Data warehousing



Han, J. and M. Kamber. Data Mining: Concepts and Techniques. 2001.
Morgan Kaufmann.
Application
  KDD process
                                           Pattern Evaluation



                               ...
Data mining is the process of discovering
interesting knowledge from large amounts
of data stored in databases, data
wareh...
Data mining and
Business Intelligence
                                                           End User
                ...
Architecture of a
typical data mining     Graphical user interface
system


                               Pattern evaluat...
Customer
 Cust_ID    Name      Address      Age        Income    Credit_info   …



Item
 item_ID    Name      Brand      ...
Queries

List of items sold in previous quarter
Total sales last month, grouped by salesperson
Number of sales transaction...
Data source in Chicago
                                                                     Client


Data source in New Yo...
ty)
                                                                                                          Multi-dimens...
Data Warehouse
 A decision support database that is maintained
 separately from the organization’s operational
 database

...
Data Warehouse — Subject-Oriented

• Organized around major subjects, such as customer,
  product, sales.
• Focusing on th...
Data Warehouse — Integrated

• Constructed by integrating multiple, heterogeneous
  data sources
   – relational databases...
Data Warehouse — Time Variant

 • The time horizon for data warehouse is significantly
   longer than that of operational ...
Data Warehouse — Non-Volatile

• A physically separate store of data transformed from the
  operational environment.
• Ope...
Data Warehouse vs. Heterogeneous DBMS

• Traditional heterogeneous DB integration
   – Build wrappers/integrators on top o...
Data Warehouse vs. Operational DBMS
• OLTP (on-line transaction processing)
   – Major task of traditional relational DBMS...
OLTP vs. OLAP
                                  OLTP                            OLAP
 users                clerk, IT profe...
Why Separate Data Warehouse?
• Maintain high performance for both systems
   – DBMS— tuned for OLTP: access methods, index...
From Tables and Spreadsheets to
 Data Cubes
• A data warehouse is based on a multidimensional data
  model which views dat...
Multi-dimensional “cube”
                      Sales by Item, Time, Location, Supplier
           Time




               ...
Cube: A Lattice of Cuboids

                                  all
                                                        ...
yearly data
                                             (keep all data)




                                             ...
Conceptual Modeling of Data Warehouses

Modeling data warehouses: dimensions & measures
  – Star schema: A fact table in t...
Star Schema Example
time
time_key                                            item
day                                     ...
Snowflake Schema example
time
time_key                                       item
day                                     ...
Fact Constellation example
  time                                                       Shipping Fact Table
  time_key    ...
Data warehouse vs. data marts
• Data warehouse
  – Enterprise-wide scope
  – Subjects that span the organization
  – Fact ...
Computing Measures
• Measure: numerical value at each point in the data cube
  e.g. <time=“Q1”, location=“Chicago”, item=“...
Measure types
• Distributive: E.g., count(), sum(), min(), max().
  Result derived by applying the function to n aggregate...
Concept Hierarchy
Example: Location dimension

all                                   all


region                Europe   ...
Concept hierarchies
Full or partial ordering          Industry Region            Year

                                  C...
OLAP examples
 Sales volume as a function of product,
 month, and region
                      Dimensions: Product, Locati...
A Sample Data Cube
                                                 Total annual sales
                            Date   ...
ty)
                                              (ci
                                                     Chicago
       ...
Dice for
                                (location in {Chicago, Toronto}
                                and time in {Q1} ...
Browsing a Data Cube
                       Visualization
                       OLAP
                       Interactive
 ...
Typical OLAP Operations
•   Roll up (drill-up): summarize data
     – by climbing up hierarchy or by dimension reduction
•...
A Star-Net Query Model
                                 Customer Orders
       Shipping Method
                           ...
Data Warehouse Design: Four Views
• Top-down view
     • selection of the relevant information necessary for the data
    ...
Data Warehouse Design Process
• Top-down, bottom-up approaches or a combination
   – Top-down: Starts with overall design ...
Multi-Tiered DW Architecture

