Define Query Processing

Query processing is the process of selecting the best plan or strategy to be employed in responding to a database request. The plan is then executed to produce a response. The component of the DBMS accountable for generating this strategy is known as a query processor. It is a step by step process. The first or initial step is to transform the query into a standard from. For instance, a query in QBE   looked into by the research community.  Query might be transformed into a relational algebraic expression; the optimization is carried out by substituting equivalent expressions for those in the query. In the next step a number of strategies known as access pans are generated for evaluating the transformed query. The physical characteristics of the data and any supporting access method are taken into account in producing alternate access pans. The cost of each access plan is calculated and the optimal one is selected and executed.