1. 1 1 Data Warehousing Decision-Support Systems  Data Analysis  OLAP  Extended aggregation features in SQL –Windowing and ranking  Implementation Techniques  View Materialization  Indexing

2. 2 2 Decision Support Systems Decision-support systems are used to make business decisions often based on data collected by on-line transaction- processing systems. Examples of business decisions:  what items to stock?  What insurance premium to change?  Who to send advertisements to? Examples of data used for making decisions  Retail sales transaction details  Customer profiles (income, age, sex, etc.)

3. 3 3 Decision-Support Systems: Overview Data analysis tasks are simplified by specialized tools and SQL extensions  Example tasks  For each product category and each region, what were the total sales in the last quarter and how do they compare with the same quarter last year  As above, for each product category and each customer category Statistical analysis packages (e.g., : S++) can be interfaced with databases  Statistical analysis is a large field will not study it here Data mining seeks to discover knowledge automatically in the form of statistical rules and patterns from Large databases. A data warehouse archives information gathered from multiple sources, and stores it under a unified schema, at a single site.  Important for large businesses which generate data from multiple divisions, possibly at multiple sites  Data may also be purchased externally

4. 4 4 Decision-Support Systems: Overview Composed of four major components:  Data Store Component  That is, DSS database  Data Extraction and Filtering Component  Used to extract and validate data taken from operational databases and external data sources  End-User Query Tool  Used to create queries that access Data Store Component  End-User Presentation Tool Component  Used to organize and present data

5. 5 5 Main Components of a Decision Support System (DSS)

6. 6 6 Transforming Operational Data Into Decision Support Data

7. 7 7 Contrasting Operational and DSS Data Characteristics

8. 8 8 The Data Warehouse Integrated, subject-oriented, time-variant, nonvolatile database that provides support for decision making 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 & dice, rollup, MOLAP, pivot,...

9. 9 9 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

10. 10 What is a Warehouse? Collection of tools  gathering data  cleansing, integrating,...  querying, reporting, analysis  data mining  monitoring, administering warehouse

11. 11 Warehouse Architecture Client Warehouse Source Query & Analysis Integration Metadata

12. 12 Why a Warehouse? Two Approaches:  Query-Driven (Lazy)  Warehouse (Eager) Source ?

13. 13 Query-Driven Approach Client Wrapper Mediator Source

14. 14 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

15. 15 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

16. 16 Data Marts Smaller warehouses Spans part of organization  e.g., marketing (customers, products, sales) Do not require enterprise-wide consensus  but long term integration problems?

17. 17 Creating a Data Warehouse

18. 18 Data Analysis and OLAP Aggregate functions summarize large volumes of data Online Analytical Processing (OLAP)  Interactive analysis of data, allowing data to be summarized and viewed in different ways in an online fashion (with negligible delay) Data that can be modeled as dimension attributes and measure attributes are called multidimensional data.  Given a relation used for data analysis, we can identify some of its attributes as measure attributes, since they measure some value, and can be aggregated upon. For instance, the attribute number of the sales relation is a measure attribute, since it measures the number of units sold.  Some of the other attributes of the relation are identified as dimension attributes, since they define the dimensions on which measure attributes, and summaries of measure attributes, are viewed.

19. 19 Integration of OLAP with a Spreadsheet Program

20. 20 OLAP Client/Server Architecture

21. 21 OLAP Server Arrangement

22. 22 OLAP Server with Multidimensional Data Store Arrangement

23. 23 OLAP Server With Local Mini Data Marts

24. 24 OLAP Implementation The earliest OLAP systems used multidimensional arrays in memory to store data cubes, and are referred to as multidimensional OLAP (MOLAP) systems. OLAP implementations using only relational database features are called relational OLAP (ROLAP) systems Hybrid systems, which store some summaries in memory and store the base data and other summaries in a relational database, are called hybrid OLAP (HOLAP) systems.

25. 25 Typical ROLAP Client/Server Architecture

26. 26 MOLAP Client/Server Architecture

27. 27 Relational vs. Multidimensional OLAP

28. 28 Warehouse Schemas Typically warehouse data is multidimensional, with very large fact tables  Examples of dimensions: item-id, date/time of sale, store where sale was made, customer identifier  Examples of measures: number of items sold, price of items Dimension values are usually encoded using small integers and mapped to full values via dimension tables  Resultant schema is called a star schema  More complicated schema structures –Snowflake schema: multiple levels of dimension tables –Constellation: multiple fact tables

29. 29 Star

30. 30 Star Schema

31. 31 Terms Fact table Dimension tables Measures

32. 32 Star Schemas A star schema is a common organization for data at a warehouse. It consists of: 1. Fact table : a very large accumulation of facts such as sales.  Often “insert-only.” 2. Dimension tables : smaller, generally static information about the entities involved in the facts.

