Within relational model systems on both mainframes and personal computers, the main mechanism for increasing the efficiency with which data is retrieved from the database is the .

index

Explanation:

A database index is a data structure that allows a query to efficiently retrieve data from a database. It improves the speed of data retrieval operations on a database table. They are related to specific tables and consist of one or more keys. A table can have more than one index built from it.

Indexes are used to quickly locate data without having to search every row in a database table every time a database table is accessed. Indexes are created using a few database columns. It is an efficient way to optimize the performance of a database by minimizing the number of disk accesses required when a query is processed.

Within relational model systems on both mainframes and personal computers, the main mechanism for increasing the efficiency with which data is retrieved from the database is the index.