Question

I want to use a database table as a queue. I want to insert in it and take elements from it in the inserted order (FIFO). My main consideration is performance because I have thousands of these transactions each second. So I want to use a SQL query that gives me the first element without searching the whole table. I do not remove a row when I read it.
Does SELECT TOP 1 ….. help here?
Should I use any special indexes?

Answer 1

I’d use an IDENTITY field as the primary key to provide the uniquely incrementing ID for each queued item, and stick a clustered index on it. This would represent the order in which the items were queued.

To keep the items in the queue table while you process them, you’d need a “status” field to indicate the current status of a particular item (e.g. 0=waiting, 1=being processed, 2=processed). This is needed to prevent an item be processed twice.

When processing items in the queue, you’d need to find the next item in the table NOT currently being processed. This would need to be in such a way so as to prevent multiple processes picking up the same item to process at the same time as demonstrated below. Note the table hints UPDLOCK and READPAST which you should be aware of when implementing queues.

e.g. within a sproc, something like this:

DECLARE @NextID INTEGER

BEGIN TRANSACTION

-- Find the next queued item that is waiting to be processed
SELECT TOP 1 @NextID = ID
FROM MyQueueTable WITH (UPDLOCK, READPAST)
WHERE StateField = 0
ORDER BY ID ASC

-- if we've found one, mark it as being processed
IF @NextId IS NOT NULL
    UPDATE MyQueueTable SET Status = 1 WHERE ID = @NextId

COMMIT TRANSACTION

-- If we've got an item from the queue, return to whatever is going to process it
IF @NextId IS NOT NULL
    SELECT * FROM MyQueueTable WHERE ID = @NextID

If processing an item fails, do you want to be able to try it again later? If so, you’ll need to either reset the status back to 0 or something. That will require more thought.

Alternatively, don’t use a database table as a queue, but something like MSMQ – just thought I’d throw that in the mix!