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what is difference between star schema and extend star
schema?
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Answer Posted / kayani
my knowledge
is sap as data is divided into masterdata(data frequently not changed)+transaction data(frequently change of data).
arranging of masterdata(m.d) and transaction data(t.d) in dataware house we use a procedure that procedure called schema. as transactiondata table is middle of masterdata table we termed it as star schema.
disadvantages of star schema :
1)when we are designing a query we have infoprovider as infocube where transaction data is stored in infocube to perform query we need both m.d and t.d so data manager has to join both m.d table and infocube it consumes time, accessing time is more performance degrading takes place.
2)in star schema the master data (m.d)and t.d table split at one pointer so in a server there are many infocubes if many cubes want to use same masterdata we cannot access the same masterdata into another cube but only copy so data redundancy takes place same data is stored in multiple places in the server.
3)star schema doesn't support extrnal hirearchy model.
finally the limitations of star schema are:
1)system performance degradation due to single pointer.
2)data redundancy.
3)starschema doesn't support external hierarchy.
to overcome this limitation we go for extended star schema model.
1).where transactiondata table resides in center followed by dimensional datatable followed by sid table(which consists of masterdata table) out side of the dimensional table. so in performing a query data is accessible from one infocube to another so we overcome from data redundancy system performance increases.
2)when performing a query as the masterdata table and trasactional data table split not at one pointer but if n masterdata table and 1 transaction data table then it splits into (n+1) table there is no join so system performance further increases.
3) it supports external hirerchy model
aftr all we can eliminate system degrading performance,dataredundancy and we can use external hirerchy models.
note:
in extended star schema the master data table (sid tables ) connects the dimensional table through sid's i.e surrogate system genrated ids and dimensional table with transactional data table through dimensional id's. where as in star schema masterdata table through dimensional ids only.as all masterdata resides in one table masterdata table.
hope this might be understand in overview.
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