Data Warehousing Questions

Bitmap indexes make use of bit arrays (bitmaps) to answer queries by performing bitwise logical operations.

Bitmap indexes are useful in the data warehousing applications.

Bitmap indexes have a significant space and performance advantage over other structures for such data.

Tables that have less number of insert or update operations can be good candidates.

The advantages of Bitmap indexes are:

   - They have a highly compressed structure, making them fast to read.

   - Their structure makes it possible for the system to combine multiple indexes together so that they can      access the underlying table faster.

The Disadvantage of Bitmap indexes is:

  - The overhead on maintaining them is enormous. 

View:

- Tail raid data representation is provided by a view to access data from its table.

- It has logical structure can not occupy space.

- Changes get affected in corresponding tables.

Materialized view

- Pre calculated data persists in materialized view.

- It has physical data space occupation.

- Changes will not get affected in corresponding tables.

Following are the data integration factors:

- Sub set of the available data should be optimal. 

- Noise/distortion estimation levels because of sensory/processing conditions at the time of data collection.

- Accuracy, spatial and spectral resolution of data.

- Data formats, storage and retrieval mechanisms.

- Efficiency of computation for integrating data sets to reach the goals.

Business facts or measures and foreign keys are persisted in fact tables which are referred as candidate keys in dimension tables. Additive values are usually provided by the fact tables which acts as independent variables by which dimensional attributes are analyzed.

Attributes that are used to constrain and group data for performing data warehousing queries are persisted in the dimension tables.

•  NO_CACHE – It is used for not caching values.


•  PRE_LOAD_CACHE – Result column preloads and compares the column into the memory, prior to executing the lookup.


•  PRE_LOAD_CACHE is used when the table can exactly fit in the memory space.


•  DEMAND_LOAD_CACHE – Result column loads and compares the column into the memory when a function performs the execution.


•  DEMAND_LOAD_CACHE is suitable while looking up the highly repetitive values with small subset of data.

Look for the existence of the following items:

- Generic Data Models

- An Enterprise Data Platform

- Identify the Data Sources

- Selection of a MDM Product

- Implementation of a Customer Master Index or appropriate alternative

- Scheduling and executing the batch jobs.

- Configuring, starting and stopping the real-time services

- Adapters configuration and managing them.

- Repository usage, Job Server configuration.

- Access Server configuration.

- Batch job publishing.

- Real-time services publishing through web services.

In a SQL Database, a 'correlated subquery' is a kind of sub query yet connected subquery is dependent on another query for a value that is returned. In case of execution, the sub query is executed first and afterwards the correlated query will be executed.