Y
1
4
1
Y
N
4
4
0
N
N
1
5
2
Y
Y
2
5
1
N
Y
5
5
0
N
Y
1
6
2
Y
Y
2
6
1
N
Y
…
…
…
…
…
Multi-Valued Dimensions
• Occasionally a dimension takes on a variable
number of multiple values
– Example: Bank accounts may be owned by one, two,
or even more customers (individual vs. joint accounts)
• Can be modeled using a bridge table
• Bank transaction fact table
– Grain: one row per transaction
– Dimensions: Date, Branch, TransType, Account,
Customer
– Including Customer dimension would violate the grain
Multi-Valued Dimensions
• Weights for each account sum to 1
• Allows for proper allocation of facts
when using Customer dimension
Fact
Table
account_id
Account
Dimension
Bridge
Table
account_id
account_id
Customer
Dimension
customer_id
customer_id
Accountrelated
attributes
weight
Customerrelated
attributes
Weighted Report vs. Impact Report
• Two formulations for customer queries
• Weighted report
– Multiply all facts by weight before aggregating
– SUM(DollarAmt * weight)
– Subtotals and totals are meaningful
• Impact report
–
–
–
–
–
Don’t use the weight column
SUM(DollarAmt)
Some facts are double-counted in totals
Each customer is fully credited for his/her activity
Most useful when grouping by customer
Loading the Data Warehouse
Data is periodically
extracted
Data is cleansed and
transformed
Users query the data
warehouse
Source Systems
(OLTP)
Data Staging Area
Data Warehouse
Staging Area vs. Warehouse
• Data warehouse
– Cleansed, transformed data
– User-friendly logical design
– Optimized physical design
• Indexes, Pre-computed aggregates
• Staging area
– Intermediate representations of data
– “Work area” for data transformations
• Same server or different?
• Separate staging server and warehouse server
– Run extraction in parallel with queries
• Staging area and warehouse both part of same database
– Less copying of data is required
Alternating Server Approach
Warehouse
Warehouse
Swap
Roles
Staging
Load
New
Data
Staging
Staging
Warehouse
Make a Copy
Load the Copy
Backup
Surrogate Key Assignment
• Maintain natural key → surrogate key mapping
– Separate mapping for each dimension table
– Can be stored in a relational database table
• One or more columns for natural key
• One column for surrogate key
• Need a separate mapping table for each data source
– Unless data sources already use unified natural key scheme
• Handling multiple dimension rows that preserve history
– 1st approach: Mapping table contains surrogate key for most
current dimension row
– 2nd approach: Mapping table lists all surrogate keys that were
ever used for each natural key
• Add additional columns to mapping table:
– Begin_date, End_date, Is_current_flag
• “Late-arriving” fact rows can use historically correct key
• Necessary for hybrid slowly changing dimension schemes
Detecting Changed Rows
• Some source systems make things easy
– All changes timestamped → nothing to do!
– Usually the case for fact tables
• Except for Accumulating Snapshot facts
– For each source system, record latest timestamp
covered during previous extraction cycle
• Some source systems just hold snapshot
– Need to detect new vs. changed vs. unchanged rows
– New vs. old rows: Use surrogate key mapping table
– Detecting changed vs. unchanged rows
• Approach 1: Use a hash function
– Faster but less reliable
• Approach 2: Column-by-column comparison
– Slower but more reliable
Handling Changed Rows
• Using a hash function
–
–
–
–
Compute a small summary of the data row
Store previous hash value in mapping table
Compare with hash value of current attribute values
If they’re equal, assume no change
• Hash table collisions are possible
– Cyclic redundancy checksum (CRC)
• Commonly used hash function family
• No collisions under local changes and byte reorderings
• Determine which attributes have changed
– Requires column-by-column comparison
– Store untransformed attribute values in mapping table
– Choose slowly changing dimension approach based on changed
attributes
Dimension Loading Workflow
Natural key in
mapping table?
Yes
No
Insert row in
mapping table
Insert row in
dimension table
Has row
changed?
No
Do nothing
Yes
Which SCD
type?
Type 2
Type 1
Update row in
mapping table
Update row in
mapping table
Update row in
dimension table/
Mark aggregates
Insert row in
dimension table
Duplicates from Multiple Sources
• Information about the same logical entity found
in multiple source systems
• Combine info into single dimension row
• Problems:
– Determine which rows reference same entity
• Sometimes it’s hard to tell!
• Referred to as the merge/purge problem
• Active area of research
– Resolve conflicts between source systems
• Matching records with different values for the same field
• Approach #1: Believe the “more reliable” system
• Approach #2: Include both values as separate attributes
Merge/Purge
• How can we determine that these are the same person?
FName
LName
Address
City
Zip
B
Babcock 3135 Campus Dr #112
San Mateo 94403
Brian
Bobcock 3135 Compass Drive Apt 112
San Mateo 94403-3205
• Fuzzy matching based on textual similarity
• Transformation rules
• Comparison to known good source
– NCOA: National Change Of Address database
• Related application: Householding
– Can we determine when two individuals/accounts belong to the
same household?
– Send one mailing instead of two
Next Week: Query Processing
• We’re done with logical database design
• Next topic: how can the database answer
queries efficiently?
• No textbook from here on
– Optional readings for each topic will be posted on the
course web page
• Mostly research papers
• Some readings from books on reserve in Math/CS library
• Tuesday’s topic: Query processing basics
– Will be review if you’ve taken CS 245
– Haven’t taken CS 245 → may want to read DSCB
Chapter 15.1-15.5, 15.8