Data Warehousing/Mining Comp 150 Data Warehousing Design (not in book)

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Data Warehousing/MiningComp 150 Data
Warehousing Design(not in book)

• Instructor Dan Hebert

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Warehouse Design

• What to materialize in the warehouse
• Which source data?
• Which summary tables?
• Which indices?
• Influenced by both querying and maintenance
• Trade storage space and update time for query
  speed

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Designing a Data Warehouse

• Data models designed to support DW require
  optimization strategies for DSS
• Design option
• Relational model in DW - ROLAP Servers for
  analysis
• Special-purpose multi-dimensional data model in
  DW (MDDB) - MOLAP Servers for analysis

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Why is DW Design Different?

• DSS few transactions, each accessing a large
  number of records
• Typical ER designs tend to be complex and
  difficult to navigate

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Multi-Dimensional Data

• Measures - numerical data being tracked
• Dimensions - business parameters that define a
  transaction
• Example Analyst may want to view sales data
  (measure) by geography, by time, and by product
  (dimensions)
• Dimensional modeling is a technique for
  structuring data around the business concepts
• ER models describe entities and relationships
• Dimensional models describe measures and
  dimensions

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Dimensional Modeling Using Relational DBMS

• Special schema design star, snowflake
• Special indexes bitmap, multi-table join
• Special tuning maximize query throughput
• Proven technology (relational model, DBMS), tend
  to outperform specialized MDDB especially on
  large data sets
• Products
• IBM DB2, Oracle, Sybase IQ, RedBrick, Informix

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Dimensional Modeling Using Special-Purpose Model
(MDDB)

• Facts stored in multi-dimensional arrays
• Dimensions used to index array
• Sometimes on top of relational DB
• Products
• Pilot, Arbor Essbase, Gentia

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Example

• Sales by product line over the past six months
• Sales by account between 1990 and 1995

Account Info
Key columns joining fact table to dimension tables
Numerical Measures
Prod Code Time Code Acct Code Sales Qty
Fact table for measures
Product Info
Dimension tables
Time Info
. . .
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Dimensional Modeling

• Dimensions are organized into hierarchies
• E.g., Time dimension days ? weeks ? quarters
• E.g., Product dimension product ? product line ?
  brand
• Dimensions have attributes
• Physical architecture describe by Star Schema

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Example Contd
Geography
Time
Geography Code Region Code Region Mgr City
Code City Name
Time Code Quarter Code Quarter Name Week Code Day
Code Day name
Sales
Geography Code Time Code Account Code Product
Code Dollar Amount Units
Product
Account
Product Code Product Name Brand Mgr Brand
Code Prod. Line Code Prod. Line Name Prod.
Name ...
Account Code Key Account Code Account
Name Account Type Account Market
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Dimensional Modeling Contd

• Fact tables are fully normalized
• Dimension tables are denormalized
• Repetitively stored for sake of simplicity and
  performance

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Extending Dimensional Modeling

• Some instances when star schema is not ideal
• Denormalized schema may require too much storage
• Very large dimension tables are affecting
  performance negatively
• Snowflake schema
• Normalized dimensions

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Advantages of Dimensional Modeling

• Define complex, multi-dimensional data with
  simple model
• Reduces the number of phycial joins a query has
  to process
• Allows the data warehouse to evolve with rel. low
  maintenance
• HOWEVER! Star schema and rel. DBMS are not the
  magic solution
• Query optimization is still problematic

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Index Structures

• Traditional access methods
• B-trees, hash tables, grid files, etc.
• Popular in warehouses
• Inverted indexes (lists)
• Bit map indexes
• Join indexes

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Inverted Index

• Index for every keyword
• Query
• Get people with age 20 and name Fred
• (1) Use age index and retrieve ids
  r4,r18,r34,r35
• (2) Use name index and retrieve ids r18,r52
• (3) Answer is intersection r18

