How does an RDBMS handle query optimisation?

An RDBMS optimises queries by analysing them and determining the most efficient way to access the requested data.

Query optimisation is a key function of a Relational Database Management System (RDBMS). It involves the process of choosing the most efficient means of executing a SQL query. The optimiser's goal is to minimise the system resources required to fulfil the query, thereby improving the system's overall performance.

The RDBMS uses a component known as the query optimiser to handle this task. The query optimiser analyses the query and the database schema, and then generates an execution plan. This plan outlines the steps the system will take to execute the query, including which indexes to use, which join algorithms to apply, and in what order to access the tables.

The optimiser uses a cost-based approach to determine the best execution plan. It estimates the cost of different execution plans based on factors such as the number of disk I/O operations, the amount of CPU time, and the amount of memory required. The optimiser then selects the plan with the lowest estimated cost.

The optimiser also takes into account statistics about the data stored in the database. These statistics, which are typically collected by the RDBMS automatically, provide information about the distribution and density of the data, the number of distinct values in a column, and the average row length. The optimiser uses these statistics to make more accurate cost estimates.

In addition to cost-based optimisation, the RDBMS may also use rule-based optimisation. In this approach, the optimiser applies a set of predefined rules to transform the query into an equivalent form that can be executed more efficiently. For example, the optimiser may rewrite a query to take advantage of an index, or to eliminate unnecessary operations.

In conclusion, query optimisation is a complex process that involves analysing the query, the database schema, and the data itself to determine the most efficient execution plan. The goal is to minimise the use of system resources and improve the performance of the RDBMS.