A crucial property of a relational DBMS is that it provides physical data independence. This allows physical structures such as indexes to change seamlessly without affecting the output of the query; but such changes do impact efficiency. Thus, together with the capabilities of the execution engine and the optimizer, the physical database design determines how efficiently a query is executed on a DBMS. The first generation of relational execution engines were relatively simple, targeted at OLTP, making index selection less of a problem. The importance of physical design was amplified as query optimizers became sophisticated to cope with.
