Database Systems: Concepts, Design and Applications

11.2. Query Processing

Query processing is the procedure of transforming a high-level query (such as SQL) into a correct and efficient execution plan expressed in low-level language that performs the required retrievals and manipulations in the database. A query processor selects the most appropriate plan that is used in responding to a database request. When a database system receives a query (using query languages discussed in chapter 5) for update or retrieval of information, it goes through a series of query complication steps, called execution plan, before it begins execution. In the first phase, called syntax-checking phase, the system parses the query and checks that it obeys the syntax rules. It then matches objects in the query syntax ...