Decision Support Chapter 23 OLAP & Data Warehousing. R.

Decision Support Chapter 23 OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke

Introduction Increasingly, organizations are analyzing current and historical data to identify useful patterns and support business strategies. Emphasis is on complex, interactive, exploratory analysis of very large datasets created by integrating data from across all parts of an enterprise; data is fairly static. Contrast such On-Line Analytic Processing (OLAP) with traditional On-line Transaction Processing (OLTP): mostly long queries, instead of short update Xacts. OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke

Three Complementary Trends Data Warehousing: Consolidate data from many sources in one large repository. Loading, periodic synchronization of replicas. Semantic integration. OLAP: Complex SQL queries and views. Queries based on spreadsheet-style operations and “multidimensional” view of data. Interactive and “online” queries. Data Mining: Exploratory search for interesting trends and anomalies. (Another lecture!) OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke

EXTERNAL DATA SOURCES Data Warehousing Integrated data spanning EXTRACT TRANSFORM long time periods, often LOAD augmented with summary REFRESH information. Several gigabytes to terabytes common. DATA Metadata WAREHOUSE Interactive response Repository times expected for complex queries; ad-hoc SUPPORTS updates uncommon. DATA MINING OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke OLAP

Warehousing Issues Semantic Integration: When getting data from multiple sources, must eliminate mismatches, e.g., different currencies, schemas. Heterogeneous Sources: Must access data from a variety of source formats and repositories. Replication capabilities can be exploited here. Load, Refresh, Purge: Must load data, periodically refresh it, and purge too-old data. Metadata Management: Must keep track of source, loading time, and other information for all data in the warehouse. OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke

locid sales timeid pid Multidimensiona l Data Model 11 1 1 25 11 2 1 8 Collection of numeric measures, which depend on a set of dimensions. 11 3 1 15 12 1 1 30 12 2 1 20 12 3 1 50 8 10 10 13 1 1 8 30 20 50 13 2 1 10 25 8 15 locid 13 3 1 10 11 pid 12 Slice locid 1 is shown: 13 E.g., measure Sales, dimensions Product (key: pid), Location (locid), and Time (timeid). 1 2 3 timeid OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke 11 1 2 35

MOLAP vs ROLAP Multidimensional data can be stored physically in a (disk-resident, persistent) array; called MOLAP systems. Alternatively, can store as a relation; called ROLAP systems. The main relation, which relates dimensions to a measure, is called the fact table. Each dimension can have additional attributes and an associated dimension table. E.g., Products(pid, pname, category, price) Fact tables are much larger than dimensional tables. OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke

Dimension Hierarchies For each dimension, the set of values can be organized in a hierarchy: PRODUCT TIME LOCATION year quarter category pname week country month date OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke state city

OLAP Queries Influenced by SQL and by spreadsheets. A common operation is to aggregate a measure over one or more dimensions. Find total sales. Find total sales for each city, or for each state. Find top five products ranked by total sales. Roll-up: Aggregating at different levels of a dimension hierarchy. E.g., Given total sales by city, we can roll-up to get sales by state. OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke

OLAP Queries Drill-down: The inverse of roll-up. E.g., Given total sales by state, can drill-down to get total sales by city. E.g., Can also drill-down on different dimension to get total sales by product for each state. Pivoting: Aggregation on selected dimensions. WI E.g., Pivoting on Location and Time 63 1995 yields this cross-tabulation: Slicing and Dicing: Equality 1996 38 and range selections on one or more dimensions. OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke 1997 75 CA Total 81 144 107 145 35 110 176 223 339 Total 1

Comparison with SQL Queries The cross-tabulation obtained by pivoting can also be computed using a collection of SQLqueries: SELECT SUM(S.sales) FROM Sales S, Times T, Locations L WHERE S.timeid T.timeid AND S.timeid L.timeid GROUP BY T.year, L.state SELECT SUM(S.sales) SELECT SUM(S.sales) FROM Sales S, Times T FROM Sales S, Location L WHERE S.timeid T.timeid WHERE S.timeid L.timeid GROUP BY T.year GROUP BY L.state OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke 1

The CUBE Operator Generalizing the previous example, if there are k dimensions, we have 2 k possible SQL GROUP BY queries that can be generated through pivoting on a subset of dimensions. CUBE pid, locid, timeid BY SUM Sales Equivalent to rolling up Sales on all eight subsets of the set {pid, locid, timeid}; each roll-up corresponds to an SQL query of the form: SELECT SUM(S.sales) Lots of recent work on optimizing the CUBE operator! FROM Sales S GROUP BY grouping-list OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke 1

Design Issues TIMES timei dat wee mont quart yea holiday fl d e k h er r ag PRODUCTS pid timei d pid pnam categor pric e y e locid sale s SALES (Fact locid city table) LOCATIONS state countr y Fact table in BCNF; dimension tables not normalized. Dimension tables are small; updates/inserts/deletes are rare. So, anomalies less important than good query performance. This kind of schema is very common in OLAP applications, and is called a star schema; computing the join of all these relations is called a star join. OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke 1

