sql-ultimate-course/scripts/26_Performance_Optimization.sql at main · sindoc/sql-ultimate-course · GitHub

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/* ==============================================================================

30x SQL Performance Tips

-------------------------------------------------------------------------------

This section demonstrates best practices for fetching data, filtering,

joins, UNION, aggregations, subqueries/CTE, DDL, and indexing.

It covers techniques such as selecting only necessary columns,

proper filtering methods, explicit joins, avoiding redundant logic,

and efficient indexing strategies.

Table of Contents:

1. FETCHING DATA

2. FILTERING

3. JOINS

4. UNION

5. AGGREGATIONS

6. SUBQUERIES, CTE

7. DDL

8. INDEXING

===============================================================================

-- ###############################################################

-- # FETCHING DATA #

-- ###############################################################

-- ============================================

-- Tip 1: Select Only What You Need

-- ============================================

-- Bad Practice

SELECT * FROM Sales.Customers

-- Good Practice

SELECT CustomerID, FirstName, LastName FROM Sales.Customers

-- ============================================

-- Tip 2: Avoid unnecessary DISTINCT & ORDER BY

-- ============================================

-- Bad Practice

SELECT DISTINCT

FirstName

FROM Sales.Customers

ORDER BY FirstName

-- Good Practice

SELECT

FirstName

FROM Sales.Customers

-- ============================================

-- Tip 3: For Exploration Purpose, Limit Rows!

-- ============================================

-- Bad Practice

SELECT

OrderID,

Sales

FROM Sales.Orders

-- Good Practice

SELECT TOP 10

OrderID,

Sales

FROM Sales.Orders

-- ###########################################################

-- # FILTERING #

-- ###########################################################

/* ==============================================================================

Tip 4: Create nonclustered Index on frequently used Columns in WHERE clause

===============================================================================*/

SELECT *

FROM Sales.Orders

WHERE OrderStatus = 'Delivered';

CREATE NONCLUSTERED INDEX Idx_Orders_OrderStatus ON Sales.Orders(OrderStatus)

/* ==============================================================================

Tip 5: Avoid applying functions to columns in WHERE clauses

===============================================================================*/

-- Bad Practice

SELECT * FROM Sales.Orders

WHERE LOWER(OrderStatus) = 'delivered'

-- Good Practice

SELECT * FROM Sales.Orders

WHERE OrderStatus = 'Delivered'

---------------------------------------------------------

-- Bad Practice

SELECT *

FROM Sales.Customers

WHERE SUBSTRING(FirstName, 1, 1) = 'A'

-- Good Practice

SELECT *

FROM Sales.Customers

WHERE FirstName LIKE 'A%'

---------------------------------------------------------

-- Bad Practice

SELECT *

FROM Sales.Orders

WHERE YEAR(OrderDate) = 2025

-- Good Practice

SELECT *

FROM Sales.Orders

WHERE OrderDate BETWEEN '2025-01-01' AND '2025-12-31'

/* ==============================================================================

Tip 6: Avoid leading wildcards as they prevent index usage

===============================================================================*/

-- Bad Practice

SELECT *

FROM Sales.Customers

WHERE LastName LIKE '%Gold%'

-- Good Practice

SELECT *

FROM Sales.Customers

WHERE LastName LIKE 'Gold%'

/* ==============================================================================

Tip 7: Use IN instead of Multiple OR

===============================================================================*/

-- Bad Practice

SELECT *

FROM Sales.Orders

WHERE CustomerID = 1 OR CustomerID = 2 OR CustomerID = 3

-- Good Practice

SELECT *

FROM Sales.Orders

WHERE CustomerID IN (1, 2, 3)

-- #######################################################

-- # JOINS #

-- #######################################################

/* ==============================================================================

Tip 8: Understand The Speed of Joins & Use INNER JOIN when possible

===============================================================================*/

-- Best Performance

SELECT c.FirstName, o.OrderID FROM Sales.Customers c INNER JOIN Sales.Orders o ON c.CustomerID = o.CustomerID

-- Slightly Slower Performance

SELECT c.FirstName, o.OrderID FROM Sales.Customers c RIGHT JOIN Sales.Orders o ON c.CustomerID = o.CustomerID

SELECT c.FirstName, o.OrderID FROM Sales.Customers c LEFT JOIN Sales.Orders o ON c.CustomerID = o.CustomerID

-- Worst Performance

SELECT c.FirstName, o.OrderID FROM Sales.Customers c OUTER JOIN Sales.Orders o ON c.CustomerID = o.CustomerID

/* ==============================================================================

Tip 9: Use Explicit Join (ANSI Join) Instead of Implicit Join (non-ANSI Join)

===============================================================================*/

-- Bad Practice

SELECT o.OrderID, c.FirstName

FROM Sales.Customers c, Sales.Orders o

WHERE c.CustomerID = o.CustomerID

-- Good Practice

SELECT o.OrderID, c.FirstName

FROM Sales.Customers AS c

INNER JOIN Sales.Orders AS o

ON c.CustomerID = o.CustomerID;

--For simple queries: There is no measurable performance difference if both ANSI and non-ANSI queries are correctly written.

