The Query Language – SQL is one of the main reasons of success of RDBMS. A user just needs to specify the query in SQL that is close to the English language and does not need to say how such query is to be evaluated. However, a query needs to be evaluated efficiently by the DBMS. But how is a query-evaluated efficiently? This unit attempts to answer this question. The unit covers the basic principles of query evaluation, the cost of query evaluation, the evaluation of join queries, etc. in detail. It also provides information about query evaluation plans and the role of storage in query evaluation and optimisation. This unit thus, introduces you to the complexity of query evaluation in DBMS.
let us begin by defining query processing. Let us take a look at the Figure
In the first step Scanning, Parsing, and Validating is done to translate the query into its internal form. This is then further translated into relational algebra (an intermediate query form). Parser checks syntax and verifies relations. The query then is optimised with a query plan, which then is compiled into a code that can be executed by the database runtime processor.
We can define query evaluation as the query-execution engine taking a query-evaluation plan, executing that plan, and returning the answers to the query. The query processing involves the study of the following concepts:
• how to measure query costs?
• algorithms for evaluating relational algebraic operations.
• how to evaluate a complete expression using algorithms on individual operations?
let us begin by defining query processing. Let us take a look at the Figure
In the first step Scanning, Parsing, and Validating is done to translate the query into its internal form. This is then further translated into relational algebra (an intermediate query form). Parser checks syntax and verifies relations. The query then is optimised with a query plan, which then is compiled into a code that can be executed by the database runtime processor.
We can define query evaluation as the query-execution engine taking a query-evaluation plan, executing that plan, and returning the answers to the query. The query processing involves the study of the following concepts:
• how to measure query costs?
• algorithms for evaluating relational algebraic operations.
• how to evaluate a complete expression using algorithms on individual operations?
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