                               Monitor
                                  &          OLAP Ser...
Three Data Warehouse Models
• Enterprise warehouse
  – collects all of the information about subjects spanning
    the ent...
Data Marts
• Data warehouse designed to meet the needs
  of a specific group of users
• Should (but may not) be designed w...
Data Warehouse Development: A
Recommended Approach
                                     Multi-Tier Data
                  ...
OLAP Server Types
• Relational OLAP (ROLAP)
   – Use relational or extended-relational DBMS to store and manage
     wareh...
Metadata Repository
Meta data is the data defining warehouse objects. It has the
following kinds
 – Description of the str...
Data Warehouse Back-End Tools, Utilities
• Data extraction:
   – get data from multiple, heterogeneous, and external
     ...
Advanced examples
Exploration of Data Cubes
Hypothesis-driven: exploration by user, huge search space
Discovery-driven
   ...
Advanced examples
Example: Discovery-driven exploration
Advanced examples
 Complex Aggregation at Multiple
 Granularities
• Ex. Total sales in 2000 by Item, Region, Month, with
 ...
Advances examples
                 Supplier                             Supplier
    Sales                            Sale...
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Data warehousing

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Data warehousing

  1. 1. Data warehousing Han, J. and M. Kamber. Data Mining: Concepts and Techniques. 2001. Morgan Kaufmann.
  2. 2. Application KDD process Pattern Evaluation Data Mining Task-relevant Data Data Warehouse Selection Data Cleaning Data Integration Databases
  3. 3. Data mining is the process of discovering interesting knowledge from large amounts of data stored in databases, data warehouses and/or other information repositories.
  4. 4. Data mining and Business Intelligence End User Making Decisions Data Presentation Business Analyst Visualization Techniques Data Mining Data Information Discovery Analyst Data Exploration Statistical Analysis, Querying and Reporting Data Warehouses / Data Marts OLAP, Multi-dimensional Analysis DBA Data Sources Paper, Files, Information Providers, Database Systems, OLTP
  5. 5. Architecture of a typical data mining Graphical user interface system Pattern evaluation Knowledge Data mining engine base Database or data warehouse server Data cleaning Filtering Data integration Data Database warehouse
  6. 6. Customer Cust_ID Name Address Age Income Credit_info … Item item_ID Name Brand Category Type Price Supplier … Employee Emp_ID Name Department Group Salary Commission … Purchases Trans_ID Cust_id Emp_id Date Time Pay_method amount … Items_sold Trans_ID Item_ID Qty A relational database fragment
  7. 7. Queries List of items sold in previous quarter Total sales last month, grouped by salesperson Number of sales transactions in December Salesperson with highest amount of sales Data warehouse Integrates data from various sources Data organized on a historical perspective Presents different levels of summarized data Multi-dimensional structure dimension: attribute cell: aggregate measures
  8. 8. Data source in Chicago Client Data source in New York Clean Transform Data Query and Integrate warehouse analysis tools Load Data source in Toronto Client Data source in Vancouver Typical data warehouse architecture
  9. 9. ty) Multi-dimensional data (ci Chicago New York ss Toronto dre Vancouver Ad Q1 Time (qtr.) Q2 Q3 Q4 T1 T2 T3 T4 T5 T6 Item-types Drill down on Roll-up ty) y) on Address (ci data for Q1 tr un ss (co dre Chicago USA ss Ad New York dre Canada Toronto Ad Vancouver Q1 Jan Time (month) Q2 Feb Q3 March Q4 T1 T2 T3 T4 T5 T6 T1 T2 T3 T4 T5 T6 Item-types Item-types
  10. 10. Data Warehouse A decision support database that is maintained separately from the organization’s operational database Collection of data this is – subject-oriented – integrated – time-variant – nonvolatile
  11. 11. Data Warehouse — Subject-Oriented • Organized around major subjects, such as customer, product, sales. • Focusing on the modeling and analysis of data for decision makers, not on daily operations or transaction processing. • Provide a simple and concise view around particular subject issues by excluding data that are not useful in the decision support process.