--For complex queries: ANSI joins are usually easier to optimize and debug because their structure makes the intent of the query clearer.

/* ==============================================================================

Tip 10: Make sure to Index the columns used in the ON clause

===============================================================================*/

SELECT c.FirstName, o.OrderID

FROM Sales.Orders AS o

INNER JOIN Sales.Customers AS c

ON c.CustomerID = o.CustomerID;

CREATE NONCLUSTERED INDEX IX_Orders_CustomerID ON Sales.Orders(CustomerID)

/* ==============================================================================

Tip 11: Filter Before Joining (Big Tables)

===============================================================================*/

-- Best Practice For Small-Medium Tables

-- Filter After Join (WHERE)

SELECT c.FirstName, o.OrderID

FROM Sales.Customers AS c

INNER JOIN Sales.Orders AS o

ON c.CustomerID = o.CustomerID

WHERE o.OrderStatus = 'Delivered';

-- Filter During Join (ON)

SELECT c.FirstName, o.OrderID

FROM Sales.Customers AS c

INNER JOIN Sales.Orders AS o

ON c.CustomerID = o.CustomerID

AND o.OrderStatus = 'Delivered';

-- Best Practice For Big Tables

-- Filter Before Join (SUBQUERY)

SELECT c.FirstName, o.OrderID

FROM Sales.Customers AS c

INNER JOIN (

SELECT OrderID, CustomerID

FROM Sales.Orders

WHERE OrderStatus = 'Delivered'

) AS o

ON c.CustomerID = o.CustomerID;

/* ==============================================================================

Tip 12: Aggregate Before Joining (Big Tables)

===============================================================================*/

-- Best Practice For Small-Medium Tables

-- Grouping and Joining

SELECT c.CustomerID, c.FirstName, COUNT(o.OrderID) AS OrderCount

FROM Sales.Customers AS c

INNER JOIN Sales.Orders AS o

ON c.CustomerID = o.CustomerID

GROUP BY c.CustomerID, c.FirstName;

-- Best Practice For Big Tables

-- Pre-aggregated Subquery

SELECT c.CustomerID, c.FirstName, o.OrderCount

FROM Sales.Customers AS c

INNER JOIN (

SELECT CustomerID, COUNT(OrderID) AS OrderCount

FROM Sales.Orders

GROUP BY CustomerID

) AS o

ON c.CustomerID = o.CustomerID;

-- Bad Practice

-- Correlated Subquery

SELECT

c.CustomerID,

c.FirstName,

(SELECT COUNT(o.OrderID)

FROM Sales.Orders AS o

WHERE o.CustomerID = c.CustomerID) AS OrderCount

FROM Sales.Customers AS c;

/* ==============================================================================

Tip 13: Use Union Instead of OR in Joins

===============================================================================*/

-- Bad Practice

SELECT o.OrderID, c.FirstName

FROM Sales.Customers AS c

INNER JOIN Sales.Orders AS o

ON c.CustomerID = o.CustomerID

OR c.CustomerID = o.SalesPersonID;

-- Best Practice

SELECT o.OrderID, c.FirstName

FROM Sales.Customers AS c

INNER JOIN Sales.Orders AS o

ON c.CustomerID = o.CustomerID

UNION

SELECT o.OrderID, c.FirstName

FROM Sales.Customers AS c

INNER JOIN Sales.Orders AS o

ON c.CustomerID = o.SalesPersonID;

/* ==============================================================================

Tip 14: Check for Nested Loops and Use SQL HINTS

===============================================================================*/

SELECT o.OrderID, c.FirstName

FROM Sales.Customers c

INNER JOIN Sales.Orders o

ON c.CustomerID = o.CustomerID

-- Good Practice for Having Big Table & Small Table

SELECT o.OrderID, c.FirstName

FROM Sales.Customers AS c

INNER JOIN Sales.Orders AS o

ON c.CustomerID = o.CustomerID

OPTION (HASH JOIN);

-- ################################################################

-- # UNION #

-- ################################################################

/* ==============================================================================

Tip 15: Use UNION ALL instead of using UNION | duplicates are acceptable

===============================================================================*/

-- Bad Practice

SELECT CustomerID FROM Sales.Orders

UNION

SELECT CustomerID FROM Sales.OrdersArchive

-- Best Practice

SELECT CustomerID FROM Sales.Orders

UNION ALL

SELECT CustomerID FROM Sales.OrdersArchive

/* =======================================================================================

Tip 16: Use UNION ALL + Distinct instead of using UNION | duplicates are not acceptable

========================================================================================*/

-- Bad Practice

SELECT CustomerID FROM Sales.Orders

UNION

SELECT CustomerID FROM Sales.OrdersArchive

-- Best Practice

SELECT DISTINCT CustomerID

FROM (

SELECT CustomerID FROM Sales.Orders

UNION ALL

SELECT CustomerID FROM Sales.OrdersArchive

) AS CombinedData

-- ##########################################################

-- # AGGREGATIONS #

-- ##########################################################

/* ==============================================================================

