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

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4.1 Dynamic index ANDing <strong>for</strong> star schema queries<br />

The new dynamic index ANDing is a new enhanced star join access method<br />

introduced in <strong>DB2</strong> 9 <strong>for</strong> z/<strong>OS</strong> as a main requirement from <strong>SAP</strong> <strong>BI</strong>. The goal of the<br />

new access method is to improve and stabilize data warehouse query<br />

per<strong>for</strong>mance and to lead to more predictable query per<strong>for</strong>mance.<br />

This is done by introducing self-automating and self-defending access paths and<br />

uses parallelism. This means that the access path used at run time is dynamic<br />

and can be changed within the query processing by the <strong>DB2</strong> engine. One further<br />

goal of dynamic index ANDing is to simplify the index design on fact tables. The<br />

main idea behind dynamic index ANDing access is to apply the most filtering<br />

dimensions or snowflake branches be<strong>for</strong>e the E- fact table and to do this in an<br />

efficient way.<br />

Dynamic index ANDing has the following characteristics:<br />

► Better exploitation of <strong>SAP</strong> <strong>BI</strong> single column fact table indexes<br />

No need <strong>for</strong> additional multi-column indexes <strong>for</strong> leveraging star join access<br />

► Consistent parallelism<br />

– Independent filtering dimension access in parallel<br />

– fact table access (and post fact table) in parallel<br />

► Adaptive query execution based upon runtime filtering<br />

– Less filtering dimensions can be discarded <strong>for</strong> pre- fact table access.<br />

– Self-automating and self-defending access path.<br />

– Runtime fallback to workfile <strong>for</strong> RID processing. This avoids RID pool<br />

failures.<br />

► RID pool overflow to workfile<br />

► Less dependent on perfect statistics<br />

– Although optimizer costing is still per<strong>for</strong>med<br />

– Better tolerance of less than perfect access path choice<br />

► Provides more predictable query per<strong>for</strong>mance<br />

36 <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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