Frequently Asked Questions (FAQ) about Multidimensional Clustering (MDC) for IBM DB2 for Linux, UNIX, and Windows Applies to: SAP NetWeaver Business Warehouse 7.0 and higher, SAP BW 3.0, 3.1, and 3.5 on DB2 for Linux, UNIX, and Windows. For more information, visit the Landscape Design and Architecture homepage Summary Multidimensional Clustering (MDC) is a feature of DB2 for Linux, UNIX, and Windows. This feature is designed for OLAP systems and has been integrated in the SAP NetWeaver BW environment. This document provides answers to frequently asked questions about MDC. Author: Carola Langwald Company: IBM Deutschland Research & Development GmbH Created on: March 5th, 2010 Author Bio Carola Langwald works as a Senior Software Engineer for the SAP DB2 for Linux, UNIX, and Windows (SAP DB2 for LUW) development team of the IBM Böblingen Lab. She was responsible for the implementation of the MDC Advisor in the SAP DBA Cockpit. In addition, she is involved in customer support for SAP NetWeaver BW environments and DB2 for LUW features like multidimensional clustering, database partitioning feature, and row compression. 2010 SAP AG 1
Table of Content Frequently Asked Questions (FAQ) about Multidimensional Clustering (MDC) for IBM DB2 for Linux, UNIX, and Windows... 3 Is MDC automatically installed with DB2 or is it an additional feature?... 3 For which objects can I define MDC?... 3 How do I use MDC in an SAP NetWeaver BW system?... 3 How can I check if there are tables on my system that are already defined for MDC?... 4 How can I identify InfoProviders or tables that are suitable candidates for MDC?... 4 Do MDC tables need to be reorganized?... 5 In which scenarios does MDC not help?... 5 Do aggregates inherit the MDC settings that are defined for the respective InfoCube?... 5 Which are the relevant SAP Notes concerning MDC in SAP BW 3.x and SAP NetWeaver BW 7.x and higher? 6 How do transports affect MDC InfoCubes?... 6 Which extent size should I use for tablespaces where MDC tables are stored? What about disk space consumption?... 7 Prerequisites:... 7 Example 1:... 7 Example 2:... 8 Related Content... 9 Disclaimer and Liability Notice... 10 2010 SAP AG 2
Frequently Asked Questions (FAQ) about Multidimensional Clustering (MDC) for IBM DB2 for Linux, UNIX, and Windows In the following, IBM DB2 for Linux, UNIX, and Windows is referred to as DB2. Is MDC automatically installed with DB2 or is it an additional feature [page 3]? For which objects can I define MDC [page 3]? How do I use MDC in an SAP NetWeaver BW system [page 3]? How can I check if there are tables on my system that are already defined for MDC [page 4]? How can I find InfoProviders or tables that are suitable candidates for MDC [page 4]? Do MDC tables need to be reorganized [page 5]? In which scenarios does MDC not help [page 5]? Do aggregates inherit the MDC settings that are defined for the respective InfoCube [page 5]? Which are the relevant SAP Notes concerning MDC in SAP BW 3.x and SAP NetWeaver BW 7.x and higher [page 6]? How do transports affect MDC InfoCubes [page 6]? Which extent size should I use for tablespaces where MDC tables are stored? What about disk space consumption [page 7]? Is MDC automatically installed with DB2 or is it an additional feature? MDC is a DB2 feature that is automatically installed with the DB2 software and that is free of charge. It was introduced with DB2 V8.2. In an SAP environment, MDC is supported for SAP NetWeaver BW 7.x and SAP BW 3.x systems. For which objects can I define MDC? You can define MDC for the following objects: InfoCube fact tables Aggregate fact tables DataStore active tables DataStore activation queue tables DataStore change log tables PSA tables How do I use MDC in an SAP NetWeaver BW system? To define MDC settings on PSA tables, DataStore activation queue, and DataStore change log tables, set the RSADMIN parameter DB6_MDC_FOR_PSA to YES. All subsequently created PSA, DataStore activation queue, and DataStore change log tables then use MDC. The default value of this RSADMIN parameter is NO. For InfoCube fact tables and DataStore active tables, you have to choose MDC dimensions manually using transaction RSA1 in your BW system. For more information about MDC, see: Improving Database Performance with Multidimensional Clustering in the Database Administration Guide: SAP NetWeaver Business Warehouse 7.0 and Higher Administration Tasks: IBM DB2 for Linux, UNIX, and Windows on SAP Service Marketplace at: http://service.sap.com/instguidesnw <your SAP NetWeaver Release> Operations Database- Specific Guides SAP Note 942909 DB6: Multi-Dimensional Clustering for SAP BW 3.x 2010 SAP AG 3
