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Data Warehousing and Bitmap Indexes - More than just some bits

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Dani Schnider, Principal Consultant bei der Trivadis AG, an der Oracle Open World 2011 in San Francisco

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Data Warehousing and Bitmap Indexes - More than just some bits

  1. 1. MORE THAN JUST SOME BITS… Data Warehousing and Bitmap Indexes Dani Schnider, Trivadis AG Oracle Open World 2011, San Francisco BASEL BERN BRUGG LAUSANNE ZÜRICH DÜSSELDORF FRANKFURT A.M. FREIBURG I.BR. HAMBURG MÜNCHEN STUTTGART WIEN 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 1
  2. 2. Dani Schnider  Principal Consultant und DWH/BI Lead Architect bei Trivadis in Zürich  Teacher for courses about Data Warehousing, SQL Optimization and Oracle Warehouse Builder  Co-author of book «Data Warehousing mit Oracle» 2014 © Trivadis 2 Fehlertolerante Ladeprozesse 18. April 2012
  3. 3. Mit über 600 IT- und Fachexperten bei Ihnen vor Ort 2014 © Trivadis 11 Trivadis Niederlassungen mit über 600 Mitarbeitenden 200 Service Level Agreements Mehr als 4'000 Trainingsteilnehmer Forschungs- und Entwicklungs-budget: CHF 5.0 / EUR 4 Mio. Finanziell unabhängig und nachhaltig profitabel Erfahrung aus mehr als 1'900 Projekten pro Jahr bei über 800 Kunden Stand 12/2012 Hamburg Düsseldorf Frankfurt Freiburg München Wien Basel Bern Zürich Lausanne 3 Stuttgart Data Warehouseing & Bitmap Indexes 02.10.2011 3
  4. 4. 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 4 SOME BITS ABOUT BITMAP INDEXES AND THEIR USAGE IN DATA WAREHOUSES
  5. 5. Bitmap Indexes – Concept Oracle® Database Concepts 11g Release 2 (11.2) 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 5
  6. 6. Bitmap Indexes – Concept 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 6
  7. 7. Bitmap Indexes – Example 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 7
  8. 8. Comparison B-tree Index / Bitmap Index 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 8
  9. 9. Comparison B-tree Index / Bitmap Index 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 9
  10. 10. Concatenated Bitmap Indexes Bitmap Indexes on two or more columns are possible, but not useful 2014 © Trivadis Why?  Concatenated bitmap index can be used only when all columns are filtered  Two or more separate bitmap indexes are more flexible and can be used for all combinations Data Warehouseing & Bitmap Indexes 02.10.2011 10
  11. 11. Index Strategy on Star Schema  Dimension Tables:  Unique b-tree index on primary key column  Bitmap indexes on additional columns (optional)  Fact Tables:  Bitmap indexes on all dimension key (foreign key) columns  Bitmap join indexes on often used filter columns (optional)  Usually no primary key on fact tables 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 11
  12. 12. Index Strategy on Star Schema 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 12
  13. 13. Query Optimization on Star Schemas  Typical Queries:  Filter criteria on (multiple) dimension 2014 © Trivadis tables  Facts are selected by join with dimensions  Problem:  Tables with restrictions should be read first  Only two tables can be joined at a time  No relationships between dimension tables Data Warehouseing & Bitmap Indexes 02.10.2011 13
  14. 14. Star Transformation – Concept 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 14
  15. 15. Star Transformation – Example What is the total revenue for product “Shorts” for all customers in Germany, grouped by male and female customers? 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 15
  16. 16. Star Transformation – Example 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 16
  17. 17. Bitmap Join Index – Concept  Bitmap index is defined on fact 2014 © Trivadis table  Indexed column is part of dimension table  Possible for star and snowflake schemas Data Warehouseing & Bitmap Indexes 02.10.2011 17
  18. 18. Bitmap Join Index – Example Same example with Bitmap Join Indexes 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 18 Demo
  19. 19. Bitmap Indexes and ETL  Bitmap indexes are efficient for set-based ETL  Index maintenance is deferred until end of each DML operation  Efficient index maintenance for Parallel DML  Avoid “hand-made” parallel ETL with multiple sessions  Locking behavior of bitmap indexes  For large data loads: Rebuild indexes after ETL operation  Set all indexes to UNUSABLE  Load data into table  Rebuild all indexes 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 19
  20. 20. Bitmap Indexes and Partitioning  Bitmap Indexes must always be LOCAL  Local indexes are recommended in Data Warehouse  Only purpose of global indexes in DWH: unique indexes without partition key   Restriction of local bitmap indexes is not a problem 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 20
  21. 21. Bitmap Indexes and Compression  Bitmap Indexes are always compressed  Zeros are compressed, Ones are not compressed  Bitmap contains number of zeros to next 1 bit  Bitmap Indexes need less disk space than B-tree Indexes  Even for columns with high number of distinct keys  Detail description see «Bitmap Index Internals», © Julian Dyke  http://www.juliandyke.com/Presentations/BitmapIndexInternals.ppt 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 21
  22. 22. Bitmap Indexes and Compression  Number of distinct keys has impact on size of index  Bitmap index is usually smaller than b-tree index  For high number of keys, size of bitmap index increases  Size of b-tree index is more or less constant  Sort order affects size of bitmap index, but not of b-tree index 2014 © Trivadis 350 300 250 200 150 100 50 Data Warehouseing & Bitmap Indexes 02.10.2011 22 Demo 0 10 100 1000 10000 100000 Bitmap (scattered) Bitmap (sorted) B-tree index
  23. 23. Bitmap Indexes and Compression  Enabling table compression  When a table is enabled for compression, all bitmap indexes must be disabled  Reason: Hakan factor of table  Maximum number of rows that can be stored in a table block  Hakan factor changes when table is enabled for compression (only first time) 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 23 Demo
  24. 24. Bitmap Indexes – Conclusion  Bitmap Indexes are a powerful Data Warehouse feature  Can be combined in ad hoc queries  Star Transformation  Useful for selective and non-selective columns  Bitmap Indexes are useful for large databases  Efficient for set-based ETL and Parallel DML  Local bitmap indexes on partitioned tables  Compression of bitmap indexes  There are almost no reasons to use b-tree indexes in a Data Warehouse  B-tree indexes are mainly used for Primary Key or Unique Key constraints 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 24
  25. 25. References  Oracle® Database Data Warehousing Guide 11g Release 2 (11.2)  Oracle Corporation, http://www.oracle.com/pls/db112/  Oracle® Database Concepts 11g Release 2 (11.2)  Oracle Corporation, http://www.oracle.com/pls/db112/  Christian Antognini: Troubleshooting Oracle Performance  Apress, http://www.apress.com/9781590599174  Jonathan Lewis: Cost-Based Oracle Fundamentals  Apress, http://www.apress.com/9781590596364  Julian Dyke: Bitmap Index Internals  http://www.juliandyke.com/Presentations/BitmapIndexInternals.ppt 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 25
  26. 26. THANK YOU. Trivadis AG Dani Schnider Europa-Strasse 5 CH-8152 Glattbrugg/Zürich Switzerland Tel. +41 44 808 70 20 Fax +41 44 808 70 21 info@trivadis.com www.trivadis.com BASEL BERN BRUGG LAUSANNE ZÜRICH DÜSSELDORF FRANKFURT A.M. FREIBURG I.BR. HAMBURG MÜNCHEN STUTTGART WIEN 2014 © Trivadis Data Warehouseing & Bitmap Indexes 02.10.2011 26

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