Скачать или смотреть 🚦SQL Server Query Tuning Series: How Table-Valued Functions Impact Query Parallelism🚦

Introduction to Table-Valued Functions 📝
Welcome to the SQL Server Query Tuning Series! In this video, we will explore an essential topic for every SQL Server developer and database administrator: Table-Valued Functions (TVFs) and their impact on query performance. 📊

Table-Valued Functions allow us to encapsulate reusable logic that returns a table (like a stored procedure but returns data in a tabular format). These functions are quite useful because they let you reuse code and handle complex logic in an organized manner. However, they can also cause severe performance problems if not used correctly, especially in large-scale databases. 🤔

In this video, we will:

Identify the differences between Inline Table-Valued Functions (iTVFs) and Multi-Statement Table-Valued Functions (mTVFs).
Explore how TVFs can cause serial execution, leading to slower query performance.
Dive into parallel execution strategies and how to tune queries using TVFs for better performance. 🔧
Provide practical examples and techniques to optimize SQL queries when using TVFs.
Let's begin our query tuning journey! 🛠️✨

What Are Table-Valued Functions (TVFs)? 📚
A Table-Valued Function (TVF) is a user-defined function that returns a table. Unlike scalar functions (which return a single value), TVFs can return multiple rows of data, similar to a regular table. There are two primary types:

Inline Table-Valued Functions (iTVFs): The body of the function is a single SELECT statement that returns a table. iTVFs are generally more performance-friendly because they behave like views or derived tables. SQL Server can optimize them, inline them into the calling query, and potentially allow parallel execution. ⚡

Example:

CREATE FUNCTION dbo.GetProductsByCategory(@CategoryID INT)
RETURNS TABLE
AS
RETURN
(
SELECT ProductID, ProductName
FROM Products
WHERE CategoryID = @CategoryID
);
Multi-Statement Table-Valued Functions (mTVFs): These involve multiple statements inside the function body and return the result as a table. Unfortunately, mTVFs have performance issues because SQL Server treats them as a "black box" during query optimization. It has limited visibility into the function's internal logic, and as a result, these functions often force serial execution. 🚨

Example:


CREATE FUNCTION dbo.GetProductsWithDetails(@CategoryID INT)
RETURNS @ProductDetails TABLE
(
ProductID INT,
ProductName VARCHAR(100),
CategoryName VARCHAR(100)
)
AS
BEGIN
INSERT INTO @ProductDetails
SELECT p.ProductID, p.ProductName, c.CategoryName
FROM Products p
JOIN Categories c ON p.CategoryID = c.CategoryID
WHERE p.CategoryID = @CategoryID;

RETURN;
END;
The Problem: Serial Execution with Table-Valued Functions 🚦
One of the biggest issues with using TVFs, particularly Multi-Statement TVFs, is that they can force serial execution instead of allowing parallel processing. 🔄

What Is Serial Execution? 🏎️
In serial execution, a query is executed on a single processor/core. This is fine for small datasets, but as the data grows, the performance degrades because the CPU isn't leveraging all the available resources. Think of it as running a marathon alone when you could have a team helping you! 🏃‍♂️

SQL Server uses parallel execution when it detects that a query can benefit from multiple processors. In parallel execution, SQL Server divides the workload into smaller tasks (threads) that are executed concurrently across multiple cores, resulting in faster query processing. 🏎️💨

However, when a Multi-Statement TVF is part of a query, it often forces the entire query to run serially, negating any potential parallelism. This leads to performance bottlenecks in queries that work with large datasets or involve complex joins and aggregations. 😱

Why Does This Happen? ❓
This happens because SQL Server treats Multi-Statement TVFs as opaque objects during query optimization. It cannot "see" inside the function to determine the cost or the data involved. As a result, SQL Server often resorts to a serial execution plan, thinking it's safer than potentially introducing parallelism that might not benefit the function. ⚠️

Case Study: Serial Execution in Action 🛠️
Let’s look at a practical example. Imagine you have a query that retrieves order details for a specific customer using an mTVF:


SELECT *
FROM dbo.GetOrderDetailsByCustomer(@CustomerID) AS od
JOIN Orders o ON od.OrderID = o.OrderID;
This query calls a Multi-Statement TVF, which returns a table of order details. However, because it's an mTVF, SQL Server treats it as a black box and decides to execute the entire query in serial mode.

Execution Plan: 👓
When you check the execution plan, you'll see that the Parallelism operators (indicating parallel execution) are missing. Instead, you'll see that the Query Cost (CPU) is higher than expected. The lack of parallelism causes a longer execution time, especially if there are thousands or millions of rows. 🚦