A virtual warehouse is a set of views over an operational database For efficient query processing, only some of the possible summaries may be materialized. A virtual data warehouse is easy to build but -requires excess capacity on operational database servers.
This model creates a virtual view of databases, allowing the creation 0f • “virtual warehouse” as opposed to a physical warehouse. In a virtual warehouse, you have a logical description of all the databases and their structures, individuals who want to get information from those databases do not have to know anything about them.
This approach creates a single “virtual database” from all the resources. The data resources can be local or remote. In this type of data warehouse, the data is not moved from the sources: Instead, the users are given direct access to the data. Direct access to the data is sometimes through simple SQL queries, view definition, or data-access middleware.
With this approach, it is possible to access remote data sources including major RDBMSs. The virtual data warehouse scheme lets a client application access data distributed across multiple data sources through a single SQL statement, a single interface. All data sources are accessed as though they are local users and their applications do not even need. to know the physical location of the data.
There is a great benefit in starting with a virtual warehouse since many organizations do not want to replicate information in the physical data warehouse. Some organizations decide to provide both by creating a data warehouse containing summary-level data with access to legacy data for transaction details.
A virtual database is easy and fast, but it is not without problems. Since the queries must compete with the production data transactions, their performance can be considerably degraded. Since there is no metadata, no summary data, or history; all the queries must be •repeated, creating an additional burden or. the system. Above all, there is no clearing and refreshing process, which involves causing the queries to become very complex.