Stored Procedures : Scenes behind Execution Plan, Compiled Once, Executed Many Times

Is Stored Proc really compiled only once, What about the execution plan?

Hello, SQL enthusiast! 👋 Today, we’re diving into the world of Stored Procedures in SQL Server — a powerful tool that can boost performance and simplify database management. When you hear “stored procedure,” you might think, “Aren’t they just saved SQL queries?” Well, they’re much more than that! Stored procedures bring benefits like pre-compilation, execution plan reuse, and enhanced security to your database applications.

Let’s unpack how stored procedures work, why they’re beneficial, and how SQL Server manages their execution plans. By the end of this blog, you’ll understand why stored procedures are often the go-to choice for SQL performance optimization. 🌟

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What is a Stored Procedure? 🤔

A Stored Procedure (SP) is a saved, reusable SQL query or set of queries that performs specific actions in the database. Think of it as a function for your database: you define it once and then call it whenever you need it.

Stored procedures can:

• Perform CRUD operations (Create, Read, Update, Delete)
• Run complex business logic involving multiple SQL statements
• Accept input parameters and return output values
• Encapsulate and reuse logic across applications

A simple stored procedure might look like this:

CREATE PROCEDURE GetEmployeeByID
    @EmployeeID INT
AS
BEGIN
    SELECT * 
    FROM Employees 
    WHERE EmployeeID = @EmployeeID;
END;

Now that we know what they are, let’s look at the magic that happens under the hood — compilation and execution plans! 🧙‍♂️

Compiled Once, Executed Many Times 🔄

When you first run a stored procedure, SQL Server compiles it and creates an execution plan. The execution plan is SQL Server’s roadmap for how it will retrieve or manipulate data. The best part? This execution plan is stored in memory, so SQL Server can reuse it each time you call the stored procedure, saving both time and resources.

Why is Compilation Important?

Compilation is a costly operation. Every time SQL Server encounters an ad-hoc query (a query that isn’t part of a stored procedure), it has to:

1. Parse the query syntax 📝
2. Optimize the query to determine the most efficient way to execute it ⚙️
3. Generate an execution plan 📈

For stored procedures, SQL Server does this once, then reuses the compiled plan every time the stored procedure is called. This means that your application skips these steps on subsequent executions, leading to faster performance. 🚀

What is an Execution Plan? 📊

An execution plan is SQL Server’s strategy for executing a query. Think of it as the database’s playbook — it outlines the order of operations, indexes to use, join types, and any other optimization details.

Here’s what happens with stored procedure execution plans:

1. First Execution: SQL Server parses, optimizes, and compiles the stored procedure into an execution plan.
2. Plan Cache: The execution plan is stored in memory (plan cache) for reuse.
3. Subsequent Executions: SQL Server retrieves the execution plan from the cache and skips the parsing and optimization steps.

With the execution plan cached, stored procedures can significantly speed up repetitive operations.

Benefits of Stored Procedures and Cached Execution Plans 🎉

Using stored procedures with cached execution plans has several key advantages:

1. Faster Execution ⚡

The pre-compiled nature of stored procedures means they execute faster than ad-hoc queries, especially when you run them multiple times.

2. Reduced Server Load 📉

Reusing execution plans means SQL Server spends less time parsing and optimizing queries, which reduces CPU and memory usage.

3. Increased Security 🔐

Stored procedures add a layer of security by controlling access and avoiding direct SQL queries in your application code, which reduces SQL injection risks.

4. Consistent Query Performance 🎯

Stored procedures offer consistent performance by using a standardized execution plan, so there are no surprises or performance variances.

Execution Plan in Action: A Quick Example 🛠️

Let’s see the execution plan in action! Here’s an example to show how SQL Server reuses execution plans for stored procedures.

First, we create a stored procedure:

CREATE PROCEDURE GetOrdersByCustomerID
    @CustomerID INT
AS
BEGIN
    SELECT * 
    FROM Orders 
    WHERE CustomerID = @CustomerID;
END;

First Execution:

When you call GetOrdersByCustomerID for the first time, SQL Server compiles it and generates an execution plan.

EXEC GetOrdersByCustomerID @CustomerID = 123;

During this first execution, SQL Server:

1. Parses the SQL statement
2. Optimizes the query to find the best execution strategy
3. Generates and stores the execution plan

Second Execution and Beyond:

On the next call to GetOrdersByCustomerID, SQL Server skips the parsing and optimization phases. It retrieves the execution plan from the cache and jumps straight to execution.

EXEC GetOrdersByCustomerID @CustomerID = 456;

With the execution plan already in place, SQL Server doesn’t need to repeat the costly compilation process. This efficiency can make a huge difference when your application calls stored procedures frequently!

Exploring Execution Plans with SQL Server Management Studio (SSMS) 🔍

To view an execution plan for a stored procedure, you can use SQL Server Management Studio (SSMS). Here’s how:

1. Open SSMS and type your stored procedure call in a new query window.
2. Click on Query > Display Actual Execution Plan.
3. Run the stored procedure.

SSMS will display a graphical representation of the execution plan, showing the steps SQL Server takes to execute the procedure. This can help you spot any inefficiencies, such as missing indexes or costly table scans.

How to Keep Execution Plans Effective ⚙️

Execution plans are not always set in stone. SQL Server automatically manages plan cache, but here are a few tips to ensure your execution plans stay effective:

1. Use Parameters Consistently 🧩: SQL Server reuses execution plans based on parameterized queries. Ensure your stored procedures use parameters, which helps SQL Server cache and reuse plans.
2. Avoid Query Hints if Possible 🧠: Query hints can override SQL Server’s optimization choices, which might lock in suboptimal plans. Let SQL Server handle optimization unless absolutely necessary.
3. Monitor for Plan Caching Issues 🛠️: Over time, the plan cache can grow, causing memory pressure. Use DBCC FREEPROCCACHE sparingly to clear the cache if you need SQL Server to re-optimize queries.
4. Regularly Recompile if Data Patterns Change 🔄: If your data changes significantly, the existing plan might become inefficient. In this case, use WITH RECOMPILE to force SQL Server to create a new plan.

Final Thoughts: Why Stored Procedures Shine 🌟

Stored procedures aren’t just a way to store SQL scripts — they’re a performance-boosting tool that can bring speed, efficiency, and security to your applications. With compilation and execution plan reuse, stored procedures help SQL Server handle repetitive tasks with ease. The next time you design your database layer, consider using stored procedures for the repetitive, performance-critical parts of your code.

Stored procedures and cached execution plans are like a well-rehearsed orchestra 🎶: once SQL Server knows the notes, it plays them flawlessly every time you call the tune. So, go ahead and make stored procedures a core part of your SQL toolbox — you’ll be glad you did! 😊