Back to top [page 2] How can I check if there are tables on my system that are already defined for MDC? You can choose between the following options: On the database, check the column CLUSTERED of the catalog table SYSCAT.TABLES: o Y = table is multidimensional clustered o NULL value = table is not multidimensional clustered In the BW metadata tables, check for the following: o InfoCubes Check the column PARTMODE of the table RSDCUBE: M = table is multidimensional clustered I or empty = table has a clustered index o DataStore objects Check the column INDEXID of the table RSDODSOIOBJIND: MDC = MDC is defined for the DataStore object o PSA and PSA-like tables For PSA and PSA-like tables, information about MDC is not stored in BW metadata tables. In your BW system, call transaction SE14 and proceed as follows: a. In the Obj. name field, enter the name of the table you want to check and choose Edit. The ABAP Dictionary: Utility for Database Tables screen appears. b. Choose Storage Parameters. The Storage parameters: (display and maintain) screen appears. If MDC is defined for the table, the ORGANIZE BY DIMENSION storage option for the table is displayed. For more information, see Setting Up MDC for DataStore Objects and Setting Up MDC for InfoCubes in the Database Administration Guide: SAP NetWeaver Business Warehouse 7.0 and Higher Administration Tasks: IBM DB2 for Linux, UNIX, and Windows on SAP Service Marketplace at: http://service.sap.com/instguidesnw <your SAP NetWeaver Release> Operations Database-Specific Guides How can I identify InfoProviders or tables that are suitable candidates for MDC? To identify candidates for MDC, check which InfoProviders are being used in the BW reporting queries and for DELETE operations: Checking the BW reporting queries: By using MDC for an InfoProvider, you can improve the performance of BW reporting queries that are defined on this InfoProvider. You can check the queries manually or by using the MDC Advisor that is integrated in the DBA Cockpit as of Enhancement Package 1 for SAP NetWeaver BW 7.0. Checking the DELETE operations: MDC speeds up DELETE operations because DELETE operations are processed on the MDC blocks and MDC block indexes. Therefore, block indexes have to be updated only once per entire block instead of once per deleted row in a record-based index. o To define MDC for large PSA tables, DataStore activation queue, and DataStore change log tables, set the RSADMIN parameter DB6_MDC_FOR_PSA to YES. MDC is then defined on the REQUEST ID field and deletion by request speeds up. o For InfoCube fact tables and DataStore object active tables: Define MDC on the columns that you are using for selective deletions. 2010 SAP AG 4
Especially for InfoCubes: if you are using BW compression, you should define MDC additionally on the package dimension. During BW compression, the data is deleted by request from the F fact table of an InfoCube. For more information, see Improving Database Performance with Multidimensional Clustering in the Database Administration Guide: SAP NetWeaver Business Warehouse 7.0 and Higher Administration Tasks: IBM DB2 for Linux, UNIX, and Windows on SAP Service Marketplace at: http://service.sap.com/instguidesnw <your SAP NetWeaver Release> Operations Database-Specific Guides Do MDC tables need to be reorganized? No, you do not have to reorganize MDC tables to establish or reestablish MDC. To define MDC for tables that already contain data, you have to recluster the table. To do so, you can use the reclustering tool in the SAP NetWeaver BW system. For more information, see Reclustering InfoCubes and DataStore Objects in the Database Administration Guide: SAP NetWeaver Business Warehouse 7.0 and Higher Administration Tasks: IBM DB2 for Linux, UNIX, and Windows on SAP Service Marketplace at: http://service.sap.com/instguidesnw <your SAP NetWeaver Release> Operations Database-Specific Guides In which scenarios does MDC not help? If you have restrictions on an InfoProvider on several different dimensions and you select them all as MDC dimensions or if your MDC dimension candidates have a high granularity, it might occur that the MDC blocks are sparsely filled and you waste disk space. To avoid this, use the MDC Advisor to identify MDC candidates or use the Space Check option during the reclustering process. For more information, see Checking the Disk Space Consumption of MDC InfoProviders in the Database Administration Guide: SAP NetWeaver Business Warehouse 7.0 and Higher Administration Tasks: IBM DB2 for Linux, UNIX, and Windows at: http://service.sap.com/instguidesnw <your SAP NetWeaver Release> Operations Database-Specific Guides Do aggregates inherit the MDC settings that are defined for the respective InfoCube? Line item aggregates inherit the MDC settings from the InfoCube. If the maximum number of characteristics (16) is not exceeded for the aggregate, dimensions of the basis InfoCube are split into a number of line item dimensions by creating a line item dimension in the aggregate for each characteristic. If you do not want this behavior, you can change the RSADMIN parameter DB6_MDC_FOR_AGGREGATES to NO. 2010 SAP AG 5