Implementation Issues New indexing techniques: Bitmap indexes, Join indexes, array representations, compression, precomputation of aggregations, etc. E.g., Bitmap index: t-vector: F sex bit for each M ossible value. any queries can e answered using t-vector ops! custid name sex rating 10 10 01 10 112 115 119 112 Joe Ram Sue Woo OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke M M F M 3 5 5 4 rating 00100 00001 00001 00010 1

Join Indexes Consider the join of Sales, Products, Times, and Locations, possibly with additional selection conditions (e.g., country “USA”). A join index can be constructed to speed up such joins. The index contains [s,p,t,l] if there are tuples (with sid) s in Sales, p in Products, t in Times and l in Locations that satisfy the join (and selection) conditions. Problem: Number of join indexes can grow rapidly. A variant of the idea addresses this problem: For each column with an additional selection (e.g., country), build an index with [c,s] in it if a dimension table tuple with value c in the selection column joins with a Sales tuple with sid s; if indexes are bitmaps, called bitmapped join index. OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke 1

Bitmapped Join Index TIMES timei dat wee mont quart yea holiday fl d e k h er r ag PRODUCTS pid timei d pid pnam categor pric e y e locid sale s SALES (Fact locid city table) LOCATIONS state countr y Consider a query with conditions price 10 and country “USA”. Suppose tuple (with sid) s in Sales joins with a tuple p with price 10 and a tuple l with country “USA”. There are two join indexes; one containing [10,s] and the other [USA,s]. Intersecting these indexes tells us which tuples in Sales are in the join and satisfy the given selection. OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke 1

Views and Decision Support OLAP queries are typically aggregate queries. Precomputation is essential for interactive response times. The CUBE is in fact a collection of aggregate queries, and precomputation is especially important: lots of work on what is best to precompute given a limited amount of space to store precomputed results. Warehouses can be thought of as a collection of asynchronously replicated tables and periodically maintained views. Has renewed interest in view maintenance! OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke 1

View Modification (Evaluate On Demand) CREATE VIEW RegionalSales(category,sales,state) AS SELECT P.category, S.sales, L.state View FROM Products P, Sales S, Locations L WHERE P.pid S.pid AND S.locid L.locid Query SELECT R.category, R.state, SUM(R.sales) FROM RegionalSales AS R GROUP BY R.category, R.stat SELECT R.category, R.state, SUM(R.sales) FROM (SELECT P.category, S.sales, L.state Modified FROM Products P, Sales S, Locations L Query WHERE P.pid S.pid AND S.locid L.locid) AS R GROUP BY R.category, R.state OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke 1

View Materialization (Precomputation) Suppose we precompute RegionalSales and store it with a clustered B tree index on [category,state,sales]. Then, previous query can be answered by an index-only scan. SELECT R.state, SUM(R.sales)SELECT R.state, SUM(R.sales) FROM RegionalSales R FROM RegionalSales R WHERE R.category “Laptop”WHERE R. state “Wisconsin” GROUP BY R.state GROUP BY R.category Index on precomputed view Index is less useful (must is great! scan entire leaf level). OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke 1

Issues in View Materialization What views should we materialize, and what indexes should we build on the precomputed results? Given a query and a set of materialized views, can we use the materialized views to answer the query? How frequently should we refresh materialized views to make them consistent with the underlying tables? (And how can we do this incrementally?) OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke 2

Interactive Queries: Beyond Materialization Top N Queries: If you want to find the 10 (or so) cheapest cars, it would be nice if the DB could avoid computing the costs of all cars before sorting to determine the 10 cheapest. Idea: Guess at a cost c such that the 10 cheapest all cost less than c, and that not too many more cost less. Then add the selection cost c and evaluate the query. If the guess is right, great, we avoid computation for cars that cost more than c. If the guess is wrong, need to reset the selection and recompute the original query. OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke 2

Top N Queries SELECT P.pid, P.pname, S.sales FROM Sales S, Products P WHERE S.pid P.pid AND S.locid 1 AND S.timeid 3 ORDER BY S.sales DESC OPTIMIZE FOR 10 ROWS SELECT P.pid, P.pname, S.sales FROM Sales S, Products P WHERE S.pid P.pid AND S.locid 1 AND S.timeid 3 AND S.sales c ORDER BY S.sales DESC OPTIMIZE FOR construct is not in SQL:1999! Cut-off value c is chosen by optimizer. OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke 2

Interactive Queries: Beyond Materialization Online Aggregation: Consider an aggregate query, e.g., finding the average sales by state. Can we provide the user with some information before the exact average is computed for all states? Can show the current “running average” for each state as the computation proceeds. Even better, if we use statistical techniques and sample tuples to aggregate instead of simply scanning the aggregated table, we can provide bounds such as “the average for Wisconsin is 2000 102 with 95% probability. Should also use nonblocking algorithms! OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke 2

Summary Decision support is an emerging, rapidly growing sub area of databases. Involves the creation of large, consolidated data repositories called data warehouses. Warehouses exploited using sophisticated analysis techniques: complex SQL queries and OLAP “multidimensional” queries (influenced by both SQL and spreadsheets). New techniques for database design, indexing, view maintenance, and interactive querying need to be supported. OLAP & Data Warehousing. R. Ramakrishnan and J. Gehrke 2