Table Variable Deferred Compilation (SQL 2019)

,

Table Variable Deferred Compilation is a Performance Improvement Feature available in the Intelligent Query Processing Family Tree from SQL Server 2019 Onward.

In the words of Microsoft -

"This feature Improves plan quality and overall performance for queries referencing table variables. During optimization and initial plan compilation, this feature will propagate cardinality estimates that are based on actual table variable row counts instead of the original one-row guess. This exact row count information will then be used for optimizing downstream plan operations.

To enable table variable deferred compilation, enable database compatibility level 150 for the database you're connected to when the query runs."

More Reading available here - https://docs.microsoft.com/en-us/sql/relational-databases/performance/intelligent-query-processing?view=sql-server-ver15#table-variable-deferred-compilation

There is also a known caveat - PERFORMANCE MAY NOT BE IMPROVED BY THIS FEATURE IF THE TABLE VARIABLE ROW COUNT VARIES SIGNIFICANTLY ACROSS EXECUTIONS.

Okay, let's run some queries and see this feature in action.

--Check Version of the server
SELECT @@VERSION
/*
This feature is available only from SQL Server 2019 onward.
Microsoft SQL Server 2019 (RTM-GDR) (KB4517790) - 15.0.2070.41 (X64)   ...
*/

--Check Compatability of the Microsoft Demo Database - WideWorldImportersDW
SELECT Compatibility_Level
FROM sys.databases
WHERE name = 'WideWorldImportersDW'
GO
--By Default when you restore WideWorldImportersDW, Compat level is 130 as it was created from SQL 2016 onwards.
--WideWorldImportersDW is the Microsoft Demo DB used these days instead of AdventureWorks and is available for free download.
--Also available is the enlarge db script required to be run on the installed WideWorldImportersDW, for certain large data demos.
/*https://github.com/microsoft/sql-server-samples/blob/master/samples/features/intelligent-query-processing/IQP%20Demo%20Setup%20-%20Enlarging%20WideWorldImportersDW.sql*/

--MICROSOFT DEMO

-- ******************************************************** --
-- Table variable deferred compilation
-- ******************************************************** --

--Bring DB Compatability to SQL 2017 and clear the db plan cache.
USE [WideWorldImportersDW]
GO
ALTER DATABASE [WideWorldImportersDW] SET COMPATIBILITY_LEVEL = 140;
GO
ALTER DATABASE SCOPED CONFIGURATION CLEAR PROCEDURE_CACHE;
GO

--Enable Exec Plan
DECLARE @Order TABLE 
	([Order Key] BIGINT NOT NULL,
     [Quantity] INT NOT NULL
	);

INSERT @Order
SELECT [Order Key], [Quantity]
FROM [Fact].[Order]
WHERE  [Quantity] > 99;

-- Look at estimated rows, speed, join algorithm
SET STATISTICS TIME ON 
SET STATISTICS IO ON
	SELECT oh.[Order Key], oh.[Order Date Key],
	   oh.[Unit Price], o.Quantity
	FROM Fact.[Order] AS oh
	INNER JOIN @Order AS o
		ON o.[Order Key] = oh.[Order Key]
	WHERE oh.[Unit Price] > 0.10
	ORDER BY oh.[Unit Price] DESC;
SET STATISTICS TIME OFF
SET STATISTICS IO OFF
GO
--You will see that the estimated rows from the table variable is only 1, because of which all downstream operations go in the wrong direction, using the nested loop join instead of hash match, etc.

--Now See the 2019 feature in action.
ALTER DATABASE [WideWorldImportersDW] SET COMPATIBILITY_LEVEL = 150
GO
ALTER DATABASE SCOPED CONFIGURATION CLEAR PROCEDURE_CACHE;
GO

DECLARE @Order TABLE 
	([Order Key] BIGINT NOT NULL,
	 [Quantity] INT NOT NULL
	);

INSERT @Order
SELECT [Order Key], [Quantity]
FROM [Fact].[Order]
WHERE [Quantity] > 99;

-- Look at estimated rows, speed, join algorithm
SET STATISTICS TIME ON 
SET STATISTICS IO ON
	SELECT oh.[Order Key], oh.[Order Date Key],
		oh.[Unit Price], o.Quantity
	FROM Fact.[Order] AS oh
	INNER JOIN @Order AS o
		ON o.[Order Key] = oh.[Order Key]
	WHERE oh.[Unit Price] > 0.10
	ORDER BY oh.[Unit Price] DESC;
SET STATISTICS TIME OFF
SET STATISTICS IO OFF
GO
--Immediate improvement with actual rows from Table Variable being used in Plan. Join operation and Index operation also changed accordingly.

