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Max or Top 1 more efficient


Max or Top 1 more efficient

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tshad
tshad
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I have a table with a month value and year value both ints.

I need two things, the latest date and the date 24 months before.

I came up with these to get the latest date and trying to find the most efficient.

SELECT MAX(CONVERT(smalldatetime, CAST(MonthValue AS varchar) + '/01/' +
CAST(YearValue AS varchar))) AS ValueDate
FROM dbo.myTable


This gets me a plan of

Stream Aggregate (Aggregate) -Cost 7%
Compute Scalar - Cost 1%
Clustered Index Scan - Cost 91%

The other option is:

SELECT TOP 1
CONVERT(smalldatetime, CAST(MonthValue AS varchar) + '/01/' +
CAST(YearValue AS varchar)) AS ValueDate
FROM dbo.myTable
ORDER BY ValueDate DESC


Sort (top N Sort) -Cost 92%
Compute Scalar - Cost 0%
Clustered Index Scan - Cost 8%

Which one is the most efficient? The costs seem equivelant.

The whole query (depending on which select I use) would be:


DECLARE @ValueDate smalldatetime

SELECT @ValueDate =
(SELECT TOP 1
CONVERT(smalldatetime, CAST(MonthValue AS varchar) + '/01/' +
CAST(YearValue AS varchar)) AS ValueDate
FROM dbo.myTable
ORDER BY ValueDate DESC)

SELECT DATEADD(month, -23, @ValueDate)



Thanks,

Tom
Lynn Pettis
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Or this:



with MaxDate as (
select
max(dateadd(month, MonthValue - 1, dateadd(year, YearValue - 1900, 0))) as MaxDate
from
dbo.DealerForecastSegmentValue
)
select
md.MaxDate,
dateadd(month, -23, md.MaxDate) as MinDate
from
MaxDate;




Cool
Lynn Pettis

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How long do they take to run?

How long does a potentially SARGable
SELECT  MAX(CONVERT(smalldatetime, CAST(MonthValue AS varchar) + '/01/' +
CAST(YearValue AS varchar))) AS ValueDate
FROM (
SELECT TOP 1 YearValue, MonthValue
FROM dbo.myTable
ORDER BY YearValue DESC, MonthValue DESC
) d

query take?


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tshad (4/24/2013)
I have a table with a month value and year value both ints.

I need two things, the latest date and the date 24 months before.

I came up with these to get the latest date and trying to find the most efficient.

SELECT MAX(CONVERT(smalldatetime, CAST(MonthValue AS varchar) + '/01/' +
CAST(YearValue AS varchar))) AS ValueDate
FROM dbo.myTable


This gets me a plan of

Stream Aggregate (Aggregate) -Cost 7%
Compute Scalar - Cost 1%
Clustered Index Scan - Cost 91%

The other option is:

SELECT TOP 1
CONVERT(smalldatetime, CAST(MonthValue AS varchar) + '/01/' +
CAST(YearValue AS varchar)) AS ValueDate
FROM dbo.myTable
ORDER BY ValueDate DESC


Sort (top N Sort) -Cost 92%
Compute Scalar - Cost 0%
Clustered Index Scan - Cost 8%

Which one is the most efficient? The costs seem equivelant.

The whole query (depending on which select I use) would be:


DECLARE @ValueDate smalldatetime

SELECT @ValueDate =
(SELECT TOP 1
CONVERT(smalldatetime, CAST(MonthValue AS varchar) + '/01/' +
CAST(YearValue AS varchar)) AS ValueDate
FROM dbo.myTable
ORDER BY ValueDate DESC)

SELECT DATEADD(month, -23, @ValueDate)



Thanks,

Tom


A better thing to do would be to add a persisted calculated column to do the datetime conversion and then index that. Things will be much faster that way.

As a bit of a sidebar, a more efficient method for doing the calculation would be to get rid of all the VARCHAR conversions and stick to integer math. It's pretty simple, too.

DateTimeValue = DATEADD(mm,YearValue*12-22801+MonthValue,0)




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RBAR is pronounced ree-bar and is a Modenism for Row-By-Agonizing-Row.
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Stop thinking about what you want to do to a row... think, instead, of what you want to do to a column.
Although they tell us that they want it real bad, our primary goal is to ensure that we dont actually give it to them that way.
Although change is inevitable, change for the better is not.
Just because you can do something in PowerShell, doesnt mean you should. Wink

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ScottPletcher
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Just in case, IF you already have an index on ( YearValue, MonthValue ), then this should be extremely fast:


SELECT
MAX(YearValue) AS YearValue, MAX(MonthValue) AS MonthValue --or some computation to turn this into a datetime
FROM dbo.myTable
WHERE
YearValue = ( SELECT MAX(YearValue) FROM dbo.myTable )



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