33. 33 Dimension Hierarchies store sType cityregion  snowflake schema  constellations

34. 34 Online Analytical Processing The operation of changing the dimensions used in a cross-tab is called pivoting Suppose an analyst wishes to see a cross-tab on item-name and color for a fixed value of size, for example, large, instead of the sum across all sizes.  Such an operation is referred to as slicing.  The operation is sometimes called dicing, particularly when values for multiple dimensions are fixed. The operation of moving from finer-granularity data to a coarser granularity is called a rollup. The opposite operation - that of moving from coarser-granularity data to finer-granularity data – is called a drill down.

35. 35 Three-Dimensional Data Cube A data cube is a multidimensional generalization of a crosstab Cannot view a three-dimensional object in its entirety but crosstabs can be used as views on a data cube

36. 36 Cube Fact table view: Multi-dimensional cube: dimensions = 2

37. 37 3-D Cube day 2 day 1 dimensions = 3 Multi-dimensional cube:Fact table view:

38. 38 Typical OLAP Queries Often, OLAP queries begin with a “star join”: the natural join of the fact table with all or most of the dimension tables. The typical OLAP query will: 1. Start with a star join. 2. Select for interesting tuples, based on dimension data. 3. Group by one or more dimensions. 4. Aggregate certain attributes of the result.

39. 39 Aggregates Add up amounts for day 1 In SQL: SELECT sum(amt) FROM SALE WHERE date = 1 81

40. 40 Aggregates Add up amounts by day In SQL: SELECT date, sum(amt) FROM SALE GROUP BY date

41. 41 Another Example Add up amounts by day, product In SQL: SELECT date, sum(amt) FROM SALE GROUP BY date, prodId drill-down rollup

42. 42 Aggregates Operators: sum, count, max, min, median, ave “Having” clause Using dimension hierarchy  average by region (within store)  maximum by month (within date)

43. 43 Cube Aggregation day 2 day 1 129... drill-down rollup Example: computing sums

44. 44 Cube Operators day 2 day 1 129... sale(c1,*,*) sale(*,*,*) sale(c2,p2,*)

45. 45 Extended Cube day 2 day 1 * sale(*,p2,*)

46. 46 Aggregation Using Hierarchies day 2 day 1 customer region country (customer c1 in Region A; customers c2, c3 in Region B)

47. 47 Pivoting day 2 day 1 Multi-dimensional cube: Fact table view:

48. 48 OLAP Implementation Early OLAP systems precomputed all possible aggregates in order to provide online response  Space and time requirements for doing so can be very high  2 n combinations of group by  It suffices to precompute some aggregates, and compute others on demand from one of the precomputed aggregates  Can compute aggregate on (item-name, color) from an aggregate on (item-name, color, size) –For all but a few “non-decomposable” aggregates such as median –is cheaper than computing it from scratch Several optimizations available for computing multiple aggregates  Can compute aggregate on (item-name, color) from an aggregate on (item-name, color, size)  Can compute aggregates on (item-name, color, size), (item-name, color) and (item-name) using a single sorting of the base data

49. 49 Cross Tabulation of sales by item-name and color The table above is an example of a cross-tabulation (cross-tab), also referred to as a pivot-table. A cross-tab is a table where  values for one of the dimension attributes form the row headers, values for another dimension attribute form the column headers  Other dimension attributes are listed on top  Values in individual cells are (aggregates of) the values of the dimension attributes that specify the cell.

50. 50 Relational Representation of Crosstabs Crosstabs can be represented as relations The value all is used to represent aggregates The SQL:1999 standard actually uses null values in place of all More on this later….

51. 51 Hierarchies on Dimensions Hierarchy on dimension attributes: lets dimensions to be viewed at different levels of detail  E.g. the dimension DateTime can be used to aggregate by hour of day, date, day of week, month, quarter or year

52. 52 Cross Tabulation With Hierarchy Crosstabs can be easily extended to deal with hierarchies  Can drill down or roll up on a hierarchy

53. 53 Extended Aggregation SQL-92 aggregation quite limited  Many useful aggregates are either very hard or impossible to specify  Data cube  Complex aggregates (median, variance)  binary aggregates (correlation, regression curves)  ranking queries (“assign each student a rank based on the total marks” SQL:1999 OLAP extensions provide a variety of aggregation functions to address above limitations  Supported by several databases, including Oracle and IBM DB2

54. 54 Extended Aggregation in SQL:1999 The cube operation computes union of group by’s on every subset of the specified attributes E.g. consider the query select item-name, color, size, sum(number) from sales group by cube(item-name, color, size) This computes the union of eight different groupings of the sales relation: { (item-name, color, size), (item-name, color), (item-name, size), (color, size), (item-name), (color), (size), ( ) } where ( ) denotes an empty group by list. For each grouping, the result contains the null value for attributes not present in the grouping.

