# Query Processor

It is responsible for parsing, translating, optimizing and evaluating queries.

![](/files/-M5-0_5FhvvEqMr82srv)

There are 2 alternative ways to evaluate a query:

* using equivalent expressions
* using different algorithms for each operation

We must choose the method that has the least cost.

There are two approaches to select the best query evaluation plan:

* **cost-based**: considers the cost of the operations; however, it needs statistical information about data in the database
* **rule-based**: uses a set of rules to determine how to execute a query


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