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Introduction
Query processing involves extracting data from a database, typically through various steps to fetch the required information efficiently.
In this article, we will discuss about query processing in DBMS. We will discuss what steps are there. Query processing is a very important component in DBMS. It allows users to retrieve data from the database very accurately and efficiently. Before moving on to the main topic, let us understand what a query in DBMS is.
What is a Query in DBMS?
A query is a kind of request that is sent to the Database for retrieval of data. In a query, we pass some specific condition, and then it matches the specific data if it is present in the database.
We can write queries in SQL( Structured Query Language). This language is a way of communication between the user and the relational database. We can perform different operations on data by writing queries. We can select the data, delete the data, insert something, etc. Let us understand a query with the help of an example.
Suppose we want to fetch data about a person. This data comes from the employee table whose salary is greater than 10000. Then the query for this condition is
Select * from employee where salary>10000;
A query can be simple or more complex. It completely depends on the complexity of the data. It also depends on the requirements of the user.
Now that you know about a query, let us understand the query processing in DBMS.
Query processing is a process of translating a user query into an executable form. It helps to retrieve the results from a database. In query processing, it converts the high-level query into a low-level query for the database. Query processing is a very important component of DBMS. It is critical to the performance of applications that rely on database operations. The flow of query processing in DBMS is mentioned below:
Let us understand the steps involved in query processing in DBMS.
Steps in Query Processing
Query processing in DBMS involves several steps. These steps are mentioned below:
Parsing and Translation
Query parsing is the first step in query processing. In this step, a query is checked for syntax errors. Then it converts it into the parse tree. So, a parse tree represents the query in a format that is easy to understand for DBMS. A parse tree is used in other steps of query processing in DBMS.
Optimization
After doing query parsing, the DBMS starts finding the most efficient way to execute the given query. The optimization process follows some factors for the query. These factors are indexing, joins, and other optimization mechanisms. These help in determining the most efficient query execution plan. So, query optimization tells the DBMS what the best execution plan is for it. The main goal of this step is to retrieve the required data with minimal cost in terms of resources and time.
Evaluation
After finding the best execution plan, the DBMS starts the execution of the optimized query. And it gives the results from the database. In this step, DBMS can perform operations on the data. These operations are selecting the data, inserting something, updating the data, and so on.
Once everything is completed, DBMS returns the result after the evaluation step. This result is shown to you in a suitable format.
Query Evaluation Plan
A query evaluation plan, also known as an execution plan, is a fundamental concept in database management systems (DBMS). It outlines the steps and methods the DBMS will use to retrieve data in response to a user's query.
When a user submits a query to a DBMS, the system analyzes the query and creates an execution plan to efficiently retrieve the requested data. This plan is a detailed roadmap that guides the DBMS through the process of accessing, filtering, and processing data from the underlying database tables.
The query evaluation plan typically includes several key components:
Access Methods: These specify how the DBMS will access the data stored in the database tables. Common access methods include sequential scans, index scans, and nested loop joins.
Join Methods: If the query involves joining multiple tables, the execution plan will outline the method used to perform the join operation, such as nested loop joins, hash joins, or merge joins.
Filtering and Sorting: The plan may include steps to filter and sort the data based on the conditions specified in the query, using techniques like filter conditions, index scans, and sort operations.
Optimization: Modern DBMSs employ various optimization techniques to improve query performance. The execution plan may include details about how the system optimizes the query, such as selecting the most efficient access paths and join strategies.
Cost Estimation: Some DBMSs estimate the cost associated with each step of the execution plan to determine the most efficient plan. This allows the system to choose the plan with the lowest estimated cost for executing the query.
Let us understand the complete query processing process with the help of an example.
Let's consider an example to show you the query processing steps. Suppose we have a database with a table “ninjas”. It contains the ninjas_id, first_name, last_name, and question_solved. The following SQL query is used to retrieve the names of all ninjas whose question_solved is greater than 50:
SELECT first_name, last_name FROM ninjas WHERE question_solved > 50;
Now let us understand how this query will give us results by following the query processing steps.
Parsing: Firstly, the query will be parsed. This will also check whether the syntax is correct or not. Then this query will be converted into a parse tree. This tree will look like this
SELECT
|
+-- first_name
|
+-- last_name
FROM
|
+-- ninjas
WHERE
|
+-- question_solved
|
+-- >
|
+-- 50
Optimization: The DBMS determines the most efficient way to execute the query by considering factors such as whether an index exists on the question_solved field. In this case, the DBMS might use an index on the question_solved field to efficiently retrieve the matching rows.
Evaluation: The DBMS executes the optimized query. It retrieves the results from the database. Then it returns the first_name and last_name of all ninjas whose question_solved is greater than 50.
Frequently Asked Questions
What are the three phases the DBMS processes a query in?
A DBMS processes queries in three phases that are parsing (checks syntax), optimization (finds efficient plan), and execution (retrieves and manipulates data).
What are the steps involved in query processing?
There are several steps involved in query processing in DBMS. The first step is known as parsing, and then the second step is query optimization, and finally, the query execution.
What is the purpose of query processing?
The purpose of query processing is to interpret, validate, and execute user queries against a database. It involves analyzing the query, creating an execution plan, and efficiently retrieving the requested data to generate the query result.
What is the main algorithm used in query processing?
The main algorithm used in query processing is the relational algebra, which provides a formal framework for defining operations on relational databases. Common operations include selection, projection, join, and aggregation, which are used to manipulate and retrieve data according to user queries.
Which is the first step in query processing?
The first step in query processing is query parsing. During this step, the DBMS analyzes the syntax of the query to ensure it adheres to the language rules. It checks for errors and breaks down the query into a parse tree or an equivalent internal representation.
What is query processing in memory?
Query processing in memory refers to the execution of database queries entirely within the computer's main memory (RAM), without accessing data from disk. This approach aims to minimize disk I/O operations, resulting in faster query execution times and improved overall system performance.
Conclusion
In this article, we have discussed the concept of query processing in DBMS. We have also discussed the various steps that are required in query processing in DBMS. You can check out our other blogs to enhance your knowledge:
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