Distributed queries are queries that involve data from data sources on multiple computers or multiple instances of SQL Server. SQL Server supports distributed queries through OLE-DB, a library that allows connections to any data sources that has an OLE DB provider.

A Query processing in a distributed database management system requires the transmission of data between the computers in a network. Generally, a query in Distributed DBMS requires data from multiple sites, and this need for data from different sites is called the transmission of data that causes communication costs.

Definition. Distributed query processing is the procedure of answering queries (which means mainly read operations on large data sets) in a distributed environment where data is managed at multiple sites in a computer network.

The four basic operations on a database are Create, Retrieve, Update and Delete. CREATE database structure and populate it with data − Creation of a database relation involves specifying the data structures, data types and the constraints of the data to be stored.

Distributed query optimization refers to the process of producing a plan for the processing of a query to a distributed database system. The plan is called a query execution plan. In a distributed database system, schema and queries refer to logical units of data.

Objectives of Distributed Query Processing The main objectives of query processing in a distributed environment is to form a high level query on a distributed database, which is seen as a single database by the users, into an efficient execution strategy expressed in a low level language in local databases.

Layers of Query Processing

• Query Decomposition. The first layer decomposes the calculus query into an algebraic query on global relations.
• Data Localization. The input to the second layer is an algebraic query on global relations.
• Global Query Optimization.
• Distributed Query Execution.

The basic function of DDBMS is to keep track of the data distribution, fragmentation and replication by expanding the DDBMS catalog. The basic function of DDBMS is basically its ability to access remote sites and to transmits queries and data among the various sites via a communication network.

A Query processing in a distributed database management system requires the transmission of data between the computers in a network. A distribution strategy for a query is the ordering of data transmissions and local data processing in a database system.

Local Query Optimization. This stage is common to all sites in the DDB. The techniques are similar to those used in centralized systems. The first three stages discussed above are performed at a central control site, while the last stage is performed locally. 2. Data Transfer Costs of Distributed Query Processing

A distribution strategy for a query is the ordering of data transmissions and local data processing in a database system. Generally, a query in Distributed DBMS requires data from multiple sites, and this need for data from different sites is called the transmission of data that causes communication costs.

The input query on distributed data is specified formally using a query language. It is then translated into an algebraic query on global relations. This translation is done by referring to the global conceptual schema and does not take into account the actual distribution and replica-tion of data.