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Query Processing - Coggle Diagram
Query Processing
Selection Operation
File scanning is the lowest operator to access data
File scanning: search algorithms that locate and retrieve records meeting a selection condition
Fast use of indexes or tree structures b+
Primary index:allows to read records in physical order
Secondary index: does not allow reading of a file in physical order.
Sorting
Can be made by building an index on the sort key
Not be efficient in terms of disk access
Important for query processing
Can improve the performance of relational operations such as joins.
Evaluation of Expressions
Evaluation of one operation at a time
Involves the creation of temporary relationships
Are written to disk, and can be inefficient
Simultaneous evaluation of several operations in a pipeline
Results in a single operation without the need for a time relationship
Measures of Query Cost
Evaluating a practice requires comparing cost plans in search of the best plan.
Estimating individual operating costs and combining them
It can be measured in disk accesses, CPU execution time, and distributed or parallel systems.