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