Query planning in RDBMS

Query planning is an important aspect of relational database management systems (RDBMS). The query planning process involves determining the most efficient way to execute a SQL query by analyzing the query and selecting the most appropriate execution plan. In this blog, we will explore the query planning process in RDBMS and provide examples to illustrate how it works.

Query Planning in RDBMS

When a user submits a SQL query to an RDBMS, the query is first parsed and then passed to the query optimizer. The query optimizer is responsible for generating an execution plan that specifies how the query will be executed. The execution plan is a series of steps that the RDBMS will use to retrieve the data requested by the query. The optimizer considers multiple plans for executing the query and selects the plan that it determines will be the most efficient.

The query optimizer uses a cost-based approach to determine the most efficient execution plan for a query. The cost of a plan is determined by estimating the resources that will be required to execute the plan, such as the number of disk I/O operations, CPU cycles, and memory usage. The optimizer generates multiple plans for executing the query and estimates the cost of each plan. The plan with the lowest estimated cost is selected as the optimal plan.

There are several steps involved in the query planning process, which are as follows:

1. Parsing: The query is parsed to ensure that it is syntactically correct and conforms to the rules of the SQL language.
2. Semantic Analysis: The query is analyzed to ensure that it is semantically correct and conforms to the rules of the database schema.
3. Query Optimization: Multiple execution plans are generated for the query, and the optimizer selects the most efficient plan based on estimated cost.
4. Query Execution: The RDBMS executes the selected plan and retrieves the data requested by the query.

Related Articles: Partitioning in RDBMS, Query optimization in RDBMS

Examples of Query Planning

Let’s consider some examples to illustrate how the query planning process works in RDBMS.

Example 1: Select Query

Consider the following SQL query:

SELECT * FROM employees WHERE department = 'Sales';

The query planning process for this query would involve the following steps:

1. Parsing: The query is parsed to ensure that it is syntactically correct.
2. Semantic Analysis: The query is analyzed to ensure that it is semantically correct and conforms to the rules of the database schema.
3. Query Optimization: The optimizer generates multiple plans for executing the query, such as using an index on the ‘department’ column or scanning the entire ’employees’ table. The optimizer estimates the cost of each plan based on factors such as the size of the ’employees’ table, the number of rows that match the ‘Sales’ department condition, and the presence of indexes on the ‘department’ column. The plan with the lowest estimated cost is selected as the optimal plan.
4. Query Execution: The RDBMS executes the selected plan and retrieves the data requested by the query.

Example 2: Join Query

Consider the following SQL query:

SELECT e.name, d.name FROM employees e INNER JOIN departments d ON e.department_id = d.id;

The query planning process for this query would involve the following steps:

1. Parsing: The query is parsed to ensure that it is syntactically correct.
2. Semantic Analysis: The query is analyzed to ensure that it is semantically correct and conforms to the rules of the database schema.
3. Query Optimization: The optimizer generates multiple plans for executing the query, such as using nested loops, hash join, or sort-merge join. The optimizer estimates the cost of each plan based on factors such as the size of the ’employees’ and ‘departments’ tables, the number of rows that match the join condition, and the presence of indexes on the join columns. The plan with the lowest estimated cost is selected as the optimal plan.
4. Query Execution: The RDBMS executes the selected plan and retrieves the data requested by the query.

Example 3: Subquery

Consider the following SQL query:

SELECT name, salary FROM employees WHERE salary > (SELECT AVG(salary) FROM employees);

The query planning process for this query would involve the following steps:

1. Parsing: The query is parsed to ensure that it is syntactically correct.
2. Semantic Analysis: The query is analyzed to ensure that it is semantically correct and conforms to the rules of the database schema.
3. Query Optimization: The optimizer generates multiple plans for executing the query, such as using a correlated subquery or a temporary table to store the result of the subquery. The optimizer estimates the cost of each plan based on factors such as the size of the ’employees’ table, the number of rows that match the ‘salary’ condition, and the complexity of the subquery. The plan with the lowest estimated cost is selected as the optimal plan.
4. Query Execution: The RDBMS executes the selected plan and retrieves the data requested by the query.

Conclusion

Query planning is a critical component of relational database management systems. The query optimizer analyzes the SQL query and generates multiple execution plans based on estimated cost. The optimizer selects the plan with the lowest estimated cost as the optimal plan for executing the query. By understanding how the query planning process works, you can write efficient SQL queries that minimize resource usage and maximize performance.