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Bit Map Index

• Developed for Model 204 DBMS in 1987

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Using Bit Maps

• Query
• Get people with age20 and name Fred
• (1) Bit map for age 20 1101100
• (2) Bit map for nameFred 0100000
• (3) Answer is intersection 0100000
• Good if domain cardinality is small
• Bit vectors can be compressed

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Join Index

• Index on one table for a quantity that involves a
  column value of a different table

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Aggregation

• Process by which low-level data is summarized in
  advanced and placed into intermediate tables
• Speeds up query processing, less ad-hoc
• Show me total US sales for 1990
• How much to aggregate?
• Data cube data model
• All possible aggregations along all dimensions
• Cells contain aggregated values
• How much of the cells in cube should be
  pre-computed?

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Aggregation Contd

• Special operators to navigate the hierarchies
• Roll-up remove a dimension element
• e.g., Roll-up products to brands
• Drill-down (opposite of roll-up),
• Slice (defines a subcube)
• Various visualization ops (e.g., pivot)

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Example
roll-up to region
Dimensions Time, Product, Geography Attributes
Product (upc, price, ) Geography
Hierarchies Product ? Brand ? Day ?
Week ? Quarter City ? Region ? Country
Geography
NY
SF
roll-up to brand
LA
10 34 56 32 12 56
Juice Milk Coke Cream Soap Bread
Product
roll-up to week
M T W Th F S S
Time
56 units of bread sold in LA on M
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Warehouse DBMSBuzzwords

• Used primarily for decision support (DSS)
• A.K.A. On-Line Analytical Processing (OLAP)
• Complex queries, substantial aggregation
• TPC-D benchmark
• Multidimensional data model
• Can be implemented either using rel. model or
  proprietary data model
• Multi-dimensional database (MDDB)
• Aggregation Data Cube
• All possible groupings and aggregations

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Warehouse DBMS Buzzwords (2)

• ROLAP vs. MOLAP
• Special purpose OLAP servers that directly
  implement multidimensional data and operations
• Roll-up aggregate on some dimension
• Drill-down deaggregate on some dimension
• ROLAP Oracle, Sybase IQ, RedBrick
• MOLAP Pilot, Essbase, Gentia

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Warehouse DBMS - Buzzwords (3)

• Clients
• Query and reporting tools
• Analysis tools
• Data mining discovering patterns of various
  forms
• Poses many new research issues in
• Query processing and optimization
• Database design
• View management

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Data Warehouse Physical Design
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Common Design Activities OLTP

• Schema design (base tables)
• Normalization (3NF, BCNF, )
• Schema design (views)
• Mostly for convenience, security
• Usually NOT for performance
• Exception View indexing Roussopolous 1982
• Materialize pointers to tuples instead of tuples
  themselves
• Index selection
• In practice, use rules of thumb
• Tool DBDSGN IBM Almaden, RDT for System R

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Relational Views

• Part of the ANSI/SPARC architecture
• Derived, virtual table
• View definition is an SQL query statement
• View update problem
• Good for logical data independence, security
• How to implement a view for querying
• Query modification modify view query into a
  query on the underlying base tables
• View materialization physically implementing
  view as table

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View Indexing ...

• In general, no need for materialized views in
  OLTP systems
• Increase in performance through indexing
• Secondary storage space used to be expensive
• New idea (N. Roussopolous 1982) - view index
• Store index whose elements point to tuples which
  comprise view
• View selection problem Find a subset of views,
  which, when indexed, minimizes the total cost of
  answering all queries as well as cost of
  maintaining the view structures

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View Indexing

• Assume N views to consider, 2N subsets
• Cant do simple enumeration (cost to answer all
  queries in a given subset)
• NP-complete problem
• Solution uses search algorithm to approximate the
  optimal view selection
• Potential exponential worst case
• Only subset of views needs to be considered
• Cost function which computes for each state (set
  of views remaining storage)
• (1) Cost to compute queries, maintenance of
  current index set
• (2) Estimate of incremental cost that must be
  incurred in extending view set (upper bound on
  actual cost)

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View Indexing

• But ...
• Algorithm does not consider index selection on
  views (view indexes)
• Indexes have impact on which view indexes to
  choose
• Very simple cost model (maintenance cost size
  of view)
• Problem Cost of maintaining view is a complex
  query optimization problem
• Cannot be estimated without knowing which subview
  indexes are chosen
• Good first treatment of subject

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Indexing ...