  12. 12. Data Warehouse — Integrated • Constructed by integrating multiple, heterogeneous data sources – relational databases, flat files, on-line transaction records • Data cleaning and data integration – Ensure consistency in naming conventions, encoding structures, attribute measures, etc. among different data sources • e.g., Hotel price: currency, tax, breakfast covered, etc. – Data is converted when moved to the warehouse.
  13. 13. Data Warehouse — Time Variant • The time horizon for data warehouse is significantly longer than that of operational systems. – Operational database: current value data. – Data warehouse: provides information from a historical perspective (e.g., past 5-10 years) • Every key structure in the data warehouse contains an element of time, explicitly or implicitly
  14. 14. Data Warehouse — Non-Volatile • A physically separate store of data transformed from the operational environment. • Operational update of data does not occur in the data warehouse environment. – Does not require transaction processing, recovery, and concurrency control mechanisms – Requires only two operations in data accessing: • initial loading of data and access of data.
  15. 15. Data Warehouse vs. Heterogeneous DBMS • Traditional heterogeneous DB integration – Build wrappers/integrators on top of heterogeneous databases – Query driven approach • When a query is posed to a client site, a meta-dictionary is used to translate the query into queries appropriate for the individual heterogeneous sites involved, and results are integrated into a global answer set • Complex information filtering, compete for resources with local processing • Data warehouse: update-driven, high performance – Information from heterogeneous sources is integrated in advance and stored in warehouses for direct query and analysis
  16. 16. Data Warehouse vs. Operational DBMS • OLTP (on-line transaction processing) – Major task of traditional relational DBMS – Day-to-day operations: purchasing, inventory, banking, manufacturing, payroll, registration, accounting, etc. • OLAP (on-line analytical processing) – Major task of data warehouse system – Data analysis and decision making • Distinct features (OLTP vs. OLAP): – Users and system orientation: transaction vs. decision support – Data contents: current, detailed vs. historical, consolidated – Database design: ER + application vs. star schema + subject – View: current, local vs. evolutionary, integrated – Access patterns: update vs. read-only but complex queries
  17. 17. OLTP vs. OLAP OLTP OLAP users clerk, IT professional knowledge worker function day to day operations decision support DB design application-oriented subject-oriented data current, up-to-date historical, detailed, flat relational summarized, multidimensional isolated integrated, consolidated usage repetitive ad-hoc access read/write multiple large scans index on primary key unit of work short, simple transaction complex query # records accessed tens millions #users thousands hundreds DB size 100MB-GB 100GB-TB metric transaction throughput query throughput, response
  18. 18. Why Separate Data Warehouse? • Maintain high performance for both systems – DBMS— tuned for OLTP: access methods, indexing, concurrency control, recovery – Warehouse—tuned for OLAP: complex OLAP queries, multidimensional view, consolidation. • Different data and function – missing data: decision support requires historical data which operational DBs do not typically maintain – data consolidation: DS requires consolidation (aggregation, summarization) of data from heterogeneous sources – data quality: different sources typically use inconsistent data representations, codes and formats which have to be reconciled
  19. 19. From Tables and Spreadsheets to Data Cubes • A data warehouse is based on a multidimensional data model which views data in the form of a data cube • A data cube, such as sales, allows data to be modeled and viewed in multiple dimensions – Dimension tables, such as item (item_name, brand, type), or time(day, week, month, quarter, year) – Fact table contains measures (such as dollars_sold) and keys to each of the related dimension tables • In data warehousing literature n-D base cube is called a base cuboid. The lattice of cuboids forms a data cube.
