Do stored procedures lock tables/rows?

Question

Quite a simple question. In SQL 2008 if I have a stored procedure (see below) do I run the risk of a race condition between the first two statements or does the stored procedure put a lock on the things it touches like transactions do?

ALTER PROCEDURE [dbo].[usp_SetAssignedTo] 
    -- Add the parameters for the stored procedure here
    @Server varchar(50), 
    @User varchar(50),
    @UserPool varchar(50)
AS
BEGIN
    SET NOCOUNT ON;

    -- Find a Free record
    Declare @ServerUser varchar(50)
    (SELECT top 1 @ServerUser = UserName from ServerLoginUsers
    where AssignedTo is null and [TsServer] = @Server)

    --Set the free record to the user
    Update ServerLoginUsers
    set AssignedTo = @User, AssignedToDate = getdate(), SourcePool = @UserPool
    where [TsServer] = @Server and UserName = @ServerUser

    --report record back if it was updated. Null if it was not available.
    select * 
    from ServerLoginUsers 
    where [TsServer] = @Server 
        and UserName = @ServerUser 
        and AssignedTo = @User
END

Answer 1

You could get a race condition.

It can be done in one statement:

• You can assign in an UPDATE
• The lock hints allow another process to skip this row
• The OUTPUT clause returns data to the caller

Try this... (edit: holdlock removed)

Update TOP (1) ServerLoginUsers WITH (ROWLOCK, READPAST)
OUTPUT INSERTED.*
SET
   AssignedTo = @User, AssignedToDate = getdate(), SourcePool = @UserPool
WHERE
   AssignedTo is null and [TsServer] = @Server   -- not needed -> and UserName = @ServerUser

If not, you may need a separate select

Update TOP (1) ServerLoginUsers WITH (ROWLOCK, READPAST)
SET
    -- yes, assign in an update
   @ServerUser = UserName,
   -- write
   AssignedTo = @User, AssignedToDate = getdate(), SourcePool = @UserPool
OUTPUT INSERTED.*
WHERE
   AssignedTo is null and [TsServer] = @Server   -- not needed -> and UserName = @ServerUser

SELECT ...

See this please for more: http://stackoverflow.com/questions/939831/sql-server-process-queue-race-condition