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Answer Posted / prodyot sarkar
The staging area is:-
* One or more database schema(s) or file stores used to “stage” data extracted from the source OLTP systems prior to being published to the “warehouse” where it is visible to end users.
* Data in the staging area is NOT visible to end users for queries, reports or analysis of any kind. It does not hold completed data ready for querying.
* It may hold intermediate results, (if data is pipelined through a process)
* Equally it may hold “state” data – the keys of the data held on the warehouse, and used to detect whether incoming data includes New or Updated rows. (Or deleted for that matter).
* It is likely to be equal in size (or maybe larger) than the “presentation area” itself.
* Although the “state” data – eg. Last sequence loaded may be backed up, much of the staging area data is automatically replaced during the ETL load processes, and can with care avoid adding to the backup effort. The presentation area however, may need backup in many cases.
* It may include some metadata, which may be used by analysts or operators monitoring the state of the previous loads (eg. audit information, summary totals of rows loaded etc).
* It’s likely to hold details of “rejected” entries – data which has failed quality tests, and may need correction and re-submission to the ETL process.
* It’s likely to have few indexes (compared to the “presentation area”), and hold data in a quite normalised form. The presentation area (the bit the end users see), is by comparison likely to be more highly indexed (mainly bitmap indexes), with highly denormalised tables (the Dimension tables anyway).
The staging area exists to be a separate “back room“ or “engine room” of the warehouse where the data can be transformed, corrected and prepared for the warehouse.
It should ONLY be accessible to the ETL processes working on the data, or administrators monitoring or managing the ETL process.
In summary. A typical warehouse generally has three distinct areas:-
1. Several source systems which provide data. This can include databases (Oracle, SQL Server, Sybase etc) or files or spreadsheets
2. A single “staging area” which may use one or more database schemas or file stores (depending upon warehouse load volumes).
3. One or more “visible” data marts or a single “warehouse presentation area” where data is made visible to end user queries. This is what many people think of as the warehouse – although the entire system is the warehouse – it depends upon your perspective.
The “staging area” is the middle bit.
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