Is this date comparison condition SARG-able in SQL?

问题

Is this condition sargable?

AND  DATEDIFF(month,p.PlayerStatusLastTransitionDate,@now) BETWEEN 1 AND 7)

My rule of thumb is that a function on the left makes condition non sargable.. but in some places I have read that BETWEEN clause is sargable. So does any one know for sure?

For reference:

What makes a SQL statement sargable?
http://en.wikipedia.org/wiki/Sargable

NOTE: If any guru ends here, please do update Sargable Wikipedia page. I updated it a little bit but I am sure it can be improved more :)

回答1:

Using AdventureWorks, if we look at these two equivalent queries:

SELECT OrderDate FROM Sales.SalesOrderHeader
WHERE DATEDIFF(month,OrderDate,GETDATE()) BETWEEN 1 AND 7;

SELECT OrderDate FROM Sales.SalesOrderHeader
WHERE OrderDate >= DATEADD(MONTH, -7, GETDATE())
  AND OrderDate <= DATEADD(MONTH, -1, GETDATE());

In both cases we see a clustered index scan:

But notice the recommended/missing index only on the latter query, since it's the only one that could benefit from it:

If we add an index to the OrderDate column, then run the queries again:

CREATE INDEX dt ON Sales.SalesOrderHeader(OrderDate);
GO

SELECT OrderDate FROM Sales.SalesOrderHeader
WHERE DATEDIFF(month,OrderDate,GETDATE()) BETWEEN 1 AND 7;

SELECT OrderDate FROM Sales.SalesOrderHeader
WHERE OrderDate >= DATEADD(MONTH, -7, GETDATE())
  AND OrderDate <= DATEADD(MONTH, -1, GETDATE());

We see much difference - the latter uses a seek:

Notice too how the estimates are way off for your version of the query. This can be absolutely disastrous on a large data set.

There are very few cases where a function or other expression applied to the column will be sargable. One case I know of is CONVERT(DATE, datetime_column) - but that particular optimization is undocumented, and I recommend staying away from it anyway. Not only because you'd be implicitly suggesting that using functions/expressions against columns is okay (it's not in every other scenario), but also because it can lead to wasted reads and disastrous estimates.

回答2:

I would be very surprised if that was sargable. One option might be to rewrite it as:

WHERE p.PlayerStatusLastTransitionDate >= DATEADD(month,1,CAST(@now AS DATE))
AND   p.PlayerStatusLastTransitionDate <= DATEADD(month,7,CAST(@now AS DATE))

Which I believe will be sargable (even though it's not quite as pretty).

来源：https://stackoverflow.com/questions/10853774/is-this-date-comparison-condition-sarg-able-in-sql

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