55. 55 Extended Aggregation (Cont.) Relational representation of crosstab that we saw earlier, but with null in place of all, can be computed by select item-name, color, sum(number) from sales group by cube(item-name, color) The function grouping() can be applied on an attribute  Returns 1 if the value is a null value representing all, and returns 0 in all other cases. select item-name, color, size, sum(number), grouping(item-name) as item-name-flag, grouping(color) as color-flag, grouping(size) as size-flag, from sales group by cube(item-name, color, size) Can use the function decode() in the select clause to replace such nulls by a value such as all  E.g. replace item-name in first query by decode( grouping(item-name), 1, ‘all’, item-name)

56. 56 Extended Aggregation (Cont.) The rollup construct generates union on every prefix of specified list of attributes E.g. select item-name, color, size, sum(number) from sales group by rollup(item-name, color, size)  Generates union of four groupings: { (item-name, color, size), (item-name, color), (item-name), ( ) } Rollup can be used to generate aggregates at multiple levels of a hierarchy. E.g., suppose table itemcategory(item-name, category) gives the category of each item. Then select category, item-name, sum(number) from sales, itemcategory where sales.item-name = itemcategory.item-name group by rollup(category, item-name) would give a hierarchical summary by item-name and by category.

57. 57 Extended Aggregation (Cont.) Multiple rollups and cubes can be used in a single group by clause  Each generates set of group by lists, cross product of sets gives overall set of group by lists E.g., select item-name, color, size, sum(number) from sales group by rollup(item-name), rollup(color, size) generates the groupings {item-name, ()} X {(color, size), (color), ()} = { (item-name, color, size), (item-name, color), (item-name), (color, size), (color), ( ) }

58. 58 Ranking Ranking is done in conjunction with an order by specification. Given a relation student-marks(student-id, marks) find the rank of each student. select student-id, rank( ) over (order by marks desc) as s-rank from student-marks An extra order by clause is needed to get them in sorted order select student-id, rank ( ) over (order by marks desc) as s-rank from student-marks order by s-rank Ranking may leave gaps: e.g. if 2 students have the same top mark, both have rank 1, and the next rank is 3  dense_rank does not leave gaps, so next dense rank would be 2

59. 59 Ranking (Cont.) Ranking can be done within partition of the data. “Find the rank of students within each section.” select student-id, section, rank ( ) over (partition by section order by marks desc) as sec-rank from student-marks, student-section where student-marks.student-id = student-section.student-id order by section, sec-rank Multiple rank clauses can occur in a single select clause Ranking is done after applying group by clause/aggregation Exercises:  Find students with top n ranks  Many systems provide special (non-standard) syntax for “top-n” queries  Rank students by sum of their marks in different courses  given relation student-course-marks(student-id, course, marks)

60. 60 Ranking (Cont.) Other ranking functions:  percent_rank (within partition, if partitioning is done)  cume_dist (cumulative distribution)  fraction of tuples with preceding values  row_number (non-deterministic in presence of duplicates) SQL:1999 permits the user to specify nulls first or nulls last select student-id, rank ( ) over (order by marks desc nulls last) as s-rank from student-marks

61. 61 Ranking (Cont.) For a given constant n, the ranking the function ntile(n) takes the tuples in each partition in the specified order, and divides them into n buckets with qual numbers of tuples. For instance, we an sort employees by salary, and use ntile(3) to find which range (bottom third, middle third, or top third) each employee is in, and compute the total salary earned by employees in each range: select threetile, sum(salary) from ( select salary, ntile(3) over (order by salary) as threetile from employee) as s group by threetile

62. 62 Windowing E.g.: “Given sales values for each date, calculate for each date the average of the sales on that day, the previous day, and the next day” Such moving average queries are used to smooth out random variations. In contrast to group by, the same tuple can exist in multiple windows Window specification in SQL:  Ordering of tuples, size of window for each tuple, aggregate function  E.g. given relation sales(date, value) select date, sum(value) over (order by date between rows 1 preceding and 1 following) from sales Examples of other window specifications:  between rows unbounded preceding and current  rows unbounded preceding  range between 10 preceding and current row  All rows with values between current row value –10 to current value  range interval 10 day preceding  Not including current row

63. 63 Windowing (Cont.) Can do windowing within partitions E.g. Given a relation transaction(account-number, date-time, value), where value is positive for a deposit and negative for a withdrawal  “Find total balance of each account after each transaction on the account” select account-number, date-time, sum(value) over (partition by account-number order by date-time rows unbounded preceding) as balance from transaction order by account-number, date-time