I <3 SET STATISTICS IO ON

Once upon a time, a senior DBA at the Salesforce Marketing Cloud had pointed out a very small performance tweak to me. I was curious about whether or not it still held true in SQL Server 2014, so I wrote up the sample below. What’s interesting to me is that a cursory examination of the Execution Plan doesn’t reveal the difference. Of course, if you dig deep enough, you’ll find it there. But it’s not obvious.

On the other hand, by using SET STATISTICS IO ON, I can easily see the difference. Hopefully, this isn’t some new earth-shattering development for you so much as a simple reminder to make use of this statistical information.


IF NOT EXISTS (SELECT 1 FROM sys.indexes WHERE object_id = OBJECT_ID(N'Person.Person') AND name = N'IX_Person_PersonType_EmailPromotion')
CREATE NONCLUSTERED INDEX IX_Person_PersonType_EmailPromotion ON Person.Person (PersonType, EmailPromotion);

SET STATISTICS IO ON; SET NOCOUNT ON;
SELECT BusinessEntityID, EmailPromotion
FROM Person.Person
WHERE PersonType = N'SC'
AND EmailPromotion IN (1, 2);
SELECT BusinessEntityID, EmailPromotion
FROM Person.Person
WHERE PersonType = N'SC'
AND EmailPromotion BETWEEN 1 AND 2;
SET STATISTICS IO OFF;

DROP INDEX IX_Person_PersonType_EmailPromotion ON Person.Person;


Table 'Person'. Scan count 2, logical reads 5, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Person'. Scan count 1, logical reads 3, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

Wondering what exactly these results mean? Look here: https://msdn.microsoft.com/en-us/library/ms184361.aspx

Wondering what the tweak was? Look closely at the IN versus the BETWEEN, and then at the number of scans and reads.

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