Subject: Distributed Database

Topic: Concept and Overview Distributed Database system

Difficulty: Medium

Objectives of Distributed Query Processing

1. 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.

2. An important point of query processing is query optimization. Because many execution strategies are correct transformations of the same high level query the one that optimizes (minimizes) resource consumption should be retained.

3. The good measure of resource consumption are:

   i. The total cost that will be incurred in processing the query. It is the dome of all times incurred in processing the operations of the query at various sites and intrinsic communication.

   ii. The resource time of the query. This is the time elapsed for executing the query. Since operations can be executed in parallel at different sited, the response time of a query may be significantly less than its cost.

4. Obviously the total cost should be minimized.

   i. In a distributed system, the total cost to be minimized includes CPU, I\O, and communication costs. This cost can be minimized by reducing the number of I\O operation through fast access methods to the data and efficient use of main memory. The communication cost is the time needed for exchanging the data between sited participating in the execution of the query.

   ii. In centralized systems, only CPU and I\O cost have to be considered.