Query Optimization in PL/SQL

This content originally appeared on DEV Community and was authored by Pranav Bakare

Query Optimization in PL/SQL and SQL involves techniques and strategies to improve the performance and efficiency of SQL queries embedded in PL/SQL code or standalone SQL statements. Efficient queries reduce response time, resource usage, and database load, especially when dealing with large datasets or complex operations. Both PL/SQL and SQL require optimization to ensure smooth, high-performance database interactions.

Key Techniques for Query Optimization in SQL and PL/SQL

1. Indexing:

Description: Indexes allow the database to quickly locate rows without scanning the entire table.

Optimization Tip: Use indexes on frequently queried columns, particularly those used in WHERE, JOIN, and ORDER BY clauses.

Example:

CREATE INDEX idx_employee_id ON employees(employee_id);
SELECT * FROM employees WHERE employee_id = 101;

Caution: Over-indexing can slow down INSERT and UPDATE operations since the database must maintain the indexes.

1. Avoiding Full Table Scans:

Description: Full table scans are costly as they involve reading every row in the table.

Optimization Tip: Use indexes, avoid functions on indexed columns in the WHERE clause, and prefer selective WHERE conditions.

Example:

-- Avoid using functions on indexed columns:
SELECT * FROM employees WHERE UPPER(last_name) = 'BAKARE'; -- Slow, index ignored.
-- Use:
SELECT * FROM employees WHERE last_name = 'Bakare'; -- Fast, index utilized.

1. Query Rewriting:

Description: Restructure or rewrite the query to improve performance.

Optimization Tip: Replace inefficient queries with equivalent, more efficient ones. For example, consider using JOIN instead of subqueries when possible.

Example:

-- Inefficient subquery:
SELECT * FROM employees WHERE employee_id IN (SELECT employee_id FROM departments);

-- More efficient JOIN:
SELECT e.* FROM employees e JOIN departments d ON e.employee_id = d.employee_id;

1. Bind Variables:

Description: Bind variables are placeholders in SQL queries that are replaced with actual values at runtime. They help reuse SQL execution plans, improving performance.

Optimization Tip: Use bind variables to avoid hard parsing and reduce database overhead.

Example:

-- Instead of:
SELECT * FROM employees WHERE employee_id = 101;
-- Use bind variables:
EXECUTE IMMEDIATE 'SELECT * FROM employees WHERE employee_id = :1' USING v_employee_id;

1. Use of Analytical Functions:

Description: Analytical functions such as ROW_NUMBER, RANK, and PARTITION BY can perform complex calculations more efficiently than using subqueries or self-joins.

Optimization Tip: Use analytical functions to replace multiple queries or window functions.

Example:

-- Instead of complex subquery:
SELECT e., (SELECT COUNT() FROM employees WHERE salary > e.salary) salary_rank FROM employees e;

-- Use analytical function:
SELECT e.*, RANK() OVER (ORDER BY salary DESC) salary_rank FROM employees e;

1. Use of EXISTS Instead of IN:

Description: EXISTS is more efficient than IN in certain cases, especially when subqueries involve large datasets.

Optimization Tip: Replace IN with EXISTS when the subquery returns many rows.

Example:

-- Inefficient query:
SELECT * FROM employees WHERE department_id IN (SELECT department_id FROM departments);

-- More efficient query:
SELECT * FROM employees WHERE EXISTS (SELECT 1 FROM departments WHERE employees.department_id = departments.department_id);

1. Query Plan Analysis (EXPLAIN PLAN):

Description: The EXPLAIN PLAN command helps analyze how Oracle executes a query, showing whether indexes are being used, whether a full table scan occurs, and other insights.

Optimization Tip: Use EXPLAIN PLAN to identify inefficiencies and adjust the query structure or add indexes accordingly.

Example:

EXPLAIN PLAN FOR
SELECT * FROM employees WHERE employee_id = 101;

SELECT * FROM table(DBMS_XPLAN.DISPLAY);

1. Use of Hints:

Description: Hints are directives that you can add to a SQL query to influence the optimizer’s behavior, such as forcing the use of an index or a specific join method.

Optimization Tip: Use hints sparingly, only when necessary to override the default behavior of the Oracle optimizer.

Example:

-- Force the optimizer to use the index:
SELECT /*+ INDEX(employees idx_employee_id) */ * FROM employees WHERE employee_id = 101;

1. Partitioning:

Description: Partitioning divides a large table into smaller, more manageable pieces (partitions), improving query performance when accessing a subset of data.

Optimization Tip: Use partitioning for large tables to enhance query performance and maintenance.

Example:

CREATE TABLE sales (
sale_id NUMBER,
sale_date DATE,
amount NUMBER
)
PARTITION BY RANGE (sale_date) (
PARTITION p1 VALUES LESS THAN (TO_DATE('2023-01-01', 'YYYY-MM-DD')),
PARTITION p2 VALUES LESS THAN (TO_DATE('2024-01-01', 'YYYY-MM-DD'))
);

1. Minimize Data Retrieval:

Description: Retrieving large amounts of unnecessary data can degrade performance.

Optimization Tip: Fetch only the required data using specific columns rather than SELECT *.

Example:

-- Inefficient:
SELECT * FROM employees;

-- Efficient:
SELECT employee_id, first_name, last_name FROM employees;

Additional Optimization Techniques in PL/SQL:

1. Bulk Collect and FORALL:

Description: When handling large datasets, using BULK COLLECT and FORALL can significantly improve performance by minimizing context switching between SQL and PL/SQL engines.

Optimization Tip: Use BULK COLLECT to fetch multiple rows at once and FORALL to perform bulk INSERT, UPDATE, or DELETE operations.

Example:

DECLARE
TYPE employee_tab IS TABLE OF employees%ROWTYPE;
l_employees employee_tab;
BEGIN
-- Use BULK COLLECT to fetch data in bulk
SELECT * BULK COLLECT INTO l_employees FROM employees;

-- Use FORALL to update data in bulk
FORALL i IN 1..l_employees.COUNT
UPDATE employees SET salary = salary * 1.1 WHERE employee_id = l_employees(i).employee_id;
END;

1. Pipelined Table Functions:

Description: Pipelined functions return rows to the calling query before the function has completed processing all rows, which can improve performance for large result sets.

Optimization Tip: Use pipelined table functions for large datasets that require transformation.

Example:

CREATE OR REPLACE FUNCTION get_employees RETURN employees_tab PIPELINED IS
BEGIN
FOR r IN (SELECT * FROM employees) LOOP
PIPE ROW (r);
END LOOP;
END;

1. PL/SQL Caching:

Description: Caching frequently accessed data in PL/SQL code (using package-level variables or the RESULT_CACHE feature) reduces the need to repeatedly query the database.

Optimization Tip: Cache frequently used data to avoid repeated database access for the same queries.

Example:

CREATE FUNCTION get_department_name(department_id IN NUMBER)
RETURN VARCHAR2
RESULT_CACHE
IS
dept_name VARCHAR2(100);
BEGIN
SELECT department_name INTO dept_name FROM departments WHERE department_id = department_id;
RETURN dept_name;
END;

Conclusion:

Optimizing SQL queries and PL/SQL code is essential for maintaining efficient and high-performing applications. By applying techniques like indexing, query rewriting, using bind variables, and employing analytical functions, you can significantly improve query performance. Tools such as EXPLAIN PLAN and DBMS_PROFILER help analyze and fine-tune queries, ensuring optimal resource utilization and faster execution times.

This content originally appeared on DEV Community and was authored by Pranav Bakare