--We can keep the comptability 150 for the Server and disable this feature on specific db's if needed.

ALTER DATABASE SCOPED CONFIGURATION SET DEFERRED_COMPILATION_TV = OFF;

ALTER DATABASE SCOPED CONFIGURATION SET DEFERRED_COMPILATION_TV = ON;

SELECT * FROM sys.database_scoped_configurations
GO

--OR Disable for specific Queries.
DECLARE @Order TABLE 
	([Order Key] BIGINT NOT NULL,
	 [Quantity] INT NOT NULL
	);

INSERT @Order
SELECT [Order Key], [Quantity]
FROM [Fact].[Order]
WHERE [Quantity] > 99;

SET STATISTICS TIME ON 
	SELECT oh.[Order Key], oh.[Order Date Key],
		oh.[Unit Price], o.Quantity
	FROM Fact.[Order] AS oh
	INNER JOIN @Order AS o
		ON o.[Order Key] = oh.[Order Key]
	WHERE oh.[Unit Price] > 0.10 
	ORDER BY oh.[Unit Price] DESC
	OPTION (USE HINT('DISABLE_DEFERRED_COMPILATION_TV'));
SET STATISTICS TIME OFF
GO

--Now, like highlighted at the very top, this is not all easy breeezy as it seems. For varying data workloads, since cached plans are used, Parameter Sniffing issues are introduced.

--Lets see.
CREATE OR ALTER PROC dbo.usp_TableVariableTest @Quantity INT AS
BEGIN

	DECLARE @Order TABLE 
		([Order Key] BIGINT NOT NULL,
		 [Quantity] INT NOT NULL
		)

	INSERT @Order
	SELECT [Order Key], [Quantity]
	FROM [Fact].[Order]
	WHERE [Quantity] > @Quantity

	SELECT oh.[Order Key], oh.[Order Date Key],
		oh.[Unit Price], o.Quantity
	FROM Fact.[Order] AS oh
	INNER JOIN @Order AS o
		ON o.[Order Key] = oh.[Order Key]
	WHERE oh.[Unit Price] > 0.10 
	ORDER BY oh.[Unit Price] DESC
END
GO

SET STATISTICS TIME ON 
SET STATISTICS IO ON
	EXEC dbo.usp_TableVariableTest @Quantity = 99
SET STATISTICS IO OFF
SET STATISTICS TIME OFF 
GO

--Uses cached plan and  estimates poorly.
SET STATISTICS TIME ON 
SET STATISTICS IO ON
	EXEC dbo.usp_TableVariableTest @Quantity = 360
SET STATISTICS IO OFF
SET STATISTICS TIME OFF 
GO

--Bring DB Compatability to SQL 2017 and clear the db plan cache.
USE [WideWorldImportersDW]
GO
ALTER DATABASE [WideWorldImportersDW] SET COMPATIBILITY_LEVEL = 140;
GO
ALTER DATABASE SCOPED CONFIGURATION CLEAR PROCEDURE_CACHE;
GO

--Create Proc with same logic as earlier, but with temp table.
CREATE OR ALTER PROC dbo.usp_TempTableTest @Quantity INT AS
BEGIN

	CREATE TABLE #Order 
		([Order Key] BIGINT NOT NULL,
		 [Quantity] INT NOT NULL
		)

	INSERT #Order
	SELECT [Order Key], [Quantity]
	FROM [Fact].[Order]
	WHERE [Quantity] > @Quantity

	SELECT oh.[Order Key], oh.[Order Date Key],
		oh.[Unit Price], o.Quantity
	FROM Fact.[Order] AS oh
	INNER JOIN #Order AS o
		ON o.[Order Key] = oh.[Order Key]
	WHERE oh.[Unit Price] > 0.10 
	ORDER BY oh.[Unit Price] DESC
END
GO

SET STATISTICS TIME ON 
SET STATISTICS IO ON
	EXEC dbo.usp_TempTableTest @Quantity = 99
SET STATISTICS IO OFF
SET STATISTICS TIME OFF 
GO

--With Temp Table, with varying data output and with 2019 feature off,
--this SP, makes the right estimations.
SET STATISTICS TIME ON 
SET STATISTICS IO ON
	EXEC dbo.usp_TempTableTest @Quantity = 360
SET STATISTICS IO OFF
SET STATISTICS TIME OFF 
GO

/*Final Verdict, Feature is good for legacy code, if lots of table variables were used and we want improvement without making any code changes. If you are anyway designing new code, its more reliable to use Temporary Tables for varying data outputs/loads, and not rely on this feature, as feature resolves one issue but brings up another with parameter sniffing.*/

--Yasub

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