Blocks the execution of a batch, stored procedure, or transaction until a specified time or time interval is reached, or a specified statement modifies or returns at least one row.
While executing the WAITFOR statement, the transaction is running and no other requests can run under the same transaction.
The actual time delay may vary from the time specified in time_to_pass, time_to_execute, or timeout and depends on the activity level of the server. The time counter starts when the thread associated with the WAITFOR statement is scheduled. If the server is busy, the thread may not be immediately scheduled; therefore, the time delay may be longer than the specified time.
WAITFOR does not change the semantics of a query. If a query cannot return any rows, WAITFOR will wait forever or until TIMEOUT is reached, if specified.
Cursors cannot be opened on WAITFOR statements.
Views cannot be defined on WAITFOR statements.
When the query exceeds the query wait option, the WAITFOR statement argument can complete without running. For more information about the configuration option, see query wait Option. To see the active and waiting processes, use sp_who.
Each WAITFOR statement has a thread associated with it. If many WAITFOR statements are specified on the same server, many threads can be tied up waiting for these statements to run. SQL Server monitors the number of threads associated with WAITFOR statements, and randomly selects some of these threads to exit if the server starts to experience thread starvation.
You can create a deadlock by running a query with WAITFOR within a transaction that also holds locks preventing changes to the rowset that the WAITFOR statement is trying to access. SQL Server identifies these scenarios and returns an empty result set if the chance of such a deadlock exists.
A. Using WAITFOR TIME
The following example executes the stored procedure sp_update_job at 10:20 P.M. (22:20).
USE msdb; EXECUTE sp_add_job @job_name = 'TestJob'; BEGIN WAITFOR TIME '22:20'; EXECUTE sp_update_job @job_name = 'TestJob', @new_name = 'UpdatedJob'; END; GO
B. Using WAITFOR DELAY
The following example executes the stored procedure after a two-hour delay.
BEGIN WAITFOR DELAY '02:00'; EXECUTE sp_helpdb; END; GO
C. Using WAITFOR DELAY with a local variable
The following example shows how a local variable can be used with the WAITFOR DELAY option. A stored procedure is created to wait for a variable period of time and then returns information to the user as to the number of hours, minutes, and seconds that have elapsed.
USE AdventureWorks2008R2; GO IF OBJECT_ID('dbo.TimeDelay_hh_mm_ss','P') IS NOT NULL DROP PROCEDURE dbo.TimeDelay_hh_mm_ss; GO CREATE PROCEDURE dbo.TimeDelay_hh_mm_ss ( @DelayLength char(8)= '00:00:00' ) AS DECLARE @ReturnInfo varchar(255) IF ISDATE('2000-01-01 ' + @DelayLength + '.000') = 0 BEGIN SELECT @ReturnInfo = 'Invalid time ' + @DelayLength + ',hh:mm:ss, submitted.'; -- This PRINT statement is for testing, not use in production. PRINT @ReturnInfo RETURN(1) END BEGIN WAITFOR DELAY @DelayLength SELECT @ReturnInfo = 'A total time of ' + @DelayLength + ', hh:mm:ss, has elapsed! Your time is up.' -- This PRINT statement is for testing, not use in production. PRINT @ReturnInfo; END; GO /* This statement executes the dbo.TimeDelay_hh_mm_ss procedure. */ EXEC TimeDelay_hh_mm_ss '00:00:10'; GO
Here is the result set.
A total time of 00:00:10, in hh:mm:ss, has elapsed. Your time is up.