Query Tuning Technique
1. The sql query becomes faster if you use the actual columns names in SELECT statement instead of than *.
SELECT id, first_name, last_name, age, subject FROM student_details;
Instead of:
SELECT * FROM student_details;
2.HAVING clause is used to filter the rows after all the rows are selected. It is just like a filter. Do not use HAVING clause for any other purposes.
SELECT subject, count(subject)
FROM student_details
WHERE subject != 'Science'
AND subject != 'Maths'
GROUP BY subject;
Instead of:
SELECT subject, count(subject)
FROM student_details
GROUP BY subject
HAVING subject!= 'Vancouver' AND subject!= 'Toronto';
3.Sometimes you may have more than one subqueries in your main query. Try to minimize the number of subquery block in your query.
SELECT name
FROM employee
WHERE (salary, age ) = (SELECT MAX (salary), MAX (age)
FROM employee_details)
AND dept = 'Electronics';
Instead of:
SELECT name
FROM employee
WHERE salary = (SELECT MAX(salary) FROM employee_details)
AND age = (SELECT MAX(age) FROM employee_details)
AND emp_dept = 'Electronics';
4.Use operator EXISTS, IN and table joins appropriately in your query.
a) Usually IN has the slowest performance.
b) IN is efficient when most of the filter criteria is in the sub-query.
c) EXISTS is efficient when most of the filter criteria is in the main query.
For Example: Write the query as
Select * from product p
where EXISTS (select * from order_items o
where o.product_id = p.product_id)
Instead of:
Select * from product p
where product_id IN
(select product_id from order_items )
5. Use EXISTS instead of DISTINCT when using joins which involves tables having one-to-many relationship.
For Example: Write the query as
SELECT d.dept_id, d.dept
FROM dept d
WHERE EXISTS ( SELECT 'X' FROM employee e WHERE e.dept = d.dept);
Instead of:
SELECT DISTINCT d.dept_id, d.dept
FROM dept d,employee e
WHERE e.dept = e.dept;
6.Try to use UNION ALL in place of UNION.
For Example: Write the query as
SELECT id, first_name
FROM student_details_class10
UNION ALL
SELECT id, first_name
FROM sports_team;
Instead of:
SELECT id, first_name, subject
FROM student_details_class10
UNION
SELECT id, first_name
FROM sports_team;
7.Be careful while using conditions in WHERE clause.
For Example: Write the query as
SELECT id, first_name, age FROM student_details WHERE age > 10;
Instead of:
SELECT id, first_name, age FROM student_details WHERE age != 10;
8.Use non-column expression on one side of the query because it will be processed earlier.
Write the query as
SELECT id, name, salary
FROM employee
WHERE salary < 25000;
Instead of:
SELECT id, name, salary
FROM employee
WHERE salary + 10000 < 35000;
9.To store large binary objects, first place them in the file system and add the file path in the database.
