Buissness Model And Mapping Layer:
ü All
columns in logical fact tables are aggregated measures, except for keys required by external clients, or
dummy columns used as a divider.
ü Outer
joins are always defined in the
Business Model and Mapping layer.
ü BMM layer has Complex joins (specified as
Outer joins).
Aggregate Table:
ü Aggregate tables store pre calculated measures
that have been aggregate over a set of dimensional attributes.
ü This is very useful technique for speeding up
query response time in decision support systems.
ü This eliminates the need of run time calculations
and delivers faster results to users.
ü The calculations are done ahead of time and
the results are stored in the tables.
Aggregate navigation:
ü The aggregate navigation capability of
the Oracle BI Server allows queries to use the information stored
in aggregate tables automatically.
ü To ensure aggregate navigation, a single logical
column will have multiple physical column table sources (one for the fact and
one for the aggregate).
ü OBI Server will know which physical column to choose
according to the granularity of the relevant table sources: aggregated table or
detail fact table.
Level Based Measures:
ü
A measure column is a column
with an aggregate function. Any column with an aggregation rule is a
measure.
ü
We can add level based
measure in dimensions. Level Based measure is a column whose values are always calculated to a specific
level of aggregation.
LTS (logical table
source):
ü Logical table sources define the mappings from
a single logical table to one or more physical tables.
ü The mapping between physical columns and logical
columns are done in this element.
ü One of the uses of multiple LTS is converting a
snow flake schema to star schema.
ü The definition of the logical table source is used:
Ø to specify transformations/calculations (in the
column mapping tab),
Ø to enable aggregate navigation and fragmentation (in
the content tab)
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