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Queries

In: Computers and Technology

Submitted By wwatson2002
Words 345
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Assignment #7 – Queries A query expressed in a high-level query language such as SQL must first be scanned, parsed, and validated. The scanner identifies the language tokens, such as SQL keywords, attributes names, and relation names in the text of the query, whereas the parser checks the query syntax to determine whether it is formulated according to the syntax rules of the query language. The query must also be validated, by checking that all attribute and relation names are valid and semantically meaningful names in the schema of the particular database being queried. An internal representation of the query is then created, usually as a tree data structure called a query tree. It is also possible to represent the query using a graph data structure called a query graph. The DBMS must then devise an execution strategy for retrieving the result of the query from the database files. A query typically has many possible execution strategies, and the process of choosing a suitable one for processing a query is known as query optimization.
Optimization in a database makes things work smoothly. One heuristic rule is to use SELECT operation before using the JOIN or other Binary operations. This is because the resultant size of the file from a Binary operation such as JOIN is usually a multiplicative function of the sizes of the input files. So, The SELECT operation reduces the size of a file so it must be used before the JOIN or other binary Operation.
Secondly, we can implement optimization by using the concept of indexes while creating the table. Each index consists of a leading column and a Boolean value which specifies whether the index is covering during the index lookup.
Lastly, we can use Group-by Clauses to implement optimization in our query. In the 3-way join that covers the group-by tables result results in a very big intermediate result. By pulling the…...

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