Best way to check for current date in where clause

问题:

I'm trying to find out the most efficient (best performance) way to check date field for current date. Currently we are using:

SELECT     COUNT(Job) AS Jobs
FROM         dbo.Job
WHERE     (Received BETWEEN DATEADD(d, DATEDIFF(d, 0, GETDATE()), 0)
                        AND DATEADD(d, DATEDIFF(d, 0, GETDATE()), 1))

回答1:

WHERE
  DateDiff(d, Received, GETDATE()) = 0

Edit: As lined out in the comments to this answer, that's not an ideal solution. Check the other answers in this thread, too.

回答2:

If you just want to find all the records where the Received Date is today, and there are records with future Received dates, then what you're doing is (very very slightly) wrong... Because the Between operator allows values that are equal to the ending boundary, so you could get records with Received date = to midnight tomorrow...

If there is no need to use an index on Received, then all you need to do is check that the date diff with the current datetime is 0...

Where DateDiff(day, received, getdate()) = 0

This predicate is of course not SARGable so it cannot use an index... If this is an issue for this query then, assuming you cannot have Received dates in the future, I would use this instead...

Where Received >= DateAdd(day, DateDiff(Day, 0, getDate()), 0) 

If Received dates can be in the future, then you are probably as close to the most efficient as you can be... (Except change the Between to a >= AND < )

回答3:

If you want performance, you want a direct hit on the index, without any CPU etc per row; as such, I would calculate the range first, and then use a simple WHERE query. I don't know what db you are using, but in SQL Server, the following works:

// ... where @When is the date-and-time we have (perhaps from GETDATE())
DECLARE @DayStart datetime, @DayEnd datetime
SET @DayStart = CAST(FLOOR(CAST(@When as float)) as datetime) -- get day only
SET @DayEnd = DATEADD(d, 1, @DayStart)

SELECT     COUNT(Job) AS Jobs
FROM         dbo.Job
WHERE     (Received >= @DayStart AND Received < @DayEnd)

回答4:

that's pretty much the best way to do it. you could put the DATEADD(d, DATEDIFF(d, 0, GETDATE()), 0) and DATEADD(d, DATEDIFF(d, 0, GETDATE()), 1) into variables and use those instead but i don't think that this will improve performance.

回答5:

I'm not sure how you're defining "best" but that will work fine.

However, if this query is something you're going to run repeatedly you should get rid of the get_date() function and just stick a literal date value in there via whatever programming language you're running this in. Despite their output changing only once every 24 hours, get_date(), current_date(), etc. are non-deterministic functions, which means that your RDMS will probably invalidate the query as a candidate for storing in its query cache if it has one.

回答6:

How 'bout

 WHERE
      DATEDIFF(d, Received, GETDATE()) = 0

回答7:

I would normally use the solution suggested by Tomalak, but if you are really desperate for performance the best option could be to add an extra indexed field ReceivedDataPartOnly - which would store data without the time part and then use the query

declare @today as datetime
set @today = datediff(d, 0, getdate())

select     
    count(job) as jobs
from         
    dbo.job
where     
    received_DatePartOnly = @today