ORDER BY and WITH(ROWLOCK, UPDLOCK, READPAST)

Question

I need to set up a queue system using some SQL tables, like the one described here. That being, and since I need to filter queued items by different critera, inside a stored

Answer 1

As expected

• The SELECT with the ORDER BY, without ROWLOCK, without index will have a table lock because of a scan/intermediate sort to work out TOP 2. So the 2nd session skips the whole table because of READPAST

• The SELECT without the ORDER BY is just picking any 2 rows, which happen to be in order of insert (pure coincidence, there is no implied order). The fact that these 2 rows are locked causes the 2nd session to skip to the next non-locked rows.

SQL Server attempts to keep locks as granular as possible but the scan means a table lock. Now, this wouldn't normally make a difference (it'd be a shared read lock) but you have UPDLOCK too which means an exclusively locked table

So, you need both of these

• 3 hints in the SELECT queries (ROWLOCK, UPDLOCK, READPAST) to control granularity, isolation and concurrency.
  Using ROWLOCK only will still cause an exclusive lock on every row to scan/sort.
• an index on Value INCLUDE TestID to make the SELECT efficient. An index only will probably fix the concurrency but it won't be guaranteed.

In one of your previous questions I linked to my answer (in a comment) to SQL Server Process Queue Race Condition where I have all 3 lock hints