data warehousing
DESCRIPTION
Data Warehousing. Objectives. Definition of terms Reasons for information gap between information needs and availability Reasons for need of data warehousing Describe three levels of data warehouse architectures List four steps of data reconciliation Describe two components of star schema - PowerPoint PPT PresentationTRANSCRIPT
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Data
Warehousing
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Objectives
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Definition of termsReasons for information gap between information
needs and availabilityReasons for need of data warehousingDescribe three levels of data warehouse
architecturesList four steps of data reconciliationDescribe two components of star schemaEstimate fact table sizeDesign a data mart
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Definition
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Data Warehouse - A subject-oriented, integrated, time-variant, non-updatable collection of data used in support of management decision-making processes Subject-oriented: customers, patients, students,
products Integrated: Consistent naming conventions,
formats, encoding structures; from multiple data sources
Time-variant: Can study trends and changes Nonupdatable: Read-only, periodically refreshed
Data Mart - A data warehouse that is limited in scope
the main repository ofan organization's historical data
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Operational Systems vs. Data Warehouse
Operational systems are optimized for simplicity and speed of modification (see OLTP) through the use of normalization and an entity-relationship model
The data warehouse is optimized for reporting and analysis (online analytical processing, or OLAP)
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Need for Data Warehousing
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Integrated, company-wide view of high-quality information (from disparate databases)
Separation of operational and informational systems and data (for improved performance)
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Data Warehouse Architectures
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Generic Two-Level ArchitectureIndependent Data MartDependent Data Mart and Operational
Data StoreLogical Data Mart and Real-Time Data
WarehouseThree-Layer architecture
All involve some form of extraction, transformation and loading (ETLETL)
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Figure 11-2: Generic two-level data warehousing architecture
E
T
LOne, company-wide warehouse
Periodic extraction data is not completely current in warehouse
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Figure 11-3 Independent data mart data warehousing architecture
Data marts:Data marts:Mini-warehouses, limited in scope
E
T
L
Separate ETL for each independent data mart
Data access complexity due to multiple data marts
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Figure 11-4 Dependent data mart with operational data store: a three-level architecture
ET
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Single ETL for enterprise data warehouse(EDW)(EDW)
Simpler data access
ODS ODS provides option for obtaining current data
Dependent data marts loaded from EDW
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ET
L
Near real-time ETL for Data WarehouseData Warehouse
ODS ODS and data warehousedata warehouse are one and the same
Data marts are NOT separate databases, but logical views of the data warehouse Easier to create new data marts
Figure 11-5 Logical data mart and real time warehouse architecture
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Three-layer data architecture for a data warehouse
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Data CharacteristicsStatus vs. Event Data
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Status
Status
Event = a database action (create/update/delete) that results from a transaction
Example of DBMS log entry
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Data CharacteristicsData CharacteristicsTransient vs. Periodic DataTransient vs. Periodic Data
With transient data, changes to existing records are written over previous records, thus destroying the previous data content
Transient operational data
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Periodic data are
never physically altered
or deleted
once they have been
added to the
store
Data CharacteristicsData CharacteristicsTransient vs. Periodic DataTransient vs. Periodic DataPeriodic
warehouse data
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Other Data Warehouse Changes
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New descriptive attributes New business activity attributes New classes of descriptive attributes Descriptive attributes become more
refined Descriptive data are related to one
another New source of data
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The Reconciled Data Layer
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Typical operational data is:Transient–not historicalNot normalized (perhaps due to denormalization for
performance)Restricted in scope–not comprehensiveSometimes poor quality–inconsistencies and errors
After ETL, data should be:Detailed–not summarized yetHistorical–periodicNormalized–3rd normal form or higherComprehensive–enterprise-wide perspectiveTimely–data should be current enough to assist
decision-makingQuality controlled–accurate with full integrity
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The ETL Process
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Capture/ExtractScrub or data cleansingTransformLoad and Index
ETL = Extract, transform, and load
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Static extractStatic extract = capturing a snapshot of the source data at a point in time
Incremental extractIncremental extract = capturing changes that have occurred since the last static extract
Capture/Extract…obtaining a snapshot of a chosen subset of the source data for loading into the data warehouse
Figure 11-10: Steps in data reconciliation
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Scrub/Cleanse…uses pattern recognition and AI techniques to upgrade data quality
Fixing errors:Fixing errors: misspellings, erroneous dates, incorrect field usage, mismatched addresses, missing data, duplicate data, inconsistencies
Also:Also: decoding, reformatting, time stamping, conversion, key generation, merging, error detection/logging, locating missing data
Figure 11-10: Steps in data reconciliation
(cont.)
