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Query Processing Query Processing would mean the entire process or activity which involves query translation into low level instructions, query optimization to save resources, cost estimation or evaluation of query, and extraction of data from the database. Evaluation Plans in DBMS. When a query is submitted to DB, it is parsed and translated to relational algebra. It is verified for its validity and correctness. Once it passes this stage, different ways of evaluating the query is generated. It is checked for various factors and its execution plan is generated. It may be based on cost of the query. The paper is devoted to the problem of effective query execution in clusterbased systems. An original approach to data placement and replication on the nodes of a cluster system is presented. Based on this approach, a load balancing method for.

A portal for computer science studetns. It hosts well written, and well explained computer science and engineering articles, quizzes and practice/competitive programming/company interview Questions on subjects database management systems, operating systems, information retrieval, natural language processing, computer networks, data mining, machine learning, and more. Approximate Query Processing in Oracle Database 12c Release 2 (12.2) The APPROXCOUNTDISTINCT function was added, but not documented, in Oracle 11g to improve the speed of calculating the number of distinct values (NDV) when gathering statistics using the DBMSSTATS package.

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The activities involved in parsing, validating, execution and optimizing a query is called Query Processing.

Steps

The steps involved in query processing and optimization are as follows −

A sequence of primitive operations that can be used to evaluate a query is called query execution plan or query evaluation plan.
The query execution engine takes a query evaluation plan, executes that plan and produces the desired output. The different execution plans for a given query can have different costs based on the number of disks. It is the responsibility of the system to construct a query evaluation plan which minimizes the cost of query evaluation. This task is called query optimization.
A query optimization is expressed in high level query language and is scanned, parsed and validated. The scanner identifies the SQL keywords, attributes and relation names in the text of the query.
The parser checks the syntax which is used to determine if the query is formulated according to syntax rules of the query language.
Finally, the query is evaluated by checking that all attributes and relation names are valid and semantically meaningful in the schema of a particular database.
An internal representation of the query is then created which is either a tree or a graph known as query tree or query graph.
If the query written SQL is translated into relational algebra, then its internal representation is a query tree. Otherwise, if TRC or DRC its internal representation is a query graph. A graph has many possible execution strategies and the process of choosing a strategy with minimum cost is called query optimization.
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The query processing and optimization in the DBMS are explained in the form of a diagram below −

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