Relational databases are based on the relational model of data which is powerful and used for a wide variety of information. You only need to specify the result you want with a query language like SQL. DBMS converts a query into an execution plan which is a tree of operators, based on certain set of rules and cost functions. Execution of the plan is that data flows from the leaves towards the root, and the root node produces the final result. Query execution performances depend on both the execution plan and performance of operator nodes.