Most Efficient Way in SQL Server to Get Date from Date+Time

Most efficient way in SQL Server to get date from date+time?

I must admit I hadn't seen the floor-float conversion shown by Matt before. I had to test this out.

I tested a pure select (which will return Date and Time, and is not what we want), the reigning solution here (floor-float), a common 'naive' one mentioned here (stringconvert) and the one mentioned here that I was using (as I thought it was the fastest).

I tested the queries on a test-server MS SQL Server 2005 running on a Win 2003 SP2 Server with a Xeon 3GHz CPU running on max memory (32 bit, so that's about 3.5 Gb). It's night where I am so the machine is idling along at almost no load. I've got it all to myself.

Here's the log from my test-run selecting from a large table containing timestamps varying down to the millisecond level. This particular dataset includes dates ranging over 2.5 years. The table itself has over 130 million rows, so that's why I restrict to the top million.

SELECT TOP 1000000 CRETS FROM tblMeasureLogv2 
SELECT TOP 1000000 CAST(FLOOR(CAST(CRETS AS FLOAT)) AS DATETIME) FROM tblMeasureLogv2
SELECT TOP 1000000 CONVERT(DATETIME, CONVERT(VARCHAR(10), CRETS, 120) , 120) FROM tblMeasureLogv2 
SELECT TOP 1000000 DATEADD(DAY, DATEDIFF(DAY, 0, CRETS), 0) FROM tblMeasureLogv2

SQL Server parse and compile time: CPU time = 0 ms, elapsed time = 1 ms.

(1000000 row(s) affected) Table 'tblMeasureLogv2'. Scan count 1, logical reads 4752, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times: CPU time = 422 ms, elapsed time = 33803 ms.

(1000000 row(s) affected) Table 'tblMeasureLogv2'. Scan count 1, logical reads 4752, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times: CPU time = 625 ms, elapsed time = 33545 ms.

(1000000 row(s) affected) Table 'tblMeasureLogv2'. Scan count 1, logical reads 4752, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times: CPU time = 1953 ms, elapsed time = 33843 ms.

(1000000 row(s) affected) Table 'tblMeasureLogv2'. Scan count 1, logical reads 4752, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times: CPU time = 531 ms, elapsed time = 33440 ms. SQL Server parse and compile time: CPU time = 0 ms, elapsed time = 1 ms.

SQL Server Execution Times: CPU time = 0 ms, elapsed time = 1 ms.

What are we seeing here?

Let's focus on the CPU time (we're looking at conversion), and we can see that we have the following numbers:

Pure-Select:  422
Floor-cast:   625
String-conv: 1953
DateAdd:      531  

From this it looks to me like the DateAdd (at least in this particular case) is slightly faster than the floor-cast method.

Before you go there, I ran this test several times, with the order of the queries changed, same-ish results.

Is this something strange on my server, or what?

What is the most efficient way to extract a date from a timestamp in PostgreSQL?

To summarize the comments, your first and third solution are identical. Casting to a date simply uses the date function according to @Nick Barnes.

Those options, plus option 2, requires a function to be run against every row of the table, so even if you have an index, it cannot be used.

Assuming there is an index on created_on, this is your best bet:

select * from foo
where created_on >= '2014/1/1 00:00:00'
  and created_on < '2014/1/2 00:00:00';

Most efficient way to compare datetime to varchar

Since this is a vendor application and you don't have control over the schema, you can refactor the WHERE clause as below for a sargable expression. This will allow an index on somedateas to be used efficiently. For example:

WHERE a.somedateas BETWEEN REPLACE(CONVERT(varchar(19), @begin, 120), '-', '_') AND REPLACE(CONVERT(varchar(19), @end, 120), '-', '_') 

How to return only the Date from a SQL Server DateTime datatype

SELECT DATEADD(dd, 0, DATEDIFF(dd, 0, @your_date))

for example

SELECT DATEADD(dd, 0, DATEDIFF(dd, 0, GETDATE()))

gives me

2008-09-22 00:00:00.000

Pros:

• No varchar<->datetime conversions required
• No need to think about locale

Best approach to remove time part of datetime in SQL Server

Strictly, method a is the least resource intensive:

a) select DATEADD(dd, DATEDIFF(dd, 0, getdate()), 0)

Proven less CPU intensive for the same total duration a million rows by someone with way too much time on their hands: Most efficient way in SQL Server to get a date from date+time?

I saw a similar test elsewhere with similar results too.

I prefer the DATEADD/DATEDIFF because:

• varchar is subject to language/dateformat issues
  
  Example: Why is my CASE expression non-deterministic?
• float relies on internal storage
• it extends to work out first day of month, tomorrow, etc by changing "0" base

Edit, Oct 2011

For SQL Server 2008+, you can CAST to date i.e. CAST(getdate() AS date). Or just use date datatype so no time to remove.

Edit, Jan 2012

A worked example of how flexible this is: Need to calculate by rounded time or date figure in sql server

Edit, May 2012

Do not use this in WHERE clauses and the like without thinking: adding a function or CAST to a column invalidates index usage. See number 2 here Common SQL Programming Mistakes

Now, this does have an example of later SQL Server optimiser versions managing CAST to date correctly, but generally it will be a bad idea ...

Edit, Sep 2018, for datetime2

DECLARE @datetime2value datetime2 = '02180912 11:45' --this is deliberately within datetime2, year 0218
DECLARE @datetime2epoch datetime2 = '19000101'

select DATEADD(dd, DATEDIFF(dd, @datetime2epoch, @datetime2value), @datetime2epoch)

What is the fastest way to get only time part of datetime

Just cast/convert it to a time, as that retain the correct typing (it's still a date and time data type):

SELECT CONVERT(time(0),datetimevalue), CAST(datetimevalue AS time(3))

As the value is a datetime, the value will be accurate to 1/300th of a second, however, choose a precision appropriate for your data.

How to get Time from DateTime format in SQL?

SQL Server 2008:

SELECT cast(AttDate as time) [time]
FROM yourtable

Earlier versions:

SELECT convert(char(5), AttDate, 108) [time]
FROM yourtable

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