  20. 20. Multi-dimensional “cube” Sales by Item, Time, Location, Supplier Time Item SALES Supplier Location
  21. 21. Cube: A Lattice of Cuboids all 0-D(apex) cuboid time item location supplier 1-D cuboids time,item time,location item,location location,supplier 2-D cuboids time,supplier item,supplier time,item,location time,location,supplier 3-D cuboids time,item,supplier item,location,supplier time, item, location, supplier 4-D(base) cuboid
  22. 22. yearly data (keep all data) quarterly data old quarterly data (up to 20 years) (archived) old monthly data retail monthly data monthly data old monthly data (archived) (up to 15 years) (up to 15 years) (archived) old weekly data weekly data special event (archived) (up to 7 years) effects (up to 30 years) old detailed data current detailed data (archived) (up to 3 years)
  23. 23. Conceptual Modeling of Data Warehouses Modeling data warehouses: dimensions & measures – Star schema: A fact table in the middle connected to a set of dimension tables – Snowflake schema: A refinement of star schema where some dimensional hierarchy is normalized into a set of smaller dimension tables, forming a shape similar to snowflake – Fact constellations: Multiple fact tables share dimension tables, viewed as a collection of stars, therefore called galaxy schema or fact constellation
  24. 24. Star Schema Example time time_key item day item_key day_of_the_week Sales Fact Table item_name month brand quarter time_key type year supplier_type item_key branch_key branch location location_key branch_key location_key branch_name units_sold street branch_type city dollars_sold province_or_state country avg_sales Measures
  25. 25. Snowflake Schema example time time_key item day item_key day_of_the_week Sales Fact Table item_name month brand quarter time_key type year item_key supplier_type branch_key branch location location location_key location_key location_key branch_key units_sold street street branch_name city_key city city branch_type dollars_sold province_or_state city_key country avg_sales city province_or_street Measures country
  26. 26. Fact Constellation example time Shipping Fact Table time_key item day time_key item_key day_of_the_week Sales Fact Table item_name item_key month brand quarter time_key type shipper_key year supplier_type item_key from_location branch_key to_location branch location_key location dollars_cost branch_key units_sold location_key units_shipped branch_name street branch_type dollars_sold city province_or_street shipper avg_sales country shipper_key Multiple fact tables, sharing dimensions shipper_name location_key Collection of stars – fact constellation or shipper_type galaxy schema
  27. 27. Data warehouse vs. data marts • Data warehouse – Enterprise-wide scope – Subjects that span the organization – Fact constellation used to model multiple, interrelated subjects • Data mart – Department-wide scope – Departmental subset of data warehouse – Star, snowflake schema Star schema is more efficient and thereby popular
  28. 28. Computing Measures • Measure: numerical value at each point in the data cube e.g. <time=“Q1”, location=“Chicago”, item=“xyz”>: avg-amount • Need to be able to efficiently compute measures
  29. 29. Measure types • Distributive: E.g., count(), sum(), min(), max(). Result derived by applying the function to n aggregate values is the same as that derived by applying the function on all the data without partitioning. • Algebraic: E.g., avg(), min_N(), standard_deviation(). Can be computed by an algebraic function with M arguments (where M is a bounded integer), each of which is obtained by applying a distributive aggregate function. • Holistic: E.g., median(), mode(), rank(). There is no constant bound on the storage size needed to describe a sub-aggregate. No constant function with M arguments (constant M) that characterizes the computation. Can be difficult to compute efficiently – approximate computation
  30. 30. Concept Hierarchy Example: Location dimension all all region Europe ... North_America country Germany ... Spain Canada ... Mexico city Frankfurt ... Vancouver ... Toronto
