Inline vs. Multi-Statement Table-Valued Functions: Which Performs Better?

Table-Valued Functions: Inline vs. Multi-Statement – A Performance Analysis

Selecting between inline and multi-statement Table-Valued Functions (TVFs) significantly impacts query performance. This comparison highlights the key differences and provides guidance on optimal usage.

Inline Table-Valued Functions (ITVF)

ITVF's, similar to views, leverage real-time table structures and statistics, often leading to superior execution plans. Here's an example:

<code class="language-sql">CREATE FUNCTION MyNS.GetUnshippedOrders()
RETURNS TABLE
AS 
RETURN SELECT a.SaleId, a.CustomerID, b.Qty
    FROM Sales.Sales a INNER JOIN Sales.SaleDetail b
        ON a.SaleId = b.SaleId
        INNER JOIN Production.Product c ON b.ProductID = c.ProductID
    WHERE a.ShipDate IS NULL
GO</code>

Multi-Statement Table-Valued Functions (MSTVF)

MSTVFs utilize a pre-defined table structure and lack access to real-time statistics. Their structure differs as follows:

<code class="language-sql">CREATE FUNCTION MyNS.GetLastShipped(@CustomerID INT)
RETURNS @CustomerOrder TABLE
(SaleOrderID    INT         NOT NULL,
CustomerID      INT         NOT NULL,
OrderDate       DATETIME    NOT NULL,
OrderQty        INT         NOT NULL)
AS
BEGIN
    DECLARE @MaxDate DATETIME

    SELECT @MaxDate = MAX(OrderDate)
    FROM Sales.SalesOrderHeader
    WHERE CustomerID = @CustomerID

    INSERT @CustomerOrder
    SELECT a.SalesOrderID, a.CustomerID, a.OrderDate, b.OrderQty
    FROM Sales.SalesOrderHeader a INNER JOIN Sales.SalesOrderHeader b
        ON a.SalesOrderID = b.SalesOrderID
        INNER JOIN Production.Product c ON b.ProductID = c.ProductID
    WHERE a.OrderDate = @MaxDate
        AND a.CustomerID = @CustomerID
    RETURN
END
GO</code>

Performance Considerations

Generally, ITVFs outperform MSTVFs because of their ability to leverage real-time statistics. However, MSTVFs might be more efficient for complex operations not easily implemented within an ITVF.

When to Use Which Function Type

Choose ITVFs when:

• Database statistics are crucial for optimization.
• The operation is simple and read-only.
• A virtual view is needed for queries.

Choose MSTVFs when:

• Complex calculations or transformations are required.
• A modifiable temporary table is necessary within the function.
• Parameters are used to filter results.

Conclusion

The optimal choice between ITVF and MSTVF depends entirely on the specific task. However, ITVFs are generally preferred for their performance advantages and better utilization of database statistics.

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