Tip 17: Use Columnstore Index for Aggregations on Large Table

===============================================================================*/

SELECT CustomerID, COUNT(OrderID) AS OrderCount

FROM Sales.Orders

GROUP BY CustomerID

CREATE CLUSTERED COLUMNSTORE INDEX Idx_Orders_Columnstore ON Sales.Orders

/* ==============================================================================

Tip 18: Pre-Aggregate Data and store it in new Table for Reporting

===============================================================================*/

SELECT MONTH(OrderDate) OrderYear, SUM(Sales) AS TotalSales

INTO Sales.SalesSummary

FROM Sales.Orders

GROUP BY MONTH(OrderDate)

SELECT OrderYear, TotalSales FROM Sales.SalesSummary

-- ##############################################################

-- # SUBQUERIES, CTE #

-- ##############################################################

/* ==============================================================================

Tip 19: JOIN vs EXISTS vs IN (Avoid using IN)

===============================================================================*/

-- JOIN (Best Practice: If the Performance equals to EXISTS)

SELECT o.OrderID, o.Sales

FROM Sales.Orders AS o

INNER JOIN Sales.Customers AS c

ON o.CustomerID = c.CustomerID

WHERE c.Country = 'USA';

-- EXISTS (Best Practice: Use it for Large Tables)

SELECT o.OrderID, o.Sales

FROM Sales.Orders AS o

WHERE EXISTS (

SELECT 1

FROM Sales.Customers AS c

WHERE c.CustomerID = o.CustomerID

AND c.Country = 'USA'

);

-- IN (Bad Practice)

SELECT o.OrderID, o.Sales

FROM Sales.Orders AS o

WHERE o.CustomerID IN (

SELECT CustomerID

FROM Sales.Customers

WHERE Country = 'USA'

);

/* ==============================================================================

Tip 20: Avoid Redundant Logic in Your Query

===============================================================================*/

-- Bad Practice

SELECT EmployeeID, FirstName, 'Above Average' AS Status

FROM Sales.Employees

WHERE Salary > (SELECT AVG(Salary) FROM Sales.Employees)

UNION ALL

SELECT EmployeeID, FirstName, 'Below Average' AS Status

FROM Sales.Employees

WHERE Salary < (SELECT AVG(Salary) FROM Sales.Employees);

-- Good Practice

SELECT

EmployeeID,

FirstName,

CASE

WHEN Salary > AVG(Salary) OVER () THEN 'Above Average'

WHEN Salary < AVG(Salary) OVER () THEN 'Below Average'

ELSE 'Average'

END AS Status

FROM Sales.Employees;

-- ##############################################################

-- # DDL #

-- ##############################################################

=============================================================================

Tip 21: Avoid VARCHAR Data Type If Possible

=============================================================================

Tip 22: Avoid Using MAX or Overly Large Lengths

=============================================================================

Tip 23: Use NOT NULL If possible

=============================================================================

Tip 24: Make sure all tables have a CLUSTERED PRIMARY KEY

=============================================================================

Tip 25: Creeate Nonclustered Index on Foreign Key if they are frequently used

=============================================================================

-- Bad Practice

CREATE TABLE CustomersInfo (

CustomerID INT,

FirstName VARCHAR(MAX),

LastName TEXT,

Country VARCHAR(255),

TotalPurchases FLOAT,

Score VARCHAR(255),

BirthDate VARCHAR(255),

EmployeeID INT,

CONSTRAINT FK_Bad_Customers_EmployeeID FOREIGN KEY (EmployeeID)

REFERENCES Sales.Employees(EmployeeID)

);

-- Good Practice Practice

CREATE TABLE CustomersInfo (

CustomerID INT PRIMARY KEY CLUSTERED,

FirstName VARCHAR(50) NOT NULL,

LastName VARCHAR(50) NOT NULL,

Country VARCHAR(50) NOT NULL,

TotalPurchases FLOAT,

Score INT,

BirthDate DATE,

EmployeeID INT,

CONSTRAINT FK_CustomersInfo_EmployeeID FOREIGN KEY (EmployeeID)

REFERENCES Sales.Employees(EmployeeID)

);

CREATE NONCLUSTERED INDEX IX_CustomersInfo_EmployeeID

ON CustomersInfo(EmployeeID);

-- ##############################################################

-- # INDEXING #

-- ##############################################################

=================================================================================================================================

Tip 26: Avoid Over Indexing, as it can slow down insert, update, and delete operations

=================================================================================================================================

Tip 27: Regularly review and drop unused indexes to save space and improve write performance

=================================================================================================================================

Tip 28: Update table statistics weekly to ensure the query optimizer has the most up-to-date information

=================================================================================================================================

Tip 29: Reorganize and rebuild fragmented indexes weekly to maintain query performance.

=================================================================================================================================

Tip 30: For large tables (e.g., fact tables), partition the data and then apply a columnstore index for best performance results

=================================================================================================================================

sql-ultimate-course/scripts/26_Performance_Optimization.sql at main · sindoc/sql-ultimate-course · GitHub

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