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Best Practices for SAP BI using DB2 9 for z/OS - IBM Redbooks

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12.8 Aggregates<br />

4. The dimension tables are checked to see whether the combination of SID<br />

values that correspond to the attribute values from the PSA record exist in the<br />

dimension tables. For example, the product dimension table is checked to<br />

see if the SID combination 2/2 exists, which corresponds to the attribute<br />

combination Aircraft A/Airplanes.<br />

If the SID combination exists, the corresponding dimension identifier is<br />

retrieved. If the SID combination does not exist, it is inserted in the dimension<br />

table and a corresponding dimension identifier is generated.<br />

5. After the dimension identifiers that correspond to the given attribute values<br />

are retrieved/generated, a new record is inserted into the E- fact table. It<br />

contains the key figures from the PSA record and the corresponding<br />

dimension identifiers.<br />

Aggregates represent another technique <strong>for</strong> improving query per<strong>for</strong>mance. They<br />

are materialized, summarized views of the data in an InfoCube, and store a<br />

subset of InfoCube data in a redundant <strong>for</strong>m. When an appropriate aggregate <strong>for</strong><br />

a query exists, the summarized data can be read directly from the database<br />

during query execution instead of having to per<strong>for</strong>m this summarization during<br />

runtime. In <strong>SAP</strong> <strong>BI</strong>, the system generates suggestions <strong>for</strong> creating optimal<br />

aggregates. The system administrator can then decide whether to create those<br />

aggregates.<br />

Aggregates reduce the volume of data to be read from the database, speed up<br />

query execution time, and reduce the overall load on the database. To use<br />

aggregates you must build and maintain them, and they require additional space<br />

in the database. Aggregates are very similar to the automatic summary tables<br />

defined in <strong>DB2</strong>.<br />

An easy way to conceptualize an aggregate is to think of it as providing a benefit<br />

similar to adding another index to a database table. Aggregates require no<br />

special knowledge by the user, and in fact are completely transparent to the<br />

user. The only way an user might recognize the existence of an aggregate is by<br />

the per<strong>for</strong>mance gain that is observed.<br />

268 <strong>Best</strong> <strong>Practices</strong> <strong>for</strong> <strong>SAP</strong> <strong>BI</strong> <strong>using</strong> <strong>DB2</strong> 9 <strong>for</strong> z/<strong>OS</strong>

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