  31. 31. Concept hierarchies Full or partial ordering Industry Region Year Category Country Quarter Product City Month Week Office Day Set-grouping hierarchy ($0..$1000] e.g. price ($0..$1000] ($0..$1000] ($0..$1000] ($0..$1000] ($0..$1000] ($0..$1000] ($0..$1000] ($0..$1000] ($0..$1000] Multiple hierarchies for an attribute price: {inexpensive, moderately_priced, expensive}
  32. 32. OLAP examples Sales volume as a function of product, month, and region Dimensions: Product, Location, Time Hierarchical summarization paths on gi Industry Region Year Re Category Country Quarter Product Product City Month Week Office Day Month
  33. 33. A Sample Data Cube Total annual sales Date of TV in U.S.A. 1Qtr 2Qtr 3Qtr 4Qtr sum t uc TV od PC U.S.A Pr VCR Country sum Canada Mexico sum
  34. 34. ty) (ci Chicago New York ss Toronto dre Vancouver Ad Q1 Time (qtr.) Q2 Drill down, Roll up Q3 Q4 T1 T2 T3 T4 T5 T6 Item-types Drill down on Roll-up ty) on Address y) data for Q1 (ci tr un ss (co dre Chicago USA ss Ad New York dre Canada Toronto Ad Vancouver Q1 Jan Time (month) Q2 Feb Q3 March Q4 T1 T2 T3 T4 T5 T6 T1 T2 T3 T4 T5 T6 Item-types Item-types
  35. 35. Dice for (location in {Chicago, Toronto} and time in {Q1} Chicago Chicago New York And Item in {T3, T8} Toronto Toronto Vancouver Q1 Q1 Time (qtr.) Q2 Q2 Q3 T3 T8 Q4 Slice T1 T2 T3 T4 T5 T6 For Time in {Q1} Item-types Chicago New York Slicing and Dicing Toronto Vancouver Slice: Selection on one dimension T1 T2 T3 T4 T5 T6 Dice; Selection on two or more dimensions
  36. 36. Browsing a Data Cube Visualization OLAP Interactive manipulation
  37. 37. Typical OLAP Operations • Roll up (drill-up): summarize data – by climbing up hierarchy or by dimension reduction • Drill down (roll down): reverse of roll-up – from higher level summary to lower level summary or detailed data, or introducing new dimensions • Slice and dice: – project and select • Pivot (rotate): – reorient the cube, visualization, 3D to series of 2D planes. • Other operations – drill across: involving (across) more than one fact table – drill through: through the bottom level of the cube to its back- end relational tables (using SQL)
  38. 38. A Star-Net Query Model Customer Orders Shipping Method Customer CONTRACTS AIR-EXPRESS ORDER TRUCK PRODUCT LINE Time Product ANNUALY QTRLY DAILY PRODUCT ITEM PRODUCT GROUP CITY SALES PERSON COUNTRY DISTRICT REGION DIVISION Location Promotion Organization
  39. 39. Data Warehouse Design: Four Views • Top-down view • selection of the relevant information necessary for the data warehouse based on current and future needs • Data source view • exposes the information being captured, stored, and managed by operational systems (E/R models, CASE, etc) • Data warehouse view • fact tables and dimension tables, pre-calculated totals, counts, etc. Source information, date, time for historical context • Business query view • perspectives of data in the warehouse from the view of end- user
  40. 40. Data Warehouse Design Process • Top-down, bottom-up approaches or a combination – Top-down: Starts with overall design and planning (mature) – Bottom-up: Starts with experiments and prototypes (rapid) • From software engineering point of view – Waterfall: structured and systematic analysis at each step – Spiral: rapid generation of increasingly functional systems, short turn around time, quick turn around • Typical data warehouse design process – Choose a business process to model, e.g., orders, invoices, etc. – Choose the grain (atomic level of data) of the business process – Choose the dimensions that will apply to each fact table record – Choose the measure that will populate each fact table record
  41. 41. Multi-Tiered DW Architecture Monitor & OLAP Server other Metadata sources Integrator Analysis Operational Extract Query Transform Data Serve Reports DBs Load Refresh Warehouse Data mining Data Marts Data Sources Data Storage OLAP Engine Front-End Tools
  42. 42. Three Data Warehouse Models • Enterprise warehouse – collects all of the information about subjects spanning the entire organization • Data Mart – a subset of corporate-wide data that is of value to a specific groups of users. Its scope is confined to specific, selected groups, such as marketing data mart • Independent vs. dependent (directly from warehouse) data mart • Virtual warehouse – A set of views over operational databases – Only some of the possible summary views may be materialized
