Aggrigates

Question

Hi Experts,
I have some doubts,
*Can we create Aggregates on Hierarchies?
*What is Match Box in Business Content?
*What is Slowly changing Dimension?
*What is Navigational Attribute in ODS?
*Will Navigational Attribute affect the Query performance?
Regards,
Siva

Answer 1

Hi
An aggregate is a rollup of fact data where a total value is sufficient and no detailed information is needed.
So aggregates are like InfoCubes except that they summarize or aggregate data from an InfoCube.
When you use an aggregate, the summarization it represents does not need to be done during
runtime.
Aggregate functions happen in the background. They are not visible to the end-user.
Aggregates can be created: are not possible for:
For BasicCubes
On dimension characteristics
On navigational attributes
On hierarchy levels
Aggregates Cannot be created on
Multiprovider
RemoteCube
Ods Object
Infoset
When we need to use aggregates:
As a rule of thumb, an aggregate is reasonable and may be created if,
Aggregation ratio >10 i.e, 10 times more records are read than are displayed ( Aggregation ratio = Number of records read from DB / Number of records transferred)
AND
Percentage of DB Time > 30% ie; the time spent on database is a substantial part of the whole query runtime
I hope for creating aggregates i think the thumrule should be successful... But where can we check that Aggregation ratio > 10% and Percentage DB time > 30%.
AGGREGATES
There are three methods for viewing this data.
1. You can see in table RSDDSTAT.
2. By using t-code ST03
( Expert mode -> BW system load -> last minute's load)
3. By implementing BW Statistics.
(AWB -> Tools -> BW statistics for info cubes
o: OLAP (front end)
w: Warehouse (loading)
-> Delete specify from date, to date
-> select cube
-> execute )
You will have the following fields
QDBSL - No. of records selected
QDBTRANS - No. of records transferred.
QTIMEOLAP - OLAP time
QTIMEDB - DB time
QTIMECLIENT - Front end time
If QDBSL/QDBTRANS RATIO > 10 AND DBTIME >30 we will use aggregates.
Why we need to use this
Rollup -Moving of data from cube to aggregate
If you have aggregates on the Cube ,data will not be available for reporting until & unless you roll up the data in aggrgates .
If you use navigational attribute in the report you can drilldown to the lowest details. You can find whether the attribute is display or navigational in the tab "Attributes" of the InfoObject screen.
Navigational attributes are part of the “extended star schema”. Navigational attributes require additional table joins at runtime (in comparison to dimension characteristics) so having an impact on performance.
– but usually the decision for dimension characteristics or navigational attributes is based on business requirements rather than on performance considerations.
Navigation attributes are joins that need to be performed at query execution. There is no limit that I know of for navigational attributes but for large queries , it is desirable to minimize navigational attributes.
Joins at runtime for query generation are very expensive and should be minimized.
Check the following link to get an idea of nav attribute.
http://help.sap.com/erp2005_ehp_03/helpdata/EN/2e/caceae8dd08e48a63c2da60c8ffa5e/frameset.htm
http://help.sap.com/saphelp_nw70/helpdata/en/80/1a63e7e07211d2acb80000e829fbfe/frameset.htm
hope it helps
Regards
gaurav