The user typically writes his requests in SQL language. This is the first step of any query processing in DBMS. There are four phases in a typical Query Processing in DBMS. This is an overview of how query processing works. The command processor then uses this execution plan to retrieve the data from the database and returns the result. The query execution plan then decides the best and optimized execution plan for execution. It generates different execution plans for the query plan. It checks in the system catalog for the constraints and indexes and decides the best query plan. The query optimizer then picks them to identify the best query plan to process. Then the tokenized queries are transformed into different possible relational expressions, relational trees and relational graphs (Query Plans). Once the tokens are generated, they are verified for their correctness by the parser.

It then scans the query and divides it into individual tokens. When a query is submitted to the database, it is received by the query compiler.

The above diagram depicts how a query is processed in the database to show the result. A query processor in DBMS performs this task. It is the step-by-step process of breaking the high-level language into a low-level language in which the machine can understand and perform the requested action for the user. All these processes are together known as query processing in DBMS. It then selects the best execution path and executes the query and gets the data from internal memory. It verifies the code written by the user and then converts them into low-level languages. Hence what DBMS does is it ask its users to write queries in SQL. But it will be difficult for any user to directly write relational algebra kinds of queries. Usually, any query written in SQL is converted into low-level language using relational algebra which the system can understand. There has to be some low-level language that these systems can understand. But the underlying systems in the DBMS will not understand SQL. This SQL is a high-level language created to build a bridge between users and DBMS for their communication.

Hence the users are asked to query the database in its language – SQL. But DBMS has its own language (SQL) which it understands. The user can request the data in a language that he understands. it should be accessed easily and quickly.īut a database is a system and the users are either another system or application or a person. Accessing and manipulating the data should be done efficiently i.e. Query Processing in DBMS – The main goal of creating a database is to store the related data at one place, access and manipulate them as and when it is required by the user. This can also be represented in relational structures like tree and graphs as below:.There are four phases in a typical Query Processing in DBMS.

Query processing in dbms