  43. 43. Data Marts • Data warehouse designed to meet the needs of a specific group of users • Should (but may not) be designed with corporate standards and accessibility in mind – incorporate standards for hardware, software, networking, DBMS, naming conventions, etc. – vendor’s attempt to bypass IT and sell directly to end-users?
  44. 44. Data Warehouse Development: A Recommended Approach Multi-Tier Data Warehouse Distributed Data Marts Enterprise Data Data Data Mart Mart Warehouse Model refinement Model refinement Define a high-level corporate data model
  45. 45. OLAP Server Types • Relational OLAP (ROLAP) – Use relational or extended-relational DBMS to store and manage warehouse data and OLAP middleware to support missing pieces – Include optimization of DBMS backend, implementation of aggregation navigation logic, and additional tools and services – greater scalability • Multidimensional OLAP (MOLAP) – Array-based multidimensional storage engine (sparse matrix techniques) – fast indexing to pre-computed summarized data • Hybrid OLAP (HOLAP) – User flexibility, e.g., low level: relational, high-level: array • Specialized SQL servers – specialized support for SQL queries over star/snowflake schemas
  46. 46. Metadata Repository Meta data is the data defining warehouse objects. It has the following kinds – Description of the structure of the warehouse • schema, view, dimensions, hierarchies, derived data defn, data mart locations and contents – Operational meta-data • data lineage (history of migrated data and transformation path), currency of data (active, archived, or purged), monitoring information (warehouse usage statistics, error reports, audit trails) – The algorithms used for summarization – The mapping from operational environment to the data warehouse – Data related to system performance • warehouse schema, view and derived data definitions – Business data • business terms and definitions, ownership of data, charging policies
  47. 47. Data Warehouse Back-End Tools, Utilities • Data extraction: – get data from multiple, heterogeneous, and external sources • Data cleaning: – detect errors in the data and rectify them when possible • Data transformation: – convert data from legacy or host format to warehouse format • Load – sort, summarize, consolidate, compute views, check integrity, and build indices and partitions • Refresh – propagate the updates from the data sources to the warehouse
  48. 48. Advanced examples Exploration of Data Cubes Hypothesis-driven: exploration by user, huge search space Discovery-driven – pre-computed measures indicate exceptions, guide user in the data analysis, at all levels of aggregation – Exception: significantly different from the value anticipated, based on a statistical model – Visual cues such as background color are used to reflect the degree of exception of each cell – Computation of exception indicator can be included in cube construction SelfExp: degree of surprise in cell, relative to values at same levels of aggregation InExp: degree of surprise somewhere beneath the cell, if we drill down PathExp: degree of surprise for each drill down path from cell
  49. 49. Advanced examples Example: Discovery-driven exploration
  50. 50. Advanced examples Complex Aggregation at Multiple Granularities • Ex. Total sales in 2000 by Item, Region, Month, with subtotals • Ex. Grouping by all subsets of {item, region, month}, find the maximum price in 2000 for each group, and the total sales generated by all maximum-price-sales • Ex. Among the max-price-sales, find the min and max shelf life. Find the fraction of the total sales due to cases that have min shelf life.
  51. 51. Advances examples Supplier Supplier Sales Sales #units, %sales $value Product Product Sales #units, Sales volume as a % of total units sold $value of Product Product Ordering Group sales by contiguous 10-day intervals. Supplier Sales 10 day Moving-avg of Sales, by Product #units, $value Order Products by Sales-$ and group into Product deciles of decreasing performance.

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