• Which type of index structure, which attribute(s)
  to index on
• Access path selection -gt DBA
• Many choices, depend on many factors
• Space-time trade-off
• Index selection problem Which ordering rule for
  stored records and which non-clustered indices
• Database practitioners use rules/guidelines
  (e.g., SYBASE manual)
• Design tools available
• Support dba during creation and maintenance of
  database, i.e., solve the index-selection problem

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Factors that Influence Index Selection

• Maintenance
• Storage cost
• Global solution depends on index selection of all
  tables combined

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Example

• ORDERS (OrderNo, SuppNo, PartNo, Date, Qty)
• PARTS (PartNo, Descrip, SuppNo, QtyOnhand, Color,
  )
• Query
• SELECT O.SuppNo
• FROM PARTS P, ORDERS O
• WHERE O.PartNo P.PartNo AND O.SuppNo 15 AND
  P.QtyOnHand BETWEEN 100 AND 150
• Situation 1 Assume PARTS clustered on Descrip
  and non-clustered index on PartNo
• Then Best clustered index for ORDERS SuppNo
• Situation 2 Assume PARTS clustered on PartNo
• Then Best clustered index for ORDERS PartNo

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Data Warehouse Design

• Schema design (base tables)
• Star schema (dimensions, measures)
• Schema design (view/index selection)
• Mostly for performance enhancement
• Physical warehouse design. Balance three costs
• (1) The cost of answering queries using warehouse
  relations and additional structures
• (2) Cost of maintaining additional structures
• (3) Cost of secondary storage

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WH Schema Design

• Tables must map efficiently to the operational
  requests
• OLTP maximize concurrency, optimize
  insert/update/delete performance
• OLAP Queries large, complex, ad-hoc,
  data-intensive, no updates
• Query centric view -gt Star schema (facts,
  dimensions)
• Widely accepted, intuitive, easy to navigate
  (query formulation)
• Problem Poor performance on OLTP db engines
• Join processing (pair-wise join problem)
• Number of pair-wise joins for N tables N!
• e.g., 7 tables -gt 5,040 combinations, 5 different
  join algorithms -gt 25,200 combinations

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Star Schema Join Problem

• Heuristic pick directly related tables doesnt
  work in star schema
• Options
• Join unrelated tables (Cartesian product)
• Parallelism (speed-up, scale-up)
• New join techniques (e.g., bit vector star joins)
  in combination with new indexing schemes (e.g.,
  bit maps, variant indexes)

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Warehouse Access Path (Physical) Design Problem

• Materialize user queries as views (reduces cost
  1)
• How to reduce cost 2 and 3?
• View Index Selection Problem VIS
• Choose a set of supporting views and a set of
  indexes to materialize such that the total
  maintenance cost for the warehouse is minimized
  (cost 2 3)

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Solutions - Relational DB Design Practices

• Rel. DB design algorithms must be adapted
• View index approach has no index selection,
  simple cost model (cannot achieve global solution
  by locally optimizing each materialized subview)
• Index selection approach can be extended - but
  trouble ahead
• Algorithms require queries and frequencies as
  input

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Solutions - Rule Condition Maintenance

• Work on rule condition evaluation
• How to evaluate trigger conditions for rules
  efficiently ( view maintenance problem rule is
  triggered whenever view that satisfies its
  condition becomes non-empty)
• Discrimination networks for each rule (view)
• RETE model materializes selection and join nodes
• TREAT materializes only selection nodes
• Incremental evaluation techniques
• Recommendations not generally applicable