Which are the relevant SAP Notes concerning MDC in SAP BW 3.x and SAP NetWeaver BW 7.x and higher? SAP Note Number Title 688647 DB6 SAP BW and SAP NetWeaver BI Administration 832621 DB6: RSADMIN parameters for MDC in SAP NetWeaver 7.0 BI 942909 DB6: Multi-Dimensional Clustering for SAP BW 3.x 971135 DB6: Changes in Multi-Dimensional Clustering 980314 DB6: Filling the aggregate in blocks with MDC 1000382 DB6: MDC for PSA tables activated automatically in BW 3.x 1019826 DB6: Supplementary note about repartitioning (reclustering) 1023410 DB6: BI Migration Conversion of range partitioning to MDC 1088079 DB6: MDC Fast Roll Out with DB2 9.5 or higher 1419738 DB6 MDC Column filled with 0 after Reclustering How do transports affect MDC InfoCubes? Source System Target System Result if InfoCube is empty Result if InfoCube is filled Index Clustering Index Clustering Index Clustering Index Clustering Index Clustering MDC Target Index Clustering MDC Target MDC Source Index Clustering MDC Source Index Clustering MDC Source MDC Target MDC Source MDC Target Rule of thumb: If the InfoCube is empty in the target system, the transported setting is used. If the InfoCube is filled in the target system, the original setting of the source system remains. 2010 SAP AG 6
Which extent size should I use for tablespaces where MDC tables are stored? What about disk space consumption? We recommend that you use an extent size of 2 for tablespaces where MDC tables are stored. The size of the MDC blocks is equal to the extent size. However, depending on your data, the size of your tables, and the MDC dimensions you choose, a larger extent size might not cause high disk space consumption. In the following, we provide two examples of how you can calculate disk space consumption. Based on these calculations, you can determine the best setting for your system. Prerequisites: The tablespace type LARGE is the default for database-managed space (DMS) and for automatic tablespaces. In a LARGE tablespace, a table can support more than 255 rows per data page. The maximum size of a LARGE DMS tablespace is 64 TB. Example 1: The data in an MDC table is stored in blocks. One MDC block equals one extent. In this example, one extent is two pages and the page size is 16 KB, that is, 16,384 bytes. This means that one extent is 32,768 bytes. We assume that one row of a table that is defined for MDC contains five key values and five data columns. Each column requires 10 bytes of storage. So one row of this table is: 5 * 10 bytes + 10 *10 bytes = 150 bytes per row This results in the following: 32,768 bytes / 150 bytes = 218.45 => one MDC block contains 218 rows If KEY1 is your MDC dimension, you perform the following query to obtain the amount of rows for each value of this MDC dimension: SELECT DISTINCT KEY1, COUNT(*) AS COUNT FROM <table> GROUP BY KEY1; KEY1 COUNT 1 700 2 300 3 250 4 800 5 1000 To obtain the number of MDC blocks per MDC value, calculate as follows: COUNT / 218 = <number of blocks>. For example, for KEY1 = 1 this is: 700 rows / 218 rows per block = 3.2 MDC blocks KEY1 = 2: 300 rows / 218 rows per block = 1.3 MDC blocks KEY1 = 3: 250 rows / 218 rows per block = 1.14 MDC blocks 2010 SAP AG 7
KEY1 = 4: 800 rows / 218 rows per block = 3.66 MDC blocks KEY1 = 5: 1000 rows / 218 rows per block = 4.58 MDC blocks In this example, you can use MDC because the disk space consumption is not high. Example 2: We assume that we have the same table as in example 1, the same page size, and the same MDC dimension, but an extent size of 16 pages. Then the size of one MDC block is as follows: 16 pages per extent * 16,385 bytes = 262,144 bytes = 256 KB We assume again that one row of the table is 150 bytes, which means that: 262,144 bytes per MDC block / 150 bytes per table row = 1747.62 That is, rounded down, 1747 rows per MDC block. To obtain the number of MDC blocks per MDC value, calculate as follows: KEY1 = 1: 700 rows / 1747 rows per block = 0.4 MDC blocks KEY1 = 2: 300 rows / 1747 rows per block = 0.17 MDC blocks KEY1 = 3: 250 rows / 1747 rows per block = 0.14 MDC blocks KEY1 = 4: 800 rows / 1747 rows per block = 0,45 MDC blocks KEY1 = 5: 1000 rows / 1747 rows per block = 0.57 MDC blocks For this small table, the disk space consumption is very high, so you should not use MDC. However, if you have a larger table with better disk space consumption, you can also use an extent size of 16 for MDC tables and tablespaces. 2010 SAP AG 8
Related Content Database Administration Guide: SAP NetWeaver Business Warehouse 7.0 and Higher Administration Tasks: IBM DB2 for Linux, UNIX, and Windows in SAP Service Marketplace at: http://service.sap.com/instguidesnw <your SAP NetWeaver Release> Operations Database-Specific Guides For more information, visit the Landscape Design and Architecture homepage. 2010 SAP AG 9
Disclaimer and Liability Notice This document may discuss sample coding or other information that does not include SAP official interfaces and therefore is not supported by SAP. Changes made based on this information are not supported and can be overwritten during an upgrade. SAP will not be held liable for any damages caused by using or misusing the information, code or methods suggested in this document, and anyone using these methods does so at his/her own risk. SAP offers no guarantees and assumes no responsibility or liability of any type with respect to the content of this technical article or code sample, including any liability resulting from incompatibility between the content within this document and the materials and services offered by SAP. You agree that you will not hold, or seek to hold, SAP responsible or liable with respect to the content of this document. 2010 SAP AG 10