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Transform = convert data from format of operational system to format of data warehouse
Record-level:Record-level:Selection–data partitioningJoining–data combiningAggregation–data summarization
Field-level:Field-level: single-field–from one field to one fieldmulti-field–from many fields to one, or one field to many
Figure 11-10: Steps in data reconciliation
(cont.)
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Load/Index= place transformed data into the warehouse and create indexes
Refresh mode:Refresh mode: bulk rewriting of target data at periodic intervals
Update mode:Update mode: only changes in source data are written to data warehouse
Figure 11-10: Steps in data reconciliation
(cont.)
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Figure 11-11: Single-field transformation
In general–some transformation function translates data from old form to new form
Algorithmic transformation uses a formula or logical expression
Table lookup–another approach, uses a separate table keyed by source record code
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Figure 11-12: Multifield transformation
M:1–from many source fields to one target field
1:M–from one source field to many target fields
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Derived Data
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ObjectivesEase of use for decision support applicationsFast response to predefined user queriesCustomized data for particular target
audiencesAd-hoc query supportData mining capabilities
CharacteristicsDetailed (mostly periodic) dataAggregate (for summary)Distributed (to departmental servers)
Most common data model = star schemastar schema(also called “dimensional model”)
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Figure 11-13 Components of a star schemastar schemaFact tables contain factual or quantitative data
Dimension tables contain descriptions about the subjects of the business
1:N relationship between dimension tables and fact tables
Excellent for ad-hoc queries, but bad for online transaction processing
Dimension tables are denormalized to maximize performance
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Figure 11-14 Star schema example
Fact table provides statistics for sales broken down by product, period and store dimensions
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Figure 11-15 Star schema with sample data
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Issues Regarding Star Schema
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Dimension table keys must be surrogate (non-intelligent and non-business related), because:Keys may change over timeLength/format consistency
Granularity of Fact Table–what level of detail do you want? Transactional grain–finest levelAggregated grain–more summarizedFiner grains better market basket analysis capabilityFiner grain more dimension tables, more rows in fact table
Duration of the database–how much history should be kept?Natural duration–13 months or 5 quartersFinancial institutions may need longer durationOlder data is more difficult to source and cleanse
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Data Mining and Visualization
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Knowledge discovery using a blend of statistical, AI, and computer graphics techniques
Goals: Explain observed events or conditions Confirm hypotheses Explore data for new or unexpected relationships
Techniques Statistical regression Decision tree induction Clustering and signal processing Affinity Sequence association Case-based reasoning Rule discovery Neural nets Fractals
Data visualization–representing data in graphical/multimedia formats for analysis
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Figure 11-16: Modeling dates
Fact tables contain time-period data Date dimensions are important
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The User InterfaceMetadata (data catalog)
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Identify subjects of the data martIdentify dimensions and factsIndicate how data is derived from enterprise data
warehouses, including derivation rulesIndicate how data is derived from operational
data store, including derivation rulesIdentify available reports and predefined queriesIdentify data analysis techniques (e.g. drill-down)Identify responsible people
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On-Line Analytical Processing (OLAP) Tools
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The use of a set of graphical tools that provides users with multidimensional views of their data and allows them to analyze the data using simple windowing techniques
Relational OLAP (ROLAP)Traditional relational representation
Multidimensional OLAP (MOLAP)CubeCube structure
OLAP OperationsCube slicing–come up with 2-D view of dataDrill-down–going from summary to more
detailed views
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Figure 11-23 Slicing a data cube
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Figure 11-24 Example of drill-down
Summary report
Drill-down with color added
Starting with summary